Cambridge IGCSE Chemistry Notes (0620) PDF

r/IGCSE Resources Topical Notes for Cambridge IGCSE™ Chemistry (0620) by C. 1st edition, for examination until 2025 Version 1 © r/IGCSE Resources 2023 Page 1 of 97  Chapter 1 1.1...

r/IGCSE Resources Topical Notes for Cambridge IGCSE™ Chemistry (0620) by C. 1st edition, for examination until 2025 Version 1 © r/IGCSE Resources 2023 Page 1 of 97  Chapter 1 1.1 States of matter The 3 states of matter Matter is a physical substance that occupies space and has mass. There are many millions of different substances known to us and all of them can be categorized as solid, liquid or gas. These are known as the three states of matter. There are more states of matter other than these three but they are not in the syllabus so they will not be covered. Property Solid Liquid Gas Fixed Volume Fixed volume Indefinite volume volume Indefinite Shape Indefinite Shape Fixed Shape Takes shape of the container Takes shape of the container shape stored in stored in Can Compress? ❌ ✅ but very little  Solids In solids, particles are packed close together. They cannot move and can only vibrated in a fixed position. Solids also have strong intermolecular forces due to which along with the particles being packed close together result in a fixed shape and volume. Solids expand only a little bit when heated. Liquids Particles in liquids are slightly further apart. They also have the ability to move past each other. Liquids have slightly weaker intermolecular forces, allowing them to flow and take the shape of their container. Liquids expand on heating more than solids, but less than gases. Gases Particles in gases are much further apart and usually don’t touch each other. Gases have really weak intermolecular forces and large inter-particle distances, due to which gases © r/IGCSE Resources 2023 Page 2 of 97  can flow easily and can take the shape of the container. Gases also expand the most when heated. Fig. 1: Arrangement of particles in solids, liquids and gases Changes of state When a solid turns into a liquid, we call it melting. When a liquid turns into a gas, we call it boiling/evaporation. When a gas turns into a liquid, we call it condensation When a liquid turns into a solid, we call it freezing. The diagram below summarizes these changes of state. Fig. 2: Changes of state Kinetic particle theory © r/IGCSE Resources 2023 Page 3 of 97  The kinetic particle theory states that all matter is made up of tiny particles which are always moving randomly When these particles have higher kinetic energy, they move faster and vice versa. Heating Heating of the object means the particles inside the object have a higher average kinetic energy and therefore move faster, once these particles are moving fast enough, they are able to break the intermolecular forces holding them together, causing a change in state. However, not all particles escape at the same time, and some may escape first while others may escape later. Cooling Cooling reduces the average kinetic energy of the particles in an object, this causes the intermolecular forces to be able to hold the particles back into a relatively fixed position. Heating and cooling curves Heating curves show how a substance’s temperature increases as it’s heated, while cooling curves show how it decreases as it cools, with flat portions on the graph indicating phase changes. (see the graphs below) Fig. 3: Diagram showing the heating curve of water from -15°C to 110°C Effect of pressure and temperature on gases © r/IGCSE Resources 2023 Page 4 of 97  Temperature Higher temperature causes gases to expand and increase in volume. This happens because when the temperature increases, so does the average kinetic energy of the particles of the object, making them move faster. When these particles collide with the walls of the container, they exert greater pressure on the walls and start to occupy more space causing the gas to expand. Pressure Higher pressure causes the volume of gases to decrease as it compresses the gas particles together. This reduces the space between particles, leading to a decrease in volume. 1.2 Diffusion As we read earlier, particles are always moving. Diffusion is a result of this movement of particles. Diffusion occurs because particles move randomly and collide with each other, spreading out from areas of higher concentration to lower concentration, equalizing the distribution over time. Effect of relative molecular mass on diffusion of a gas Relative molecular mass is the sum of the masses of the atoms in a molecule, giving the molecule’s weight. You’ll read more about relative molecular mass in chapter 3. The higher the relative mollecular mass of a gas, the slower it will diffuse and it will move slower than the gases which are lighter. © r/IGCSE Resources 2023 Page 5 of 97  Chapter 2 2.1 Elements, compounds and mixtures Elements Elements consist of only one type of atom They cannot be broken down into simpler substances by chemical means Each element has it’s own symbol (for example, H for hydrogen, O for oxygen) All elements can be classified as either metals or non-metals. Metals usually have high melting and boiling points while non-metals have low melting and boiling points. Compounds Compounds are formed when two or more different elements chemically combine in fixed ratios to form a new substance. Their properties are different than that of the compounds that they are made from Compounds can only be separated into their constituent elements through chemical reactions. Forming a compound is a chemical change Examples include: H2 SO4 , HCl, CO2 etc. Mixtures Mixtures are combinations of 2 or more substances (elements or compounds) not chemically bonded together. Mixtures can be separated by physical methods, such as filtration or distillation, without breaking chemical bonds. The properties of mixtures are based on the proportions of its constituents. Examples include: sea water, air, alloys (brass) Mixture Compound It contains two or more substances It is only one substance No chemical change when mixture is Formation of new substances involves chemical formed change The properties of the mixture are of The properties of the new substance are very those of the individual different from either of the elements used to form the elements/compounds in it substance © r/IGCSE Resources 2023 Page 6 of 97  2.2 Atomic structure and the Periodic Table In chapter one, you learned that matter is made up of very tiny particles, these particles are known as atoms. Atoms are also known as the smallest particles. Each atom contains a nucleus and electrons around the nucleus. The nucleus holds protons and neutrons. These electrons are placed in shells as shown by the diagram below. Fig. 1: Atomic structure of a carbon atom As you can tell by the diagram above, electrons are negative while protons are positive. Neutrons however are neutral and have no charge. Particle Symbol Relative mass Charge Proton p 1 +1 © r/IGCSE Resources 2023 Page 7 of 97  Particle Symbol Relative mass Charge Neutron n 1 0 Electron e 1/1837 -1 Atomic number The atomic number (also known as the proton number) of an element is the number of protons in one atom of that element. For example, in the diagram above (figure 1), carbon has 6 protons, which means its atomic number is 6. The atomic number is also known as Z. Mass number The mass number (also known as nucleon number) of an element is the total number of protons and neutrons in the nucleus of one atom of that element. For example, in the diagram above (figure 1), carbon has 6 protons and 6 neutrons, which means its mass number is 12. The mass number is also known as A. Electronic configuration Electronic configuration is the arrangement of electrons in the electron shells of an atom which tells us about the distribution of electrons in that atom. The first shell cannot have more than 2 electrons and the 2nd and 3rd shell cannot have more than 8 electrons. For example, in figure 1 (yes again), carbon has 6 electrons, 2 of which are in the first shell and 4 of which in the second shell. Therefore the electronic configuration of the atom would be 2, 4. Remember Group VIII elements are known as noble gases and have a full outer shell, meaning they are unreactive. The number of outer shell electrons is equal to the group number in Groups I to VII. The number of electron shells is equal to the period number © r/IGCSE Resources 2023 Page 8 of 97  2.3 Isotopes Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons. Isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration. To write isotopes, we use the following notation: AX Z where A is the mass number, Z is the proton number and X is the symbol of the element itself. For example, we can write carbon as: 12 C 6 Calculate relative atomic mass of an element from its isotopes To calculate the relative atomic mass (Ar ) of an element from its isotopes, we can 2 things: the relative mass of its isotopes abundance of each isotope (percentage) Using this information, we can use the following formula and calculate the relative atomic mass of an element: abundance abundance Ar = ( mass of isotope 1 × ) + ( mass of isotope 2 × ) 100 100 2.4 Ions and ionic bonds Ions Ions are atoms or molecules that have a charge (are not neutral; number of protons is not equal to number of electrons) due to the gain or loss of electrons. © r/IGCSE Resources 2023 Page 9 of 97  Remember Positive ions are known as cations. They are formed when atoms lose electrons, resulting in a positive charge due to more protons than electrons. Negative ions are known as anions. They are formed when atoms gain electrons, resulting in a negative charge due to more electrons than protons. Ionic Bonding An ionic bond is a strong electrostatic attraction between oppositely charged ions. Ionic bonds are usually found in compounds that contain metals combined with non-metals. Electrons are transferred from the metal atoms to the non-metal atoms. This makes the atom stable because of their full outer shell. We use a dot and cross diagram to show these bonds Fig. 2: Diagram showing bonding in NaCl using a dot (blue) and cross (red) diagram In the diagram above, one electron is transferred from the sodium atom to the chlorine atom, forming sodium ion and chloride ion which together form sodium chloride, an ionic compound. Structure of an ionic compound Ionic compounds have a repeating, closely packed structure where positively charged ions (cations) and negatively charged ions (anions) alternate, and they are held together by strong electrostatic forces. This arrangement is known as a lattice. Many millions of ions would be arranged in this way to make up the giant ionic lattice structure. © r/IGCSE Resources 2023 Page 10 of 97  Properties of an ionic compound Properties of ionic compounds include: high melting points and boiling points good electrical conductivity when aqueous or molten and poor when solid usually solid at room temperature High melting points and boiling points These compounds have strong electrostatic forces of attraction between oppositely charged ions throughout the entire lattice, which require a lot of energy to break/overcome which leads to higher melting and boiling points Good electrical conductivity when aqueous or molten, poor when solid First we need to know what conducts electricity and what is electricity. In one simple line, electricity is the flow of electrons and therefore is conducted by either electrons or ions. Solid ionic compounds are held firmly in place in the lattice and hence ions in them cannot move, making them a bad conductor. Meanwhile when this ionic compound is dissolved in water or molten, the ions in the substance disassociate and are able to move, which means that they can now conduct electricity. Forming ionic compounds To form a stable compound, the charges on the opposing ions must always cancel out each other so that the overall charge of the compound is 0. For example, + − One Na ion + one Cl ion NaCl and similarly: 2+ − One Mg ion + two Cl ions MgCl2 To figure out the charge of the ion of an element, we can refer to it’s oxidation number. Transition elements have multiple oxidation numbers, and they have names like copper(ii) which means that its oxidation number is 2 To find out the oxidation number for other elements however, we use the number of valence electrons (which is simply the number of electrons in the last shell of an atom of that element). If number of valence electrons (electrons in last shell) is more than 4 (so 5, 6, 7, 8) then we subtract the number of valence electrons by 8. (valence electrons - 8) © r/IGCSE Resources 2023 Page 11 of 97  As we read earlier in 2.2 that the number of outer shell electrons is equal to the group number, which means that Group I elements will have an oxidation number of 1, Group II will have an oxidation number of 2 and Group VI will have a oxidation number of -2. Usually, metals have positive oxidation numbers and non-metals have negative oxidation numbers. 2.5 Simple molecules and covalent bonds Covalent bonds are bonds in which atoms share pairs of electrons to form stable compounds. A simple example would be Hydrogen gas (H2 ) In hydrogen gas, 2 hydrogen atoms share electrons to become stable and form a compound. (see diagram below) Fig. 3: Electronic configuration of hydrogen gas Similarly, oxygen gas (O2 ) also has covalent bonds along with: chlorine gas (Cl2 ) water (H2 O) methane (CH4 ) ammonia (NH3 ) hydrochloric acid (HCl) metanol (CH3 OH) ethylene (C2 H4 ) carbon dioxide (CO2 ) and many more. © r/IGCSE Resources 2023 Page 12 of 97  Fig. 4: Structures of oxygen, carbon dioxide and methanol Properties of simple covalent compounds Properties of simple covalent compounds include: Low melting points and boiling points Poor electrical conductivity Low melting points and boiling points Simple covalent compounds have weak intermolecular forces (forces that hold atoms together within a molecule) which means that less energy is required to break these forces which means that they will have low melting and boiling points. Poor electrical conductivity As you read earlier, to conduct electricity, we need either free ions or delocalized electrons and covalent compounds have none of these therefore they cannot conduct electricity. 2.6 Giant covalent structures © r/IGCSE Resources 2023 Page 13 of 97  Giant covalent structures are basically “networks” of atoms bonded together by strong covalent bonds, examples include diamond, graphite and silicon(IV) oxide. Structure of graphite Graphite consists of many layers carbon atoms arranged in a hexagonal lattice structure where each carbon atom forms three covalent bonds with three neighboring carbon atoms in the same layer. These layers have weak forces of attraction holding them together, allowing the layers to slide over each other. Graphite is a non-metal, however, it does conduct electricity. As you read above, carbon atoms form only three covalent bonds which means that an unbonded electron is available on each carbon atom. These spare/delocalized electrons between the layers allow graphite to conduct electricity. Graphite is used as a lubricant since it is slippery thanks to the weak forces holding the layers together. It is also used as an electrode because of its good electrical conductivity and how cheap it is. Structure of diamond Diamond is composed of a network of carbon atoms, each bonded to four other carbon atoms through strong covalent bonds in a tetrahedral arrangement. This network results in a very hard and rigid structure, making diamond one of the hardest substances known. Due to its strong covalent bonds, diamond has an exceptionally high melting point Diamond does not conduct electricity because it does not have free electrons or ions. Diamond is used as a cutting tool (it is really hard) as well as in jewellery (it is shiny and looks nice) Structure of silicon(IV) oxide (SiO2) Silicon(IV) oxide, SiO2, is also known as silica or quartz (calm your horses fellow minecraft players). It has a giant covalent structure where each silicon atom is bonded to four oxygen atoms through strong covalent bonds in a tetrahedral arrangement. These tetrahedral units form a network throughout the crystal lattice, creating a hard and rigid structure with a high melting point. Silicon dioxide is also not a conductor of electricity as there are no free ions or electrons in its structure. © r/IGCSE Resources 2023 Page 14 of 97  Now, the structure may seem similar to that of diamond, except for a key difference, diamond is made up of entirely carbon while silicon(IV) oxide is made up of both silicon and oxygen atoms and does not contain any carbon. There are also many similiarities between their structures such as: Hardness High melting points Do not conduct electricity 2.7 Metallic bonding Metallic bonding is the electrostatic attraction between the positive ions in a giant metallic lattice and a sea of delocalised, mobile electrons. Fig. 5: Metallic bonding Properties of metals Metals have good electrical conductivity, and are ductile (drawn into wires) and malleable (hammered into thin sheets). Good electrical conductivity As we read earlier, to conduct electricity we need free electrons and metals do indeed have free electrons which means that they can conduct electricity. Malleability and ductility © r/IGCSE Resources 2023 Page 15 of 97  Metal bonds are indeed rigid but they aren’t fixed unlike bonds in diamond which means that when a force is applied, layers of metal ions can easily slide past each other without breaking the metallic bonds allowing them to be drawn into wires (ductility) or thin sheets (malleability). © r/IGCSE Resources 2023 Page 16 of 97  Chapter 3 3.1 Formulae Molecular formula: The molecular formula of a compound tells you how many and what kinds of atoms are in one molecule of that substance. Empirical formula: The empirical formula of a compound tells you the simplest ratio of atoms in one molecule of that substance. you can figure out the compound’s molecular formula by counting the atoms in it’s structure using which you can figure out it’s empirical formula by reducing the ratios to their simplest whole numbers. Fig. 1: Displayed formula of ethane ()NEUROtiker, Public domain, via Wikimedia Commons For example, we can count that there are 2 carbon atoms and 6 hydrogen atoms in the figure above, which means it’s molecular formula would be C2 H6. Now to get the empirical formula, we can simply simplify this to get CH3. Similarly with ionic compounds, you can deduce the formula by balancing the charges on the ions in a model or diagram, ensuring that the total charge is neutral (0), to find the compound’s formula. Constructing equations Word equations and symbol equations are used to describe chemical reactions, including the reactants, products, and their physical states. A simple example of a word equation would be: Hydrogen gas + Oxygen gas Water vapor While the symbol equation for the same reaction would be: © r/IGCSE Resources 2023 Page 17 of 97  H2 (g) + O2 (g) H2 O (g) Notice the (g) next to the symbols? This is known as a state symbol which tells us about the physical state of the substance. (g) means gas (l) means liquid (s) means solid (aq) means aqueous which means that the substance is dissolved in water Constructing ionic equations Now let’s say we have this equation right here: NaCl (aq) + AgNO3 (aq) AgCl (s) + NaNO3 (aq) which is a double displacement reaction (two compounds exchange ions to form two new compounds) in which sodium chloride and silver nitrate react to form silver chloride and sodium nitrate. From the equation above, we can tell that the ions undergoing change are chloride ions (from NaCl) and silver ions (from AgNO3 ). So for this reaction, we can write that the ionic equation for this reaction is: Cl− (aq) + Ag+ (aq) AgCl (s) 3.2 Relative masses of atoms and molecules There are 118 different known elements in the periodic table. All have a different mass. The masses of atoms of these elements are really, really small and hard to work with which is why we use something called relative atomic mass (Ar). In this scale, an atom of carbon is given a relative atomic mass of 12. In simple words, relative atomic mass, Ar, is the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of carbon-12. Relative molecular mass, Mr, is the sum of the relative atomic masses of all the atoms in a compound. For example we have this equation: C + O2 CO2 The relative atomic mass of C is 12 and O is 16. Now there are 2 oxygen atoms, which means that the relative atomic mass of O2 will be 32. © r/IGCSE Resources 2023 Page 18 of 97  Using this, we can find the relative molecular mass of CO2 by adding 12 and 32 which equals to 44. Therefore the mass of CO2 is 44. Now we can add units to this to make these into actual masses, so if 12g of carbon was burned in air (another way of saying reacted with excess oxygen; don’t get confused with decomposition) and 44g of CO2 was produced. Using this statement, we can figure out that when 6g of carbon is burned in air, 22g of CO2 will be produced. 3.3 The mole and the Avogadro constant A mole is a unit of measurement used to express the quantity of atoms, molecules, ions, or particles in a substance. One mole contains 6.02 × 1023 particles (atoms, ions, molecules). This number is known as the Avogadro’s constant. Calculating moles To calculate moles, we can use the formula: mass moles = molar mass Where moles is in mol , mass is in g and molar mass is in g/mol. We can further multiply this number by 6.02 × 1023 to find out the number of particles in the substance. For example, to calculate the number of particles in 24g of carbon, we need to do the following: The mass given is 24g, and the molar mass of carbon is 12 g/mol. Now we need to use the equation above to calculate the number of moles: mass 24 moles = = =2 molar mass 12 which comes out to 2 mol. Now we can simply multiply this number by 6.02 × 1023 to find the number of particles: 2 × 6.02 × 1023 = 1.204 × 1024 particles Gases and moles © r/IGCSE Resources 2023 Page 19 of 97  One mole of any substance takes up a volume of 24 dm3 at room temperature and pressure (r.t.p.). Using this statement, we can deduce the following formula: volume of gas in dm3 number of moles in a gas = 24 dm3 Remember, 1 dm3 = 1000 cm3. Moles and solutions Sometimes concentration is measured in g/dm3 but more often it is measured in mol/dm3. number of moles concentration (in mol/dm3 ) = volume (in dm3 ) The same equation above can be rearranged to find out the number of moles: number of moles = concentration (in mol/dm3 ) × volume (in dm3 ) Tip An easy way to remember formulas is using the units. For example the unit of concentration is mol/dm3, and the formula to calculate concentration is moles/volume. The unit of moles is mol and the unit of volume is dm3 Calculating formulae To get the molecular formula of the compound, we use the formula below relative molecular mass of compound ( ) × empirical formula mass of empirical formula You read about the molecular and empirical formula of ethane earlier. Ethane’s molecular formula is C2 H6 while it’s empirical formula is CH3. Now we can find the relative molecular mass of the compound by adding the relative atomic mass of all the atoms together which is: (2 × 12) + (6 × 1) = 30 And the relative molecular mass of the empirical formula would be: (1 × 12) + (3 × 1) = 15 © r/IGCSE Resources 2023 Page 20 of 97  Now we can use the formula above to find the molecular formula of ethane: 30 × CH3 = 2 × CH3 = C2 H6 15 Moles and chemical equations The law of conservation of mass states that mass can neither be created nor destroyed. This means that the mass on the left side of the equation has to be equal to the right side of the equation. Percentage Yield Chemical reactions are not 100% efficient, they do not always produce the expected amount of product. The percentage yield of the reaction is based on the amount of product that is actually produced against what should have been produced if the reaction was 100% efficient. It can be calculated using the formula below: actual yield percentage yield = ( ) × 100 theoretical yield Percentage composition Percentage composition is used to describe the percentage by mass of each element in the compound. The formula to calculate it is: Mass of element Percentage of element = ( ) × 100 Total mass of compound Percentage purity The purity of a substance is very important. It can be obtained using the formula below: Mass of the pure product Percentage Purity = ( ) × 100 Mass of the impure product obtained The reactants that are left over at the end of reaction are known as excess reactants © r/IGCSE Resources 2023 Page 21 of 97  Chapter 4 4.1 Electrolysis Electrolysis is the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of electricity. Electrolysis occurs in an electrolytic cell. A simple electrolytic cell consists of: the anode, the positive electrode the cathode, the negative electrode the electrolyte as the molten or aqueous substance that undergoes electrolysis Important Electrodes are usually made from unreactive metals, such electrodes are known as inert electrodes. Graphite and Platinum are the commonly used inert electrodes. Cathode (negative) attracts cations (positive) Anode (positive) attracts anions (negative) Metals or hydrogen (exception) are formed at the cathode (because metal ions are positive/cations and cathodes are negative) and non-metals (except hydrogen) are formed at the anode. In electrolysis, an electric current is passed through an electrolyte causing the electrolyte to decompose. The electricity is carried through the electrolyte by ions which is why the ionic compound needs to be in a molten or aqueous state as the ions cannot move if it’s in a solid state. In this process, the transfer of charge occurs because of: the movement of electrons from the cathode to the anode the loss or gain of electrons at the electrodes the movement of ions in the electrolyte © r/IGCSE Resources 2023 Page 22 of 97  Important Oxidation and reduction Reduction is a process in which an atom, ion, or molecule gains electrons. Oxidation is the opposite of reduction, i.e. an atom, ion or molecule loses electrons. To remember this, use this mnemonic OIL RIG: Oxidation Is Loss Reduction Is Gain Something called reducing agents and oxidising agents also exist. The substance being oxidised is the reducing agent while the substance being reduced is the oxidising agent. Electrolysis of lead(II) bromide Precaution: must be done in a fume cupboard Binary compounds are those that contain two elements chemically combined. Lead(II) Bromide is an example of this. To carry out the electrolysis of lead(II) bromide, we first need to melt it since it is an ionic compound We use an electrolytic cell like the one below: © r/IGCSE Resources 2023 Page 23 of 97  Fig. 1: An electrolytic cell showing the electrolysis of lead(ii) bromide Here is the equation of the electrolysis of Lead(II) Bromide: PbBr2 (l) Br2 (g) + Pb (l) In the process, the lead ions are reduced to the lead atom (gain of electrons), here’s the half-ionic equation for it: Pb2+ (l) + 2 e− Pb (l) The lead metal is deposited at the cathode since it’s a cation as it has a positive charge. The bromide ions lose an electron at the anode to become stable and become bromine atoms Br− (l) Br + e− Then two bromine atoms combine to form a bromine molecules The bromide ions are oxidised in this case (they lose electrons). During this process, you will notice you will see that an orange-red gas is being produced at the anode, that is bromine. © r/IGCSE Resources 2023 Page 24 of 97  You will also notice that lead is being formed at the cathode. Electrolysis of aluminium oxide Bauxite (aluminium ore) is an impure form of aluminium oxide. It is first treated with sodium hydroxide to obtain pure aluminium oxide, this is done to improve the conductivity of the molten aluminium oxide This is then dissolved in molten cryolite (Na3 AlF6 ), this is done to reduce the melting point of pure aluminium oxide (which is 2017°C ) down to between 800°C and 1000°C. Then, the Hall-Heroult cell is used for the process of electrolysis. Fig. 2: Diagram of a Hall-Heroult cell showing the electrolysis of aluminium oxide During this process, Oxide ions are oxidised and Aluminium ions are reduced. The half-ionic equations for both are given below. 2 O2− (l) O2 (g) + 4 e− Al3+ + 3 e− Al (l) Approximately 15 kWh of electricity is used to produce 1 kg of aluminium. The overall equation for this is: 2 Al2 O3 (l) 4 Al (l) + 3 O2 (g) As you may notice oxygen gas is being produced and since the electrodes are made of graphite, which is a form of carbon, the oxygen reacts with the carbon producing carbon dioxide as it has enough activation energy for the reaction to occur (due to the temperature of the cell). © r/IGCSE Resources 2023 Page 25 of 97  This means that eventually, the graphite anode will have to be replaced as it will burn away. (only the anode because oxygen will be formed at the anode, not the cathode). Electrolysis of concentrated aqueous sodium chloride Concentrated aqueous sodium chloride is a strong saltwater solution with a high amount of salt/sodium chloride dissolved in water. Since sodium chloride is an ionic compound, we have to dissolve it in water so that it can conduct electricity. By doing this, the ions in sodium chloride disassociate in water. There are 4 ions in this solution: H+ and OH− from water Na+ and Cl− from sodium chloride + + When the current flows, the H ions and Na ions are both attracted to the cathode. + But as hydrogen has stronger electrostatic forces, it accepts electrons more easily than the Na ions and that is why hydrogen gas is produced at the cathode instead of sodium metal. 2 H+ (aq) + 2 e− H2 (g) − − − Again, both OH ions and Cl ions are attracted to the anode. The Cl ions lose electrons more easily as they have weaker electrostatic forces (due to more number of shells) and chlorine gas is produced at the anode. 2 Cl− (aq) Cl2 (g) + 2 e− Electrolysis of dilute sulfuric acid Pure water is a very poor conductor of electricity because there are so few ions in it. It can be made to decompose using a Hoffman voltameter. © r/IGCSE Resources 2023 Page 26 of 97  Fig. 3: A hoffman voltameter, the anode and cathode is made of platinum To enable water to conduct electricity better, some dilute sulfuric acid or sodium hydroxide solution is added to it. When the power is turned on, gases are seen to be produced at both the electrodes. Twice as much gas is produced at the cathode than at the anode. The gas at the cathode burns with a pop, proving that it is hydrogen. In this process, positively charged hydrogen ions moved to the cathode and is being oxidised. 4 H+ (aq) + 4 e− 2 H2 (g) The gas collected at the anode can relight a glowing splinter, proving it to be oxygen. 4 OH− (aq) 2 H2 O (l) + O2 (g) + 4 e− Electrolysis guidelines At an inert anode, chlorine, bromine and iodine (all the halogens) are prefered over oxygen. At an inert cathode, hydrogen is prefered over metals unless an unreactive metal is present. Electrolysis of aqueous copper(II) sulfate Copper(II) sulfate solution (CuSo4 (aq)) may be electrolysed by using inert graphite electrodes. During this process, copper metal and oxygen gas are formed. © r/IGCSE Resources 2023 Page 27 of 97  There are 4 ions present in the solution: H+ and OH− from water Cu2 + and SO4 2− from sodium chloride Copper ions are preferred over Hydrogen ions due to preferential discharge. Preferential discharge in electrolysis means that certain ions in a solution are more likely to react and be discharged (or “stick”) to the electrode compared to others, based on their chemical properties. This leads to a more efficient reaction and greater production of desired products. The copper metal is therefore deposited at the cathode. Cu2 + (aq) + 2 e− Cu (s) Similarly with hydroxide and sulfate ions, the hydroxide ions release electrons more easily, so oxygen gas and water are produced at the anode 4 OH− (aq) O2 (g) + 2 H2 O (l) + 4 e− Purification of copper Copper is a very good conductor of electricity and heat. Even small amounts of impurities can affect this conductivity noticeably, whether in fine wires or larger cables. To ensure purity, newly extracted copper has to be purified by electrolysis. The impure copper is used as the anode and the cathode is made from very pure copper. When the current flows, the copper moves from the impure anode to the pure cathode. Any impurities fall to the bottom of the cell and collect below the anode. This usually consists of precious metals such as gold and silver. Half-ionic equations Anode: Cu (s) Cu2+ (aq) + 2 e− 2+ Cathode: Cu (aq) + 2 e− Cu (s) Electroplating Electroplating is the process in which one metal is plated/coated with another. The purpose of this is to improve the appearence of the object and give a protective coating to the metal underneath it. For silver plating, the electrolyte is a solution of a silver salt. The item to be plated is made the cathode in the cell so that the metal ions move to it when the current is switched on. Ag+ (aq) + e− Ag (s) © r/IGCSE Resources 2023 Page 28 of 97  4.2 Hydrogen–oxygen fuel cells A hydrogen–oxygen fuel cell uses only hydrogen and oxygen to produce electricity with water as the only product produced. The NaOH (aq) electrolyte is used for this reaction and the electrodes are porus. O2 (g) + 2 H2 O (l) + 4 e− 4 OH− (aq) Then the hydroxide ions formed are removed from the fuel cell by a reaction with hydrogen. H2 (g) + 2 OH− (aq) 2 H2 O (l) + 2 e− Advantages and disadvantages of fuel cells Advantages: Uses hydrogen and oxygen to make non-polluting water while generating electricity (compared to petrol engines which produce many pollutants) Similar to battery, but no external charging required Capable of producing electricity as long as hydrogen and oxygen are supplied Disadvantages: Hydrogen is difficult to store in a car as it is a gas at room temperature There are no existing infrastructure (refueling stations) for this Electric motors and fuel cells are less durable than petrol or diesel engines Fuel cells are very expensive at the moment. © r/IGCSE Resources 2023 Page 29 of 97  Chapter 5 5.1 Exothermic and endothermic reactions Exothermic reactions An exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings. Examples of exothermic reactions include combustion and neutralization. Combustion When a fuel, for example natural gas, burns in excess air, it produces a large amount of energy. methane + oxygen carbon dioxide + water + ΔH ΔH means heat released. However if not enough air is available for this reaction, it will not produce as much energy and also will produce carbon monoxide, which is poisonous. Fig. 1: Reaction pathway diagram for an exothermic reaction © r/IGCSE Resources 2023 Page 30 of 97  Endothermic reactions An endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings. Examples include: Thermal decomposition and Photosynthesis Fig. 2: Reaction pathway diagram for an endothermic reaction Enthalpy change The transfer of thermal energy during a reaction is called the enthalpy change, ΔH , of the reaction. ΔH is negative for exothermic reactions and positive for endothermic reactions. Activation energy Activation energy, Ea , is the minimum energy that colliding particles must have to trigger a reaction. If the particles have less energy than this amount and they collide, they will not react. More about this in chapter 6 (6.2). Making and breaking bonds Bond breaking is an endothermic process (to break bonds, you need to take in energy). While bond making is an exothermic process. In a reaction, there is both bond breaking and bond making. © r/IGCSE Resources 2023 Page 31 of 97  A reaction in which breaking the bonds uses more energy than making the bonds produce, is known as an endothermic reaction. While a reaction in which breaking the bonds uses less energy than the amount of energy making the bonds produce, is known as an exothermic reaction. Calculating enthalpy change To calculate enthalpy change, ΔH of a reaction, we need to first list out the bonds in the reactants and products side. For example, let’s take the equation: CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2 O (l) from the combustion reaction we talked about above. From this equation, we can figure out that there are: 4 C−H bonds 2 O=O bonds on the reactants side while there are: 2 C=O bonds 4 H−O bonds on the products side. Now, we’d be given a table listing the bond energies like: Bond Bond energy/kJ/mol C−H 435 O=O 497 C=O 803 H−O 464 C−C 347 C−O 358 Using this table we can calculate the energies on both the sides and use that to calculate the enthalpy change: Reactants (bond breaking) © r/IGCSE Resources 2023 Page 32 of 97  4 C−H bonds = 4 × 435 = 1740 kJ 2 O=O bonds = 2 × 497 = 994 kJ Which comes to a total of 2734 kJ of energy which is required to break the bonds in the reactants. Products (making bonds) 2 C=O bonds = 2 × 803 = 1606 kJ 4 H−O bonds = 4 × 464 = 1856 kJ Which comes out to a total of 3462 kJ of energy given out when the bonds are made. Calculating enthalpy change To calculate enthalpy change, we use the following formula: ΔE = energy required to break bonds − energy given out when bonds are made now subsitute the values: ΔE = 2734 − 3462 = −728 The negative sign shows that the reactants are losing energy to their surroundings. The enthalpy change is negative, which means that the reaction is exothermic. © r/IGCSE Resources 2023 Page 33 of 97  Chapter 6 6.1 Physical and chemical changes Physical Change Chemical Change Does not result in formation of a new substance Forms a completely new substance Only the state changes The chemical composition changes Usually reversible Usually cannot be reversed 6.2 Rate of reaction The rate of reaction is the speed at which reactants are converted into products in a chemical reaction. Factors affecting rate of reaction There are various factors which affect this, such as: changing the concentration of solutions (the more the concentration, the higher the rate of reaction) changing the pressure of gases (the more the pressure, the higher the rate of reaction) changing the surface area of solids (the larger the surface area, the higher the rate of reaction) changing the temperature (the higher the temperature , the higher the rate of reaction) adding or removing a catalyst, including enzymes (adding a catalyst increases the rate of reaction) A catalyst is a substance that alters/changes the rate of reaction without changing the products of the reactions, it itself also is unchanged at the end of the reaction. Collision theory The collision theory tells us that for a reaction to occur, the particles of the reactants must collide with each other and have enough energy (activation energy) to trigger a reaction. Collisions which have enough activation energy and result in the formation of products are known as successful collisions. All the factors affecting rate of reaction can be explained by the collision theory. Higher concentration Higher concentration of a substance means that there are more particles of that substance, therefore more successful collisions. © r/IGCSE Resources 2023 Page 34 of 97  Pressure of gases A higher pressure means there are more gas molecules in a given volume of gas, which means that there will be more frequent collisions and hence more successful collisions. Surface area In reactions involving solids, only the exposed surface area of reactants is available for collisions. When you increase the surface area by grinding or cutting the solid into smaller pieces, more reactant particles are exposed, allowing for a greater number of collisions which means more successful collisions. Temperature A higher temperature means that particles will have more kinetic energy which means that they will move faster. If the particles move faster that means there will be more collisions leading to more successful collisions. Additionally, collisions are more likely to have enough activation energy if the temperature is higher which increases the chance of a successful collision. Catalyst Catalysts decrease the amount of the activation energy required for a reaction to take place, which means that there is a higher chance of a successful collision. © r/IGCSE Resources 2023 Page 35 of 97  Experiment To find out the rate of reaction To measure the rate of reaction, we can measure: the volume of gas produced in a specific amount of time the “loss” of mass (due to gas being released) in a specific amount of time Volume of gas produced In such an experiment, we use a gas syringe attached to a beaker/flask/test-tube with a lid (diagram below). Fig. 1: Diagram showing a gas syringe attached to a test-tube By measuring the volume of gas produced in a specific amount of time, we can calculate the rate of reaction for that reaction. Loss of mass In such an experiment, mass is "lost" because the gas produced by the reaction is released. © r/IGCSE Resources 2023 Page 36 of 97  Fig. 2: Diagram showing a beaker kept on a balance during a reaction By measuring the loss of mass in a specific amount of time, we can calculate the rate of reaction for that reaction. Now you may notice in the diagram, there’s cotton wool used to cover the beaker, this is done to prevent acid spray. Now why wasn’t a bung/lid used? because then the gas produced in the reaction would not have been released and would’ve been trapped inside. This method is not usually preferred as gas is released into the atmosphere. Plotting graphs Once we are done with our observations and after noting down the loss of mass/volume of gas produced, we can now plot a rate of reaction graph by plotting the loss of mass/volume of gas on the y axis and time on the x axis. Rate of reaction graphs As we read earlier that the surface area of the solid reactants affect the speed of a reaction, we can see this by plotting rate of reaction graphs for the same reaction but using different sizes of solids in each of them. Note that we need to use the same mass/volume of reactants and the temperature should be kept the same. The graph would look something like this: © r/IGCSE Resources 2023 Page 37 of 97  Fig. 3: A rate of reaction graph showing the effect of surface area on rate of reaction As you can see, both reactions have an equal loss in mass, however, the one with powdered solid is faster. 6.3 Reversible reactions and equilibrium Reversible reactions are chemical reactions that can go both forward and backward unlike traditional chemical reactions which can only go forward. We use the symbol to denote a reversible reaction. A reversible reaction in a closed system is at equilibrium when the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products remain constant/do not change. Two examples for reversible reactions are: hydrated copper(II) sulfate anhydrous copper(II) sulfate + water The color change in this reaction is from blue to white. and anhydrous cobalt(II) chloride + water hydrated cobalt(II) chloride The color change in this reaction from blue to pink. © r/IGCSE Resources 2023 Page 38 of 97  Changing conditions for a reversible reaction Changing the conditions can change the direction of a reversible reaction. The effect of heat on hydrated compounds The effect of heat on hydrated compounds, such as copper(II) sulfate or cobalt(II) chloride, can change the direction of a reversible reaction by moving it towards dehydration. When heat is applied, the hydrated compounds lose their water molecules and become anhydrous, shifting the equilibrium from the hydrated form to the anhydrous form. Let’s take the reaction we talked about above as an example hydrated copper(II) sulfate anhydrous copper(II) sulfate + water In this reaction, when heat is applied to hydrated copper(II) sulfate, it starts to become anhydrous and releases water, when heat is no longer being applied, the reaction starts moving in the opposite direction and hydrated copper(II) sulfate is once again formed. The addition of water to anhydrous compounds The addition of water to anhydrous compounds like copper(II) sulfate or cobalt(II) chloride can drive the reversible reaction in the opposite direction, causing hydration. As water is added, the anhydrous compounds can absorb the water molecules and transform back into their hydrated forms, shifting the equilibrium towards the hydrated compounds. Factors affecting position of equilibrium The position of equilibrium can be affected by the following factors: changing temperature changing pressure changing concentration using a catalyst Changing temperature The position of equilibrium is affected by temperature changes differently depending on whether the reaction is exothermic or endothermic. For exothermic reactions (which release heat), increasing the temperature shifts the equilibrium towards the reactants (left) because it opposes the heat-producing direction to neutralize the temperature rise, promoting the reverse reaction. © r/IGCSE Resources 2023 Page 39 of 97  For endothermic reactions (which absorb heat), increasing the temperature shifts the equilibrium towards the products (right) because it favors the heat-absorbing direction to neutralize the temperature rise, promoting the forward reaction. Basically, reversible reactions always try to maintain equilibrium. Therefore when a reaction requires heat (endothermic reactions), they favour the direction which absorbs heat to avoid an increase in temperature and the opposite occurs for reactions which give out heat. Changing pressure The effect of pressure change depends on the number of moles of gas molecules involved in the reaction. Increasing pressure generally shifts the equilibrium towards the side with fewer moles of gas molecules. Decreasing pressure shifts the equilibrium towards the side with more moles of gas molecules. Changing concentration Increasing the concentration of reactants shifts the equilibrium towards the products, favoring the forward reaction. Increasing the concentration of products shifts the equilibrium towards the reactants, favoring the reverse reaction. Using a catalyst A catalyst doesn’t affect the position of equilibrium; instead, it speeds up the attainment of equilibrium. It facilitates both the forward and reverse reactions equally, making the system reach equilibrium more rapidly but with the same equilibrium position. Haber process The haber process is used for manufacturing ammonia. This reaction requires nitrogen and hydrogen. Obtaining nitrogen The nitrogen needed for this reaction is obtained from the atmosphere via fractional distillation of liquid air. Obtaining hydrogen The hydrogen for this reaction is obtained from a method known as steam re-forming. This process involves reacting methane with steam to produce hydrogen and carbon monoxide. © r/IGCSE Resources 2023 Page 40 of 97  CH4 (g) + H2 O (g) 3 H2 (g) + CO (g) This process needs to be carried out at a temperature of 750°C , a pressure of 3000 kPa ( 30 atmospheres ) with a catalyst of nickel. The carbon monoxide produced from this reaction is then used to react with steam again to form even more hydrogen and carbon dioxide. CO (g) + H2 O (g) H2 (g) + CO2 (g) Making ammonia In this reaction, for every one mole of nitrogen, 3 moles of hydrogen are needed. The ratio of nitrogen to hydrogen is 1:3. N2 (g) + 3 H2 (g) 2 NH3 (g) The conditions required for this reaction are: a temperature of 450°C a pressure of 20 000 kPa / 200 atm a catalyst of freshly cut and finely divided iron. Contact process Contact process is a way to produce sulfuric acid. The process goes like this Obtaining sulfur dioxide First, sulfur dioxide is produced by burning sulfur with excess oxygen in air. sulfur + oxygen sulfur dioxide S (s) + O2 (g) SO2 (g) Sulfur dioxide can also be produced by roasting sulfide ores (for example, zinc sulfide) in air. Obtaining sulfur trioxide Now to produce sulfur trioxide, we need to react sulfur dioxide with oxygen, which is a reversible reaction. sulfur dioxide + oxygen sulfur trioxide 2 SO2 (g) + O2 (g) SO3 (g) The conditions required for this reaction are: © r/IGCSE Resources 2023 Page 41 of 97  a temperature of approximately 450°C a catalyst of vanadium(V) oxide a pressure of 200 kPa / 2 atmospheres Making sulfuric acid Sulfuric acid is manufactured by reacting sulfur trioxide with water. sulfur trioxide + water sulfuric acid SO3 (g) + H2 O (l) H2 SO4 (l) Precautions for this reaction are: Ensure sulfur dust explosions are prevented by wetting down the sulfur Carefully managing and neutralising acid sprays/leaks if any. All the conditions for all reactions are done to maximize the yield of the products in both the processes. 6.4 Redox Oxidation numbers As talked about in 2.4, all elements have an oxidation number. Transition elements have multiple oxidation numbers. We use roman numeral to denote oxidation number, for example, cobalt(II) chloride tells us that the oxidation number of cobalt is 2. Oxidation and reduction Oxidation and reduction have been briefly talked about in Chapter 4. © r/IGCSE Resources 2023 Page 42 of 97  Important Oxidation and reduction Reduction is a process in which an atom, ion, or molecule gains electrons. Oxidation is the opposite of reduction, i.e. an atom, ion or molecule loses electrons. To remember this, use this mnemonic OIL RIG: Oxidation Is Loss Reduction Is Gain Something called reducing agents and oxidising agents also exist. The substance being oxidised is the reducing agent while the substance being reduced is the oxidising agent. Potassium iodide is a very common reducing agent. When it is oxidised, the iodide ion is oxidised to form iodine, resulting in a color change from colourless to yellow-brown. Oxidation is also the gain of oxygen and Reduction is also the loss of oxygen. When a substance is oxidised, there’s an increase in its oxidation number. When a substance is reduced, there’s a decrease in its oxidation number. It is important to note the following: The oxidation number of the free element is always 0, for example in metals such as zinc and copper. In simple monatomic ions, the oxidation number is the same as the charge on the ion. So for example, Iodine has an oxidation number of 0 in I2 (charge is 0) but an oxidation number of -1 in I− (charge is -1) Compounds have no overall charge. The sum of the oxidation numbers in an ion is equal to the charge on the ion. This is very important 2− and useful. So for example you have the sulfate ion, SO4 and imagine you do not know the charge on it, you can simply calculate it by adding the oxidation numbers of the atoms together. So the oxidation number of S is +6 and the oxidation number of oxygen is -2 and there are 4 oxygen © r/IGCSE Resources 2023 Page 43 of 97  atoms which means that the sum of the oxidation number would be: 6 + (−2 × 4) which is equal to -2, which is the charge on the sulfate ion. Redox reactions Redox reactions are reactions in which both oxidation and reduction occur at the same time. © r/IGCSE Resources 2023 Page 44 of 97  Chapter 7 7.1 The characteristic properties of acids and bases The pH scale The pH scale is a measurement system that quantifies the acidity or alkalinity (basicity) of a solution. It ranges from 0 to 14, where: pH below 7 indicates acidity, with 0 being the most acidic. pH 7 is considered neutral. Pure water is neutral. pH above 7 indicates alkalinity, with 14 being the most alkaline (basic). Acids + Acids are substances that release hydrogen ions (H ) when mixed in water. Acids have a pH of less than 7. The lower the pH, the stronger the acid. + The stronger the acid, the higher the concentration of hydrogen ions (H ). Acids are also known as proton donors. A strong acid is an acid that completely dissociates in an aqueous solution and a weak acid is an acid that partially dissociates in aqueous solution. An example of strong acid is hydrochloric acid, HCl (equation: HCl (aq) H+ (aq) + Cl− (aq)) while an example of a weak acid is ethanoic acid CH3 COOH (equation: CH3 COOH (aq) H+ (aq) + CH3 COO− (aq); reversible reaction indicating partial disassociation) Bases − Base are substances that release hydroxide ions (OH ) when mixed in water. Bases have a pH of more than 7. The higher the pH, the stronger the base. − The stronger the base, the higher the concentration of hydroxide ions (OH ). Bases are usually oxides or hydroxides of metals. Bases which are soluble are known as alkalis. Bases are also known as proton acceptors. Amphoteric substances © r/IGCSE Resources 2023 Page 45 of 97  Some substances behave as both, proton acceptors and donors, these are known are amphoteric substances. Examples include, aluminium oxide (Al2 O3 ) and zinc oxide (ZnO). Indicators Indicators are substances that change color to show if a solution is acidic or basic. They can be used to identify acids and bases. Indicator Colour in Acid Colour in Base Litmus Red Blue Methyl Orange Pink Yellow Thymolphthalein Colourless Blue Another really popular indicator is the universal indicator. Fig. 1: pH scale for the universal indicator solution Acid reactions Acids usually react with: metals bases carbonates Acid + metal Acids react with metals to produce salt and hydrogen gas. Precaution: This can only be done with less reactive metals. The metals used are MAZIT metals: Magnesium, © r/IGCSE Resources 2023 Page 46 of 97  Aluminium, Zinc, Iron, Tin Example: Add excess magnesium ribbon to dilute HCl. This will produce Hydrogen gas. In this reaction, hydrogen ions from HCl gain electrons from the metal atoms. Mg + 2 HCl MgCl2 (aq) + H2 Acid + base Acids and bases can neutralize each other in a reaction, forming water and a salt. Example: HCl + NaOH NaCl + H2 O + − In this reaction, the hydrogen ion (H ) and the hydroxide ion (OH ) react to form water. H+ (aq) + OH− (aq) H2 O (l) Acid + carbonate Acids react with carbonate compounds to produce carbon dioxide gas, water, and a salt. Carbon dioxide gas can be tested by passing it through limewater, if it turns milky, then it is indeed carbon dioxide. Example: 2 HCl + Na2 CO3 2 NaCl + H2 O + CO2 Base reactions Bases usually react with: acids ammonium salts Base + Acid This is the same neutralization react as the one described above (acid + base). Base + ammonium salt Bases can react with ammonium salts to produce ammonia gas (NH3), water, and a salt. The Haber process is preferred over this process in industry as the haber process has a higher yield. © r/IGCSE Resources 2023 Page 47 of 97  Ca(OH)2 (s) + 2 NH4 Cl CaCl2 + H2 O + 2 NH3 Ammonia can be detected by its pungent odour and by turning red litmus blue. 7.2 Oxides Oxides can be acidic, basic or amphoteric. Acidic oxides are usually the non-metal oxides, for example sulfur dioxide (SO2 ) and carbon dioxide ( CO2 ) Basic oxides are usually the metal oxides, for example copper(II) oxide (CuO) and calcium oxide ( CaO). Amphoteric oxides are oxides which can behave as both acids and bases and can react with both to produce a salt and water. Examples include aluminium oxide (Al2 O3 ) and zinc oxide (ZnO). 7.3 Preparation of salts Before preparing salts, we must know which salts are soluble and insoluble. Soluble Salts All Sodium, Potassium and Ammonium salts are soluble. All nitrates are soluble. All chlorides are soluble except lead and silver. All sulfates are soluble except barium, calcium and lead All carbonates and hydroxides are insoluble except sodium, potassium and ammonium Calcium hydroxide is partially soluble Preparing soluble salts There are many ways to prepare soluble salts. Acid + alkali (soluble base; titration) Choose the acid and the alkali. For example, to form sodium chloride (NaCl), use hydrochloric acid (HCl) and sodium hydroxide (NaOH). Add a known volume of the acid to a flask and add a few drops of an indicator like phenolphthalein or universal indicator. © r/IGCSE Resources 2023 Page 48 of 97  Then, titrate by slowly adding the alkali from a burette until the indicator changes color, signaling that the acid has been neutralized. The resulting solution contains the soluble salt along with water, now you can just seperate the salt by evaporating the water using an evaporating basin. Acid + excess metal This is the same as the Acid + metal reaction described in 7.1, with one extra step which is to filter out the salt from the unreacted reactants. Acid + excess insoluble base Combine the acid (e.g., hydrochloric acid, HCl) with an excess of an insoluble base (e.g., copper oxide, CuO). The base reacts with the acid to form the soluble salt and water. Filter the solution to filter out the leftover insoluble base. Now evaporate the water to get salt crystals. Acid + excess insoluble carbonate This is the same as Acid + carbonate in 7.1, except there are 2 more steps: Filter the solution to filter out the leftover insoluble carbonate. Now evaporate the water to get salt crystals. Preparing insoluble salts The method of preparing an insoluble salt is called precipitation. In this method, 2 solubles salt (aqueous) react to make 1 insouble salt (solid) and 1 soluble salt (aqueous). soluble salt + soluble salt insoluble salt + soluble salt for example, BaCl2 (aq) + Na2 SO4 (aq) BaSO4 (s) + 2 NaCl (aq) Then the insouble salt is filtered out, washed with water and then left to dry. Water of crystallisation A hydrated substance is a material that is chemically bonded to water molecules, while an anhydrous substance contains no water. Water of crystallization is the water trapped in crystals. For example © r/IGCSE Resources 2023 Page 49 of 97  CuSO4 ⋅ 5 H2 O This shows that copper(II) sulfate has five water molecules as water of crystallisation. This water is trapped within the crystals. To remove water of crystallization from a compound, you can heat it gently, causing the water molecules to evaporate and leaving the anhydrous form of the compound. © r/IGCSE Resources 2023 Page 50 of 97  Chapter 8 8.1 Arrangement of elements Fig. 1: The perioidic table The Periodic Table is an arrangement of elements in periods and groups and in order of increasing proton number / atomic number. Periods in the periodic table Properties of elements moving across a period (horizontal) in the perioidic table: Gradual change from metal to non-metal Increase in number of electron Change in structure (Giant ionic to giant covalent to simple molecular) Groups in the periodic table Groups are vertical while periods are horizontal. There are many groups in the periodic table, such as: Group 1 - alkali metals Group 2 - alkaline earth metals Group 6 - chalcogens © r/IGCSE Resources 2023 Page 51 of 97  Group 7 - halogens Group 8 - noble gases The group numbers tell us about the oxidation number of an ion of that element and helps us figure out the charge on the ion of the element. For example, the oxidation number of Lithium (Li) is 1, and therefore the charge on its ion would be +1. This is only applicable for the oxidation numbers: 1, 2, 3 and 4. For oxidation numbers above 4 like 5, 6, 7, we simply subtract 8 from the oxidation number to calculate its charge. For example, oxygen (O) has an oxidation number of 6 which is obviously above 4. 6-8 = -2 which means that the charge on an ion of oxygen will be -2. Elements with the oxidation number 8 are unreactive and do not form ions. Elements of a certain group have similar chemical properties because they all have a similar electronic configuration. The electrons in the last shell of the atom is equal to the group number. Predicting the properties of elements using the position in the periodic table It is possible to predict the properties of an element using its position in the perioidic table, for example: The mass number tells us about how heavy the element is. The period number tells us about the number of shells an atom of that element has. The group number tells us about the element’s chemical properties and if it is a metal or not. 8.2 Group I properties They are good conductors of heat and electricity, are soft and have a low density. When they are freshly cut, they’re very shiny They have a low melting and boiling point. They burn in oxygen or air with specific flame colors. They react vigorously with water and with halogens to form metal halides. As you go down the group: the melting point decreases the density increases the reactivity increases © r/IGCSE Resources 2023 Page 52 of 97  8.3 Group VII properties These elements are colored and become darker down the group They exist as diatomic molecules At r.t.p, they show a gradual change from gas (Cl2 ), to liquid (Br2 ) to solid (I2 ) They form covalent compounds with non-metallic elements (such as (HCl)) They react with hydrogen to form hydrogen halides, which dissolve in water to form acidic solutions. They react with metals to produce ionic metal halides such as 2 Fe (s) + 3 Cl2 (g) 2 FeCl3 (s) Chlorine gas can bleach moist indicator paper. Colors of some halogens: Halogen Color Chlorine Yellow-green gas Bromine Red-Brown liquid Iodine Gray-black solid As you go down the group, the density increases and the reactivity decreases. Displacement reactions More reactive elements displace (replace) less reactive elements when added to a substance containing the less reactive element. Example: Potassium iodide + Chlorine Potassium chloride + Iodine 2 KI (aq) + Cl2 (g) 2 KCl (aq) + I2 (aq) 8.4 Transition elements The transition elements are metals that: have high densities have high melting points (except Mercury which is liquid at r.t.p) form coloured compounds often act as catalysts as elements and in compounds They are also: harder, stronger and have a higher density than the metals in group I and II © r/IGCSE Resources 2023 Page 53 of 97  As discussed in earlier chapters (2.4), transition metals can have ions with multiple different oxidation numbers. 8.5 Noble gases This is the most unusual group of elements, called the noble gases. It is made up of Helium, Neon, Krypton, Xenon, and Radon (radioactive). They are colorless gases They are monatomic gases They are very unreactive Use of Argon: the gas used to fill light bulbs to prevent the tungsten filament from reacting with air. Use of neon: Advertising signs and lasers These gases are unreactive as their last shell is complete. © r/IGCSE Resources 2023 Page 54 of 97  Chapter 9 9.1 Properties of metals Property Metal Non-metal Conduct Heat and Electricity ✅ ❌ Malleable  ❌ - usually soft or brittle Ductile  ❌ - usually soft or brittle Brittle   Melting and boiling point Usually high Usually low Metal reactions Metals + Dilute Acids Most metals react with dilute acids (e.g., hydrochloric acid, sulfuric acid) to produce salt and hydrogen gas. During such a reaction, you will see effervescence which is caused by bubbles of hydrogen gas being formed as the reaction proceeds. A simple reaction between magnesium and hydrochloric acid can be taken as an example: Mg (s) + 2 HCl (aq) MgCl2 (aq) + H2 (g) We can figure out which metals are the most reactive and least reactive depending upon their reaction with acids. Metals + Oxygen Many metals react with oxygen to form oxides. For example, reacting magnesium with oxygen by burning it in excess oxygen to form magnesium oxide: 2 Mg (s) + O2 (g) 2 MgO (s) Metals + Cold Water/Steam Very reactive metals Very reactive metals such as potassium, sodium and calcium react with cold water to make metal hydroxides and hydrogen gas. Example: © r/IGCSE Resources 2023 Page 55 of 97  2 Na (s) + 2 H2 O (l) 2 NaOH (aq) + H2 (g) Less reactive metals Less reactive metals such as magnesium, zinc and iron react slowly with water, however they react faster with steam. When metals react with steam, they produce metal oxide and hydrogen. Mg (s) + H2 O (g) MgO (s) + H2 (g) These experiments can be dangerous. Some precautions include: Wear eye protection Using these experiments, we can figure out an order of reactivity showing us which metals are more reactive. A figure of the reactivity series can be seen in 9.4. 9.2 Uses of metals Less reactve metals usually have more uses. Some uses of metals include: aluminium in the manufacture of aircraft because of its low density aluminium in the manufacture of overhead electrical cables because of its low density and good electrical conductivity aluminium in food containers because of its resistance to corrosion copper in electrical wiring because of its good electrical conductivity and ductility 9.3 Alloys and their properties An alloy is a mixture of a metal with other elements, designed to enhance or modify its properties for specific applications. Examples of alloys include: brass, a mixture of copper and zinc stainless steel, a mixture of iron along with chromium, nickel and carbon Alloys can be harder and stronger than the pure metals and are more useful. These properties can be explained simply by the fact that alloys are made up of metals and different elements. All these different elements have atoms of different sizes, and these atoms being different in sizes means that it is harder for the layers in alloy to slide over each other. Alloys have many uses, one of the major uses of stainless steel is in cutlery because of its hardness, and resistance to rusting. © r/IGCSE Resources 2023 Page 56 of 97  Fig. 1: The structure of an alloy In the structure above, we can see that there are two different sized atoms (blue ones and purple ones) which means that the structure is of an alloy. 9.4 Reactivity series The reactivity series is a ranking of metals in order of their reactivity with water and acids, from most reactive (e.g., potassium) to least reactive (e.g., gold). © r/IGCSE Resources 2023 Page 57 of 97  Reactivity Reaction with dilute Reaction with Ease of Reaction with water series acid air/oxygen extraction Potassium Produce hydrogen gas (K) when reacted with cold Burn very brightly water Difficult to extract and very strongly with decreasing Sodium (Na) strength Calcium (Ca) Magnesium Produce hydrogen (Mg) gas with decreasing strength Burn to form an oxide Aluminium as you with React with steam (Al) go down decreasing strength with decreasing as strength Easier to extract Carbon (C) you go down Zinc (Zn) Iron (Fe) Hydrogen (H) React slowly to form an oxide Copper (Cu) Do not react with either Do not react with cold water or steam Found as the Silver (Ag) dilute acids element Do not react Gold (Au) However, aluminium is special. It appears to be unreactive but actually isn’t. When exposed to air/oxygen, it reacts very quickly with the oxygen and forms a protective oxide layer on it’s surface. Displacement reactions Displacement reactions in metals involve a more reactive metal displacing a less reactive metal from a compound, leading to the formation of a new compound. For example, a reaction between zinc and copper(ii) nitrate. Zinc is more reactive than copper which means that it will displace copper and form zinc nitrate. zinc + copper(II) nitrate zinc nitrate + copper Zn (s) + Cu(NO3 )2 (aq) Zn(NO3 )2 (aq) + Cu (s) This can be seen as there is a color change from blue to colorless. © r/IGCSE Resources 2023 Page 58 of 97  Experiment To find out the reactivity series from a set of metals To do this, we can perform displacement reactions. We can take a metal solution, for example zinc nitrate. Now we can add various metals to this to see if the metal we added is more reactive than zinc or not, we can repeat this with solutions of other metals as well to make a reactivity series. 9.5 Corrosion of metals Corrosion is when metals slowly rust or wear away due to chemical reactions with their surroundings. Basically it is when metals/alloys are chemically attacked by oxygen and water in the environment. The more reactive a metal is, the faster it will corrode. This is why highly reactive metals like sodium and potassium are stored in oil as they will instantly corrode when exposed to air and moisture. Rust is a form of corrosion that specifically refers to the reddish-brown, flaky coating that develops on iron or steel surfaces when they react with oxygen and moisture in the presence of air. Rust usually consists of hydrated iron(III) oxide (Fe2 O3 ⋅ x H2 O) which is reddish-brown in color. The requirements for rusting are water (in the form of moisture or water itself) and oxygen (from the air). Both of the above are required for rusting. If either of them is not present, the metal (iron/steel) will not rust. Note that rusting only occurs on iron or steel surfaces. Preventing rust Rusting is a problem because it weakens the structural integrity of metal objects and structures. To prevent rusting, it is important to stop either oxygen or moisture (or both) from coming into contact with the metal. All of the methods involve isolating the metal from the surroundings using a protective shield (paint, oil, plastic etc.). Painting © r/IGCSE Resources 2023 Page 59 of 97  Applying a layer of paint to the metal surface forms a protective barrier, preventing direct contact with oxygen and moisture. A positive to this is that the object will look better however, if the paint is scratched, the metal beneath it will start to rust which will eventually spread under the paint. Greasing Coating the metal with grease or oil creates a protective layer that repels water and air, reducing the chances of rust formation. Greasing is often used for moving parts, machinery, and equipment. Coating with plastic Plastics can be used as a coating to isolate the metal from the corrosive elements, preventing rust. This is usually done in pipes. Galvanising Zinc is applied as a protective coating over the iron or steel surface, forming a physical barrier. To do this, the object is dipped into molten zinc and a thin layer of zinc is formed. Zinc is more reactive than iron/steel and this layer also corrodes but when this zinc layer corrodes, it transfers/loses electrons to the iron, protecting it. This also protects the iron even if the zinc layer has been mostly scratched away. Sacrificial protection Bars of zinc are usually attached to ships to prevent iron from rusting. This is done because zinc is more reactive than iron and is corroded instead of iron. So as long as some of the zinc stays in contact with the iron, the iron will be protected. When the zinc runs out however, it must be replaced. This is the same concept as galvanisation, where when the zinc corrodes, it transfers/loses electrons to the iron, protecting it as shown by the equation below: Zn (s) + Fe2+ (aq) Zn2+ (aq) + Fe (s) 9.6 Extraction of metals Metals are found in the Earth’s crust primarily as ores because of how reactive they are. Some unreactive metals such as gold and silver can be found as pure metals. Some common ores include: Bauxite (aluminium ore) Copper pyrites (copper ore) Hematite (iron ore) Rock salt (sodium ore) Zinc blende (zinc ore) © r/IGCSE Resources 2023 Page 60 of 97  (and yes, ores have names fellow minecraft players, better learn them) To figure out how to extract the metal from these ores, we can refer to the metal’s reactivity along with the table below. Position in the reactivity series Method Under hydrogen Exist as pure metals Above hydrogen but under carbon Metal oxide reacted with carbon Above carbon Electrolysis Extraction of iron from hematite Iron is extracted from its ore, hematite, in a blast furnace. This process uses the following: carbon in the form of coke (made by heating coal) limestone hematite (iron ore) © r/IGCSE Resources 2023 Page 61 of 97  Fig. 2: A blast furnace being used to extract iron from hematiteJ Navas, CC BY-SA 3.0, via Wikimedia Commons Hot air is sent in blasts through the bottom of the furnace (see diagram above) which heats up the iron ore as the coke burns in the hot air, forming carbon dioxide. C (s) + O2 (g) CO2 (g) The limestone then begins to decompose due to the temperature CaCO3 (s) CaO (s) + CO2 (g) The carbon dioxide produced earlier reacts with more hot coke/carbon in the furnace, producing carbon monoxide in an endothermic reaction. CO2 (g) + C (s) 2 CO (g) Carbon monoxide is a reducing agent, which means that it will reduce the iron(III) oxide in the ore and oxidise itself to form carbon dioxide and also forms iron. iron(III) oxide + carbon monoxide iron + carbon dioxide © r/IGCSE Resources 2023 Page 62 of 97  Fe2 O3 (s) + 3 CO (g) 2 Fe (s) + 3 CO2 (g) This molten iron goes to the bottom of the furnace due to its high density. The calcium oxide produced earlier by the decomposition of limestone is a base and reacts with acidic impurities such as silicon(IV) oxide in the iron to form something called slag. Slag is basically calcium silicate. CaO (s) + SiO2 (s) CaSiO3 (s) This slag also goes to the bottom of the furnace, but it is above iron since it is less dense than iron. Molten iron and slag is then piped out separately at regular intervals. The waste gases produced such as nitrogen and oxides of carbon escape from the top of furnace. These waste gases also heat the air and reduce energy costs. Slag produced by this reaction can be used for foundations of buildings and roads. Extraction of aluminium The main ore of aluminium is bauxite. Aluminium is extracted by the process of electrolysis. This has already been covered in chapter 4 here. © r/IGCSE Resources 2023 Page 63 of 97  Chapter 10 10.1 Water Water has many uses apart from sustaining life. Such as: Home Cooking Cleaning Drinking Industry As a solvent As a coolant For cleaning As a chemical reactant Pure Water is a colorless liquid which (at r.t.p) boils at 100°C and freezes at 0°C. Note that tap water is NOT pure water. It contains many impurities, therefore does not boil at 100°C. Distilled water is extremely pure and is therefore used in experiments over tap water. We can also use the information above to test for the purity of water. Test for the presence of water There are two methods to test for the presence of water. Anhydrous cobalt(II) chloride Anhydrous copper(II) sulfate Anhydrous cobalt(II) chloride Anhydrous cobalt(II) chloride can be used to test for the presence of water. When anhydrous cobalt(II) chloride (blue in color) comes into contact with water, it undergoes a chemical reaction (hydration) and forms a pink-colored hydrated cobalt(II) chloride. This change in color indicates the presence of water. Anhydrous copper(II) sulfate Anhydrous copper(II) sulfate can also be utilized to test for the presence of water. When anhydrous copper(II) sulfate (white in color) is exposed to water, it readily absorbs the water © r/IGCSE Resources 2023 Page 64 of 97  molecules and forms a blue-colored hydrated copper(II) sulfate. The change in color from white to blue indicates the presence of water. Unique Properties of water Water is a pretty unique substance. It is an excellent solvent for many ionic substances and has many unusual properties such as It has a fairly high boiling point which is unusual for it’s relatively low molecular mass. It has greater specific heat capacity (energy required to increase temperature basically) than almost any other liquid. Its density decreases when it freezes. Water pollution and treatment Water from natural sources/tap water can have things like oxygen, metals, plastics, sewage, harmful germs, nitrates, and phosphates. We need to make sure water is clean and safe. Some of these substances have benefits: a. Dissolved oxygen is good for aquatic life to survive. b. Some metal compounds provide essential minerals for life. However, some substances can be harmful: a. Certain metal compounds can be toxic. b. Plastics can harm aquatic life. c. Sewage contains harmful microbes that cause disease. d. Nitrates and phosphates can reduce oxygen in water and harm aquatic life. Treating water We need to treat water so that we can remove the harmful substances and make it drinkable. 1. Water is filtered to remove debris and solid particles. 2. Aluminum sulfate is added to clump together small particles. 3. Coarse sand filtration further removes particles. 4. Water enters a sedimentation tank for settling. 5. Fine sand filtration removes remaining impurities. 6. Chlorine is added to kill microbes and ensure safety. 10.2 Fertilisers Before even starting this topic, you need to know what fertilisers are. © r/IGCSE Resources 2023 Page 65 of 97  In simple words, fertilizers are substances that help plants grow better by providing them with the nutrients they need. They basically enrich the soil with essential nutrients. NPK elements These essential nutrients are the NPK elements (Nitrogen, Phosphorus and Potassium). Each NPK element is responsibile for the healthy growth of the plant but in different ways. Nitrogen (really important for exams) As you may know that the process of photosynthesis requires chlorophyll, a green pigment which absorbs sunlight. Nitrogen is a major constituent of chlorophyll. Therefore, without nitrogen, the plant would lose it’s green color (chlorophyll) and will not able to produce food, and therefore will die. Phosphorus Plants have roots which help secure the plant into the ground. Phosphorus promotes the growth of roots, therefore making stronger roots. Potassium Potassium basically contributes to the overall health and growth of the plant. Artificial fertilisers Even though the soil naturally contains some nutrients, it may not always have enough for healthy growth of the plants. Therefore, we might need to add artificial fertilisers. We use ammonium salts and nitrates as fertilisers. The most commonly known artificial fertiliser is ammounium nitrate (NH4 NO3 ) which is formed by the following reaction: ammonia + nitric acid ammonium nitrate NH3 (g) + HNO3 (aq) NH4 NO3 (aq) Ammounium nitrate is a nitrogenous fertilizers. As you may have guessed, nitrogenous fertilizers are fertilizers that contain nitrogen n. Some nitrogenous fertilizers include: Ammonium Nitrate - NH4 NO3 © r/IGCSE Resources 2023 Page 66 of 97  Ammonium Phosphate - (NH4 )3 PO4 Ammonium Sulfate - (NH4 )2 SO4 Urea - CO(NH2 )2 10.3 The air Composition of the air Clean, dry air is a mixture of several compounds and elements. With 78% of the air being Nitrogen, 21% being Oxygen, 0.9% Argon, 0.04% Carbon dioxide and the rest is a mixture of noble gases. Air pollution The air can become polluted in multiple ways but the syllabus states 5 air pollutants, therefore we’ll cover only those five. Carbon Dioxide The primary source of carbon dioxide in the air is from the complete combustion of fossil fuels such as coal, oil, natural gas, petroleum etc. Higher levels of carbon dioxide leads to a rise in global warming, therefore causing climate change. Carbon Monoxide and particulates Carbon monoxide and particulates are formed due to the incomplete combustion of a carbon- containing fuel (fossil fuel). The combustion is incomplete due to the lack of oxygen. Carbon monoxide is a toxic gas. Breathing in carbon monoxide is dangerous because it sticks to our red blood cells and stops them from carrying oxygen to our body parts, which can harm important organs like the brain and heart. While particulates increase the risk of respiratory problems and cancer Methane Methane is produced through the decomposition of vegetation and the digestion process in animals. Similarly to carbon dioxide, higher levels of methane leads to a rise in global warming, therefore causing climate change. Carbon dioxide and methane, as greenhouse gases, contribute to global warming by trapping heat in the atmosphere. They act like a blanket, absorbing and re-emitting thermal energy, which prevents it from escaping into space. This leads to an increase in global temperatures as more heat is retained rather than being released back into space. © r/IGCSE Resources 2023 Page 67 of 97  Nitrogen oxides Car engines produce nitrogen oxides (NOx), and catalytic converters help reduce these emissions. Nitrogen oxides can still contribute to environmental problems like the formation of acid rain and photochemical smog. These pollutants can also cause respiratory problems for people who are exposed to high levels of NOx. Sulfur Dioxide When fossil fuels containing sulfur compounds are burned, sulfur dioxide (SO2 ) is released. Sulfur dioxide contributes to the formation of acid rain when it reacts with moisture in the atmosphere. Reducing the effect of air pollution Climate Change Planting trees: trees carry out photosynthesis, which uses carbon dioxide and produces oxygen, you will read more about this below. Decreasing use of fossil fuels: Use cleaner energy sources like wind, solar. Acid Rain Catalytic converters: Installing catalytic converters in all cars, you’ll read more about this below. Reducing emissions of sulfur dioxide: Using low sulfur fuels and flue gas desulfurization units (more about them below). Flue Gas Desulfurization Units Flue gas desulfurization (FGD) is a method that removes sulfur dioxide (SO2) from industrial emissions by using substances like calcium oxide or calcium carbonate to capture and remove the sulfur dioxide before it is released into the atmosphere. This helps reduce the formation of acid rain. Catalytic converters Catalytic converters are devices installed in car exhaust systems that contain catalysts, such as platinum, to facilitate chemical reactions that convert harmful pollutants, like nitrogen oxides and carbon monoxide, into less harmful substances. Example equation: Carbon monoxide + Nitrogen(II) oxide Carbon dioxide + Nitrogen 2 CO + 2 NO 2 CO2 + N2 Photosynthesis © r/IGCSE Resources 2023 Page 68 of 97  Photosynthesis is the reaction between carbon dioxide and water to produce glucose and oxygen in the presence of chlorophyll and using energy from light. Carbon dioxide + Water Glucose + Oxygen 6 CO2 + 6 H2 O C6 H12 O6 + 6 O2 © r/IGCSE Resources 2023 Page 69 of 97  Chapter 11 11.1 Formulae, functional groups and terminology Displayed Formulas Displayed formulas show the arrangement of molecules and bonds in a compound. An example is shown below. Fig. 1: Displayed formula