Year 8 Chemistry A5 (Notes only) PDF

**[End of Topic Test]** Mid-Topic course objectives plus the following additional course objectives: Identify substances as either compounds or elements. Identify elements as monatomic, molecular or metallic. Using the particle model, describe the differences between elements, compounds and mix...

**[End of Topic Test]** Mid-Topic course objectives plus the following additional course objectives: Identify substances as either compounds or elements. Identify elements as monatomic, molecular or metallic. Using the particle model, describe the differences between elements, compounds and mixtures. Explain the difference between physical changes and chemical changes using examples. Observe and identify exothermic and endothermic reactions. Explain that when a chemical change occurs, a chemical reaction takes place. Identify evidence that a chemical change has occurred. Identify the reactants and products in a chemical reaction. Page 6 of 50 **Vocabulary** **Vocabulary for Chemistry Mid-Topic Test** +-----------------------------------+-----------------------------------+ | **Term** | **Definition** | +===================================+===================================+ | Matter | Anything that takes up space and | | | has mass | +-----------------------------------+-----------------------------------+ | Solid | A state of matter where the | | | particles are packed closely | | | together in a highly organized | | | manner. | +-----------------------------------+-----------------------------------+ | Liquid | A state of matter where the | | | particles are still quite close | | | together but not as tightly | | | packed together as solids. | +-----------------------------------+-----------------------------------+ | Gas | A state of matter where the | | | particles are very far apart and | | | can move about freely. | +-----------------------------------+-----------------------------------+ | Diffusion | The process by which a gas | | | spreads itself around its | | | container. | +-----------------------------------+-----------------------------------+ | Melting | The process by which a substance | | | in the solid-state changes to the | | | liquid state. | +-----------------------------------+-----------------------------------+ | Solidification | The process by which a substance | | | in the liquid state changes to | | (or freezing) | the solid state. | +-----------------------------------+-----------------------------------+ | Condensation | The process by which a substance | | | in the gaseous state changes to | | | liquid. Occurs by way of cooling | | | down a gas. | +-----------------------------------+-----------------------------------+ | Sublimation | The process by which a substance | | | in the solid state changes | | | directly into a gaseous state | | | (skipping the liquid state). | +-----------------------------------+-----------------------------------+ | Melting point | The temperature at which a | | | substance changes from solid to | | | liquid. | +-----------------------------------+-----------------------------------+ | Boiling point | The temperature at which a | | | substance changes from liquid to | | | gas. | +-----------------------------------+-----------------------------------+ Page 7 of 50 **Vocabulary for Chemistry End-of-Topic Topic Test** +-----------------------------------+-----------------------------------+ | **Term** | **Definition** | +===================================+===================================+ | Atoms | The tiny particles that make up | | | matter, they are the building | | | blocks of matter. | +-----------------------------------+-----------------------------------+ | Molecule | A group of two or more atoms | | | chemically bonded together. | +-----------------------------------+-----------------------------------+ | Compound | A substance made up of more than | | | one type of atom chemically | | | combined. | +-----------------------------------+-----------------------------------+ | Physical | A change to any matter which DOES | | | NOT involve the production of a | | change | new substance. e.g. When water is | | | boiled, it changes from liquid to | | | gas | +-----------------------------------+-----------------------------------+ | Chemical | A change to any matter which | | | results in the formation of a | | change | Chemical new substance (or | | | substances). When a chemical | | | change occurs, we say that a | | | chemical reaction has taken | | | place. | +-----------------------------------+-----------------------------------+ Page 8 of 50 **1. The Particle Model** **What is matter made of?** The particle model states that matter is made of very small particles that are constantly moving. The particles that make up matter exist in three arrangements (or states) known as solids, liquids and gases. **Properties of State of Matter** Solid Liquid Gas Particles vibrating in place. Fixed volume and shape. Can't be compressed. Particles can't flow. **Kinetic Theory**  Fixed volume, Take the shape of the bottom of the container. Can't be compressed. Particles slowly flow. No fixed volume or shape. Fill any container they are held in. Can be compressed. Particles can rapidly flow. Page 9 of 50 Particles are constantly moving around in random directions. These moving particles contain [kinetic energy]. This helps to explain the different properties seen in solids, liquids and gases. [Solids:] Particles in solid substances contain only a small amount of kinetic energy. This means the particles can only vibrate in place, and stay in their fixed positions. As the particles are fixed in place close to one another, they are unable to be compressed. [Liquids:] Particles in liquid substances have enough kinetic energy to move around, but only enough to flow over the top of each other. The particles are still close to each other, and so are still unable to be compressed. [Gases:] Particles in gaseous substances contain much more kinetic energy than solids and liquids, and so they can move around freely within the container. As there is much empty space between particles, gases can be compressed. Page 10 of 50 **2. Changes of State** **Changing State is a Physical Change** When a substance changes state (e.g., from a solid to a liquid) it is a physical change. This means that the substance still consists of the same particles, but it looks different. You will learn more about physical changes later in this topic. **Changing State and the Particle Model** The particle model (and the kinetic theory) helps us explain what happens when substances change state by thinking about the relationships between the particles in the substance. Each of these changes of state is caused by an increase in temperature, as it directly affects the amount of kinetic energy the particles contain. When particles are exposed to temperature changes, their kinetic energy changes as a result. This occurs through an energy transformation: thermal energy → kinetic energy As temperature increases, the kinetic energy of the particles increase also, as more thermal energy is being transferred into the particles. Conversely, as temperature decreases, the kinetic energy of the particles also decrease, as thermal energy is being removed from the substance. Each change of state is described on the next pages: Page 11 of 50 **Melting** In a solid the particles vibrate and are held  in position by bonds (forces of attraction) between the particles. As we add energy to a solid the particles increase in their vibrations until the bonds can no longer hold the particles in a fixed position. When this happens, the substance becomes a liquid. **Evaporation** The particles in a liquid are able to move freely, but are held close together by weak forces of attraction. As heat is added to the liquid the particles move faster until they have enough energy to escape from the surface of the liquid to form a gas. **Condensation** As heat is removed from the substance, the  particles move slower until they reach a point where the forces of attraction between the particles can hold them close together. When this happens, a gas becomes a liquid. **Solidification (Freezing)** Sometimes called freezing, solidification is when heat is removed from a liquid. The particles move slower until the forces of attraction between the particles can hold them in a fixed position. When this happens, a liquid becomes a solid. Page 12 of 50 **Sublimation** Sublimation is when substances turn  straight from a solid to a gas. It happens when the particles on the surface of the solid gain enough energy to overcome the bonds keeping them in a fixed position. The particles are then able to move freely in all directions. The substance has changed from a solid directly to a gas. **Deposition** Deposition is when substances turn straight from a gas to a solid. When the particles in the gas lose enough energy the bonds between the particles are strong enough that the particles go straight into a fixed position. **Boiling and Melting points** Boiling point: the temperature at which a liquid turns to gas Melting point: the temperature at which a solid turns to liquid Water has a boiling point of 100^O^C. This means that at this temperature, the liquid water particles contain enough kinetic energy to become gaseous. Likewise, water has a melting point of 0^O^C, which means that at this temperature the solid water particles contain enough kinetic energy to become liquid. Page 13 of 50 **3. Properties of Matter** **What are properties?** Understanding how particles move can help us explain the physical properties of matter. Properties refer to the qualities or traits that can be measured or observed. These properties include colour, density, volume, mass, melting and boiling points, strength, viscosity, compressibility and hardness. **Strength** Strength can be thought of in two main ways - tensile strength and compressional strength. Substances with high tensile strength, such as an elastic band, can be stretched without breaking. Substances with high compressional strength, such as concrete, can withstand a large amount of force without breaking.  Elastic bands have a high tensile strength because they can be stretched a long way without breaking Concrete and stone columns have a high compressional strength because they can withstand a large amount of force without breaking. Page 14 of 50 **Viscosity** Viscosity is a measure of how easily particles can slide over one another. It can also be described as the runniness or gooeyness of a liquid. A substance like honey has a high viscosity, while water has a low viscosity. Honey has a has a high viscosity. This means that it is gooey and does not flow easily.  **Hardness** Water has a has a low viscosity. This means that it is runny and not flows easily. is a measure of how easily a substance can be scratched. It is [not] the same as strength. For example, glass is a very hard substance, but it is not very strong as it will shatter easily if a force is applied to it. Substances that are hard often have higher melting points, as they have strong forces (bonds) between their particles. Diamonds are the hardest substance on Earth.  They cannot easily be scratched. Page 15 of 50 **4. Density** **What is Density?** Density is the amount of mass within a certain volume of a substance is known as the density of that substance. The density of a substance depends on two factors: 1\. How closely packed the particles of a substance are - the more tightly packed the higher the density of the substance. 2\. How heavy (the mass of) the individual particles that make up that substance. In most cases, this means that a substance in a solid state will be denser than the same substance in a liquid state, which will be denser than the same substance in a gaseous state. Water in one of the only substances that does not follow this rule. It has an extraordinary property that wen it freezes (turns to ice) it is less dense that when it was water. This unique property is vital for life on Earth in the following ways When water seeps into the cracks of rocks and freezes, its expansion can cause the rock to break. This process helps break down rock and helps to create soil to grow food. When water in oceans, rivers and lakes freezes, the water floats protecting them from freezing solid. This is important for aquatic life (organisms that live in water) to survive. Page 16 of 50 **Measuring Mass and Volume** In chemistry we measure mass in grams ( ), but you will sometimes be given the mass of an object in kilograms ( ). To convert from kilograms to grams multiply the number of kilograms by 1000. **Example 1:** Convert a mass of 2 kilograms to grams: ( ) = 2 × 1000 = 2000 **Example 2:** Convert a mass of 0.4 kilograms to grams: ( ) = 0.4 × 1000 = 400 **Measuring Volume** In chemistry we measure volume in cubic centimetres ( ^!^). One cubic centimetre is equal to one millilitre ( ). You will sometimes be given the volume of an object in litres. To convert from litres to millilitres (and therefore also to cubic centimetres) multiply the number of litres by 1000. **Example 3:** Convert a volume of 3 litres to cubic centimetres: ( ^!^) = 3 × 1000 = 3000 ^!^ **Example 4:** Convert a volume of 0.7 litres to cubic centimetres: ( ^!^) = 0.7 × 1000 = 700 ^!^ Page 17 of 50 **Density Calculations** Density is calculated by dividing the volume of a substance (in ^!^) by the mass of the substance (in ). This can be represented mathematically by the formula: ( / ^!^~)\ =~ ( ) ( ^!^) **Example 5:** An object has a mass of 200 and volume of 50 ^!^. Calculate its density. ~ \ =~ ~ \ =~ 200 50 = 4 / ^!^ = 200g = 50 ^!^ **Example 6:** An object has a mass of 1.5 and volume of 50 ^!^. Calculate its density. ~ \ =~ ~ \ =~ 1500 50 = 30 / ^!^ = 1.5 g *Mass is in kg, so we need to convert from kg to g* mass = 1500 g = 50 ^!^ Page 18 of 50 **Example 7:** An object has a mass of 700 and volume of 0.2 litres. Calculate its density. ~ \ =~ ~ \ =~ 1500 50 = 30 / ^!^ **Extension: Rearranging formulas** = 700 = 0.2 *volume is in litres, so we need to convert from litres to cm^3^.* = 50 ^!^ The density formula can used to calculate mass or volume. The easiest way to do this is to first add the values you have in the question, then solve for the unknown variable. This is shown in the examples below. **Example 8:** A material has a density of 2.1 / ^!^. What is the mass of 300 ^!^ of this material? Equation Substitute values Rearrange equation to find mass Calculate answer ~ \ =~ ~2.1\ =~ 300 = 2.1 × 300 = 630 g = 2.1 / ^!^ = 300 ^!^ = ? Page 19 of 50 **Example 9:** A material has a density of 2.15 / ^!^. What volume of this material is required to make a 200 sample of this material? Equation Substitute values Rearrange equation to find mass Calculate answer ~ \ =~ ~2.15\ =~ 200 ~ \ =~ 200 2.15 = 93.0 g = 2.15 / ^!^ =? = 200 Page 20 of 50 **5. Extension: Density and Buoyancy** Density can be used to predict and explain why some objects float and other objects sink. Water has a density of 1.0 g/cm^3^. This means that an object that has a higher density than 1.0 g/cm^3^ will sink, and an object with a higher density than than 1.0 g/cm^3^ will float. For example, olive oil has a density of 0.9 g/cm^3^. If water and olive oil are mixed, the olive oil with **The Archimedes Principle** Archimedes was born 287BC in Syracuse,  Sicily (modern day Italy). He was the most famous mathematician and inventor in ancient Greece, and his is most famous for the Archimedes principle. The Archimedes Principle states that when an object is partially or fully immersed in a fluid (such as water), the fluid exerts an upwards force on the object equal to the weight of the fluid displaced. Put simply, if an object is less dense than a fluid, it will float in that fluid because the upwards force from the liquid equals the weight of the object. If an object is more dense than a fluid, it will sink in that fluid because the upwards force from the liquid is less than the weight of the object. Page 21 of 50 This upwards force is known as upthrust. This explains why, when swimming, you feel like you have less weight. Your weight is being supported by upthrust. This also explains why large Boat Weight of boat = 1000 N Displaced Water weight = 1000 N ships can float. Even though they are made of steel, they are hollow (full of air). This means that, overall, they have a lower density than water and float. The weight of water displaced by the boat equals the weight of the boat. This means that the boat is less dense than water and so it will float instead of sink. Page 22 of 50 **6. Extension: Mixing** **What is Mixing?** Mixing occurs when two or more substances are combined together. Some mixtures can be clearly seen, but others occur on the particle level. For example, combining flour, sugar, cocoa and baking powder creates a mixture of dry ingredients to make a chocolate cake. While the flour and baking powder will be hard to tell apart, the other components can be distinguished from each other. When mixing sugar and water together, the sugar seemingly disappears. What has happened, however, is a mixing on the particle level.  While the sugar particles cannot be seen, they are evenly dispersed throughout the water. This type of mixture is called a solution, and is created by a mixing process called dissolving. Page 23 of 50 **Diffusion** Another type of mixing occurs when two gases or two liquids are mixed together, in a process called diffusion. This process causes the two substances to mix together evenly, even without stirring. As gas substances are combined within the same container, the gas particles will continuously move throughout, bouncing off each other and the walls of the container, until there is an even spread of both gases. This explains how food cooking in the kitchen can be smelt from the loungeroom. Or if the toilet door is left open, a nasty smell can work its way through the house. The following diagram gives a visual representation of a perfume particles diffusing around a room. Science Quest 8: p300 Page 24 of 50 Diffusion can also be seen in the mixture of two liquids. For example, when a dye is added to water, the dye particles will gradually diffuse through the water until the solution is a uniform colour. [https://www.sciencefacts.net/diffusion.html] Diffusion is also one process that cell membranes use to allow certain particles in and out of the cell. [https://www.sciencefacts.net/simple-diffusion.html] Page 25 of 50 **7. Atoms** **What is an Atom?** So far in chemistry we have been talking about particles. If we look closely at these particles we see that they are made up of atoms. Atoms are the basic building blocks of all matter, and all matter is made up of either a single type of atom, or more likely, a combination of multiple types of atoms. For example:  Pure gold is a solid made up of only gold atoms which are bonded together **Subatomic Particles** A water particle is a molecule made up of one oxygen atom and two hydrogen atoms bonded together Atoms are made up of three smaller particles known as [subatomic] [particles]. These subatomic particles are called protons, neutrons and electrons. They are arranged in an atom as shown below: Page 26 of 50 **Atoms and Elements** An element is a pure substance made of only one type of atom. There are 90 different elements that are found naturally on Earth. Elements are shown on the Periodic Table of Elements **Understanding the Periodic Table of Elements** The periodic table contains boxes with the details for each element. The **atomic number** is the number of protons in the nucleus of the atom. The number of electrons in an atom is the same as the number of protons. The **atomic mass** is the average number of protons and neutrons in the nucleus. This means that the element carbon has: 6 protons (from the atomic number) 6 electrons (same as number of protons) 6 neutrons (atomic mass subtract the atomic number) How many protons, neutrons and electrons in the flowing elements? 2 protons  2 electrons 2 neutrons 29 protons 29 electrons 35 neutrons (round to nearest whole number) Page 27 of 50 **8. The Periodic Table** **The Periodic Table** The periodic table, like any table, has rows running left to right, and columns running up and down. The rows are called PERIODS and, and numbered from top to bottom. The columns are called GROUPS, and numbered left to right. This is shown on the diagram below for a part of the periodic table. ***Group*** ***4*** ***Period 1*** ***Period 2*** ***Period 3*** ***Period 4*** ***Period 5*** ***Period 6*** ***Period 7*** ***Group 1*** ***Group 2*** ***Group 3*** For example, the element magnesium, shown by the symbol Mg on the Periodic Table, is located in Group 2 and Period 3. Page 28 of 50 **Metals and Non-Metals** The two main types of atoms in the periodic table are metals and non metals. Most of the elements on the periodic table are metals, with a small amount being non-metals. We will learn what makes an element be defined in each category in the next lesson. **Metalloids** There is one more type of element - the metalloids. Metalloids are a small group of elements that show characteristics of metals and non metals. In other words, they don't belong in either category, but belong somewhere in between. Suitably then, they also exist between the metals and the non-metals on the periodic table. Page 29 of 50 **Periodic Table Highlighting Metals and Metalloids *~Elements\ in\ Grey:\ Unknown~ ~if\ metal\ or\ non-meta~l*** ***~Elements\ that\ are~ ~faded\ are\ Non-Metal~s*** ***~Elements\ with\ dotted~ ~lines\ are\ metalloid~s*** ***~Elements\ shown\ that\ are\ not~ ~faded\ are\ metal~s*** Page 30 of 50 **Periodic Table Highlighting Non-Metals and Metalloids *~Elements\ in\ Grey:\ Unknown~ ~if\ metal\ or\ non-meta~l*** ***~Elements\ that\ are\ not~ ~faded\ are\ Non-Metal~s*** ***~Elements\ with\ dotted~ ~lines\ are\ metalloid~s*** ***~Elements\ shown\ that\ are~ ~faded\ are\ metal~s*** Page 31 of 50 **9. Metals and Non-metals** **What is a metal?** A metal (also called a metallic substance) is a substance made up of only metal atoms. For example, Copper (Cu), Lead (Pb) and Iron (Fe) are all metal substances. **Metallic bonds** A Metallic substance is made up of  metal atoms which are closely packed together in an ordered arrangement called a lattice. The metal atoms have a "sea" of delocalised electrons moving around the metal atoms. **Properties of Metallic substances:** The properties of metallic substances come from the way that the atoms have bonded together. These properties are described below: **High density** Metals are dense substances because the metal atoms are so tightly packed together in the lattice structure. **Excellent conductors of heat and electricity** Metals have a 'sea' of delocalised electrons which are free to move around. These electrons act as charge carriers and make metal very good at conducting electricity. These free electrons also mean that metals are very good conductors of heat. Page 32 of 50 **Malleable and ductile** Metal atoms arrange themselves into layers. This means that when the metal is bent or hammered into shape, the atoms slide over one another. This makes metals malleable (they can be beaten into a new shape) and ductile (they can be drawn into a wire). **Lustrous** Metals are shiny (lustrous) because the free moving electrons reflect light from the surface of the metal. **Applications of metals:** Below are some applications of metals: **Electrical Wire** **BBQ Hot Plate**  Copper is used in electrical wires because it is ductile, malleable and conducts electricity. Iron is used for BBQ hot plates because they are good conductors of heat. **~Jewellery~** Gold is used in jewellery because it is lustrous and malleable Page 33 of 50 **What is a non-metal?** A non-metal (often called a covalent substance) is a substance made up of only non-metal atoms. For example, Oxygen (O~2~), Helium (He) and Sulfur (S~8~) are all non-metal substances. **Properties of Non-metals:** The properties of non-metals come from the way that the atoms have bonded together. These properties are described in the table below: **Poor conductors of heat and electricity** Covalent substances are made up of non-metals which cannot conduct electricity in solid, liquid or gaseous state. This is because the way that the atoms bond means that there are no free moving charged particles. An exception of this is graphite (made from carbon) which is the only non-metal that can conduct electricity. **Dull, soft and waxy** The molecules that make up non-metal substances have very weak intermolecular forces holding the molecules together. This means that when they are found in solid form they are soft and waxy. This means that they are easily scratched and broken apart. In addition to this, non metals do not have free moving electrons to reflect light so they often appear dull. Page 34 of 50 **Applications of non-metals:** Below are some applications of non-metals: Paraffin wax is a combination of **Candles**  **Wire** **coating** **Insulators**  carbon and hydrogen atoms. It is dull and waxy. PVC is a combination of carbon, hydrogen and chlorine atoms. It is used to coat wires as it is a poor conductor of electricity. Polystyrene is used as cups and insulators as it is a poor conductor of heat. Page 35 of 50 **10. Elements and Compounds** **Elements and Compounds** An element is a substance made up of only one type of atom. There are three types of elements; monatomic elements, molecular elements and metallic elements. A **compound** is a substance made up of more than one type of atom.  Pure gold is an element, because it is made up of only gold atoms. **Monatomic Element** A water molecule is a compound because it is made up of two different atoms (oxygen and hydrogen). A monatomic element is made up of only one of the same type of atom. The noble gasses (Group 18 elements) are the only monatomic elements because they are unreactive (don't bond easily with other atoms) and mostly gasses at room temperature. **Molecular elements** Molecular elements are made up of the same type of non-metal atom joined together in groups called molecules. For example, hydrogen gas is a molecular element because it is found as a molecule of two hydrogen atoms bonded together (H~2~). Other examples are nitrogen gas (N~2~), Sulfur (S~8~) and Phosphorus (P~4~). Page 36 of 50 **Metallic Element**  Metallic elements are made up of the same type of metal atom joined together in a lattice structure. Magnesium (Mg), Sodium (Na), Gold (Au) and Iron (Fe) are all examples of metal elements. **Chemical Formula** In science we use the chemical symbols for elements and numbers to write down the formula for elements and compounds. For example, the chemical formula for carbon monoxide is shown below: Carbon dioxide is slightly different to carbon monoxide, in that it has two oxygen atoms. The chemical formula for carbon dioxide is shown below: Page 37 of 50 Water has two hydrogen atoms and one oxygen atom. The chemical formula for water is shown below: **Example 1** Write the chemical formula for hydrofluoric acid which is a substance made up of one hydrogen atom and one fluorine atom. 1 × ~1\ ×\ \ ~ : **Example 2** Write the chemical formula for potassium bicarbonate which is a substance made up of one potassium atom, one hydrogen atom, one carbon atom and three oxygen atoms. 1 × 1 × 3 × : ~!~ Page 38 of 50 **11. Physical and Chemical Change** **Physical Change** Physical change is a change where a substance looks different but still consists of the same elements and compounds. An example of this is ice melting to water, water changing to steam, or salt dissolving into water.  **Chemical Change** Chemical change occurs when a chemical reaction has taken place. In every chemical reaction, one or more substances are changed into new, different substances. For example, when an iron nail rusts, iron atoms (Fe) react with oxygen in the air to form a new compound called iron oxide (Fe~2~O~3~). An iron nail is made up of iron atoms (Fe). Rusting When iron rusts it turns to a new compound called iron oxide (Fe~2~O~3~) Page 39 of 50 **Observing Chemical Change** We know that a chemical change has taken place because we can observe one or more of the following: Gas is produced (we see bubbles or fizz). An irreversible colour change. Heat is absorbed or released. light is released. **Examples of Chemical Change** Page 40 of 50 **12. Reactants and Products** In chemical reactions the substances that we start with are called reactants, and the substances we end with are called products. The total number of each type of atom does not change between the reactants and products, but the atoms rearrange to form new substances. For example: When a sodium chloride (NaCl) solution is mixed with a silver nitrate (AgNO~3~) solution, white silver chloride (AgCl) is produced, leaving behind a sodium nitrate (NaNO~3~) solution. Reactants: Sodium chloride (NaCl) and silver nitrate (AgNO~3~) solution (these are the substances that we started with). Products: Silver chloride (AgCl) and sodium nitrate (NaNO~3~) solution (these are the substances that we finish with). We can write chemical reactions as equations. In chemical equations we don't use an equal sign (=), instead we use an arrow (→). The reactants are written on the left side of the arrow and the products are written on the right side of the arrow. For the reaction above the equation can be written as a word equation and a formula equation. Word Equation: ℎ + → ℎ + Formula Equation: + ~!~ → + ~!~ Page 41 of 50 **13. Endothermic and Exothermic Reactions** **Exothermic and Endothermic Reactions** Most chemical reactions either absorb or release heat. We use the terms exothermic and endothermic to describe this. Endothermic: Heat is absorbed (temperature goes down) ------------------------------- reactants + energy → products ------------------------------- Exothermic: Heat is released (temperature goes up) ------------------------------- reactants → products + energy ------------------------------- Page 42 of 50 **14. Rates of Reactions** **Rate of reaction** The speed at which a chemical reaction happens is called the rate of reaction. For example, an explosion is a chemical reaction with a fast rate of reaction (it happens over a short period of time). Rusting is an example of a chemical reaction with a slow rate of reaction (it happens over a long period of time). **Chemical Reactions and Collision Theory** Particles can only react if the collide with enough energy for the reaction to take place. This is called collision theory. There are four factors that can change the rate of a chemical reaction; temperature, concentration of reactants, surface area of solids and agitation (mixing or stirring). **Temperature and Rate of Reaction** Increasing temperature causes particles to have more energy and move around faster. This means that the rate if reaction will increase because particles will collide more often and with greater energy. Page 43 of 50 **Concentration and Rate of Reaction** Increasing the concentration of a reactant means there are more particles that could react. This means that the rate if reaction will increase because more particles means more collisions.  **Surface Area and Rate of Reaction** When you break down a substance into smaller parts you increase the surface area of that substance. Using a chemical powder (lots of tiny pieces of a substance) instead of a solid lump (one large piece of a substance) means the surface area is greater. This means that the rate if reaction will increase because more area of reactant is available for collisions. Page 44 of 50 **Agitation (mixing) and Rate of Reaction** Agitation is the scientific word for stirring. Increasing agitation causes particles to move around faster. This means that the rate if reaction will increase because particles will collide more often. Page 45 of 50 **15. Extension: Types of Reactions** **Combustion Reactions** One of the most important chemical reactions are combustion reactions, which occur when a fuel source combines with oxygen (from the air) and burns rapidly. When this happens lots of heat and light is produced. The word equation for a combustion reaction is: ---------------------------------------- Fuel + oxygen → carbon dioxide + water ---------------------------------------- Many examples of combustion reactions are found in everyday life. They include: \- burning of petrol or diesel within engines of transport vehicles - burning wood \- gas stoves \- fireworks There are two types of combustion that can occur. **[Complete combustion]** occurs when there is excess oxygen (more oxygen than needed) in the reaction. Word equation: Chemical equation: +-----------------------------------------------------------------------+ | methane + oxygen ~→~ carbon | | | | ~dioxide~ + water | +=======================================================================+ | CH~4~ + 2O2 → CO~2~ + 2H~2~O | +-----------------------------------------------------------------------+ Page 46 of 50 **[Incomplete combustion]** occurs when insufficient oxygen (less oxygen than needed) is present, and so carbon or carbon monoxide is produced instead of carbon dioxide. +-----------------------------------------------------------------------+ | Fuel + oxygen ~→~ carbon | | | | ~monoxide~ + water | +-----------------------------------------------------------------------+ The product of this reaction, carbon monoxide is a poisonous gas that is very harmful if inhaled. Most combustion reactions will produce small amounts of carbon monoxide, but larger quantities can be found from car exhaust fumes and small space heaters. **Carbon monoxide poisoning** Our body uses haemoglobin to transport oxygen to the cells around our body. If we breathe in carbon monoxide as well as oxygen, the carbon monoxide attaches to the haemoglobin before oxygen can, and so the cells in our body aren't able to gain the oxygen they need to function. Symptoms of carbon monoxide  poisoning include drowsiness, dizziness, headaches, shortness of breath and loss of brain function. It is treated by giving the person pure oxygen to breathe in. This essentially forces the haemoglobin to recommence transporting oxygen around the body instead of carbon monoxide. Page 47 of 50 **Aerobic Respiration** Humans require energy to move, or to just generally survive. The reaction of glucose and oxygen creates energy. The glucose is gained from food and oxygen is breathed in from the atmosphere. The waste products of the reaction, carbon dioxide and water, are exhaled from the body. glucose + oxygen → carbon dioxide + water ------------------------------------------- C~6~H~12~O~6~ + 6O2 → 6CO~2~ + 6H~2~O **Photosynthesis** Photosynthesis is a crucial reaction without which life on Earth would not survive. Plants are able to absorb carbon dioxide from the atmosphere to produce glucose and oxygen, which is then released back into the atmosphere. This is why rainforests are referred to as the "lungs of the Earth". You will notice that the reaction is the reverse of aerobic respiration. +-----------------------------------------------------------------------+ | carbon | | | | ~dioxide~ + water → glucose + oxygen | +=======================================================================+ | 6CO~2~ + 6H~2~O → C~6~H~12~O~6~ + 6O2 | +-----------------------------------------------------------------------+ Page 48 of 50 **Corrosion** Corrosion is a reaction where metals are broken down by water, air or other chemicals. It is the process where a metal reacts with oxygen to become a metal oxide. ------------------------------ metal + oxygen → metal oxide ------------------------------ A common example of corrosion is rusting. ------------------------------------------------- iron + oxygen → iron oxide 4Fe + 3O2 → 2Fe~2~O3 ------------------------------------------------- Page 49 of 50