Science Chemistry Notes PDF - Balancing Chemical Reactions

Science Notes Semester 1 2025 Balancing Chemical Reactions Writing Chemical Equations Word Equation: Reactants (Initial Substances, always go on the left)  Products (Substances formed during a reaction, always go on the right) Example: Magnesium + Oxygen  Magnesium oxide Symbol Equations: Magne...

Science Notes Semester 1 2025 Balancing Chemical Reactions Writing Chemical Equations Word Equation: Reactants (Initial Substances, always go on the left)  Products (Substances formed during a reaction, always go on the right) Example: Magnesium + Oxygen  Magnesium oxide Symbol Equations: Magnesium + oxygen  magnesium oxide Balanced Chemical Equations  Atoms can’t be created or destroyed in a reaction o All the atoms we start with we must end up with  A balanced equation has the same number of each element on both sides of the equation  You can not change a compound or element to balance an equation Examples: Science Notes Semester 1 2025 Rules for balancing: 1. Assemble the correct formulas for all the reactants and products, using “+” and “” 2. Count the number of atoms of each type appearing on both sides 3. Balance the elements one at a time by adding coe icients (the numbers in front) where you need more – save balancing the H and O until LAST! 4. Double- Check to make sure it is balanced by counting the atoms on both sides Never:  Never change a subscript to balance an equation (You can only change coe icients) o If you change the subscript (formula) you are describing a di erent chemical o H2O is a di erent compound than H2O2.  Never put a coe icient in the middle of a formula; they must go only in the front o 2NaCl is okay, but Na2Cl is not Balancing Chemical Reactions: 1. Identify the elements within the compounds on each side of the reaction and write them in a list below the reactants side and the products side 2. Count how many atoms of each element are on each side of the reaction 3. Identify the elements that don’t balance from the reactant side to the product side. Decide on an order in which to balance each element one by one (if any element is by itself, do this one last!) 4. Add numbers to the front of the compounds to balance the element. This symbolizes that more than one molecule of that compound is participating in the reaction. 5. Adjust the count below 6. Repeat for other elements until all are balanced Science Notes Semester 1 2025 The crossover rule for writing ionic formulae 1. Identify the metal and nonmetal 2. Write the symbols 3. Write the charges 4. Crossover the charges from top to bottom 5. Make sure you remove the + and – symbols 6. Simplify the numbers and remove the 1s Science Notes Semester 1 2025 Chemical Reactions: -Synthesis Reactions Chemical Reactions: Chemical reactions are the process compounds (or elements) breaking bonds between the reactant species and then re-forming bonds in the product species. Common Chemical Reactions: Synthesis Reactions:  A synthesis reaction combines all the reactants of the reaction to form a product.  To recognise a synthesis reaction, look for a product that contains all the reactant atoms  E.g. (make sure it is balanced) When predicting the products of a synthesis reactions: 1. Write a word equation for all species involved 2. Write down the chemical symbol for species present 3. Balance the chemical equation Examples are:  Synthesis of ammonia  Commercial production of slaked lime (calcium hydroxide)  Production of sodium chloride or common salt Science Notes Semester 1 2025  Preparation of hydrochloric acid and ammonium chloride Other examples include:  Rusting  Photosynthesis Atomic interactions – Ionic Bonding Bonding  Bonding is a type of interaction between element on the periodic table  These are 3 types of bonding: o Ionic bonding o Covalent bonding o Metallic Bonding  Atoms bonds to try and achieve a more stable configuration Ionic Bonding  Involves the transfer of electrons from one element to another.  This cause the formation of a charged species or an ion  In general: o The metal will lose electrons. Forms a Cation (+) o The nonmental will gain electrons. Forms an Anion (-)  Only valence (outside) electrons will take part in bonding  Atoms are most stable when they have a full outer shell of electrons. Unstable atoms (unfilled outer shells) gain or lose electrons to become more stable (filled out of shells)  The resulting charge of the ion is known as its valency  An example of an ionic bond o Formation of Sodium Chloride (NaCl) or Salt from Sodium metal and chlorine gas. Properties of Ionic Substances Science Notes Semester 1 2025  Ionic substances – a metal bonded with a nonmetal through transfer of electrons and formation of respective ions  Properties o Solid crystals at room temperature o Have extremely high melting and boiling points o Conduct electricity when in solution (Mixed with water) or melted (at very high temperature) o Form a strong rigid lattice like structure o They are hard and brittle o Good insulators when solid Ionic Compound Structure  The structure of an ionic compound depends on the relative sizes of the cations and anions  Ionic compounds include salts, oxides, hydroxides, sulphides and the majority of inorganic compounds.  Ionic solids are held together by the electrostatic attraction between the positive and negative ions.  The ions are held together tightly in these giant structures it takes a lot of energy to break all the bonds – ionic compounds have high melting points and boiling points. Ionic Compound Properties 1. Physical propertied of ionic compounds  Due to the presence of the strong force of attraction between the positive and negative ions, ionic compounds are solids and are hard to break  They generally break into pieces when pressure is applied, hence they are considered brittle Science Notes Semester 1 2025 2. Melting and boiling points of ionic compounds  Due to the presence of electrostatic forces of attraction between ions, a large amount of energy is required to break the ionic bonds between the atoms  Thus, ionic compounds have high melting and boiling points 3. The solubility of ionic compounds  Ionic compounds are generally soluble in polar solvents such as water whereas solubility tends to decrease in non – polar solvents such as petrol, gasoline etc. 4. Conduction of Electricity  Ionic compound do not conduct electricity in the solid state but are good conductors in a molten state  Conduction of electricity involves the flow of charge from one point to another  In the solid state, as the movement of ions is not possible, ionic compounds don’t conduct electricity  Whereas in the molten state, ionic compounds conduct electricity as electrostatic forces of attraction between the ions are overcome by the heat released Decomposition Reactions  A decomposition reaction is a type of chemical reaction in which a compound breaks down into two or more substances. It is the opposite of a combination reaction.  An example of a decomposition reaction is the breakdown of carbonic acid (H2CO3) to carbon dioxide (CO2) and water (H2O) Science Notes Semester 1 2025  Most decomposition reactions require energy either in the form of heat, light, or electricity. This makes them endothermic.  Absorption of energy causes the breaking of the bonds present in the reaction substances which decomposes to give the product  Examples: o Industry  Production of calcium oxide or quicklime  Production of lithium oxide  Preparation of oxygen and carbon dioxide  In the mining, for the extraction of metals from their oxides and chlorides through electrolytic decomposition o Daily life  When a fizzy drink is opened, carbonic acid breaks down to produce water and carbon dioxide, which causes the fizz  During the digestion of food in our body, carbohydrates, fats and proteins decompose to form many simpler substances Single Displacement  A single displacement reaction is a chemical reaction in which one element is substituted for another element in a compound, generating a new element a new compound as products  There must always be a metal (solid) and an ionic compound (solution)  Some general rules for writing and prediction single displacement reactions. 1. Write a word equation for all species involved 2. Write down the chemical symbol for the metal the ionic formula for the solution 3. Balance the chemical equation Science Notes Semester 1 2025  The starting materials are always pure elements plus an aqueous compound  When a replacement reaction occurs, a new aqueous compound and a di erent pure element will be generated as products  In predicting whether a single – displacement reaction will occur, compare the reactivity of an element using an activity series table. In general, a metal can displace any metal lower in the activity series (cations)  The metal reactivity series to the right should be consulted when deciding if a single displacement reaction will occur Acid & Metal  When an acid and metal come into contact there are predictable products that will form.  These are: Metal + Acid  Salt + Hydrogen  An example might be Zinc metal reacts with Hydrochloric acid Acid & Metal Carbonate  When an acid and metal carbonate come into contact there are predictable products that will form  These are: a salt, water and carbon dioxide gas  An example might be Magnesium Carbonate reacts with Hydrochloric acid Neutralisation Reactions  When an acid and a base come into contact there are predictable products that will form  These are: a salt and water  An example might be Sodium Hydroxide reacts with Hydrochloric acid.