Balancing Chemical Equations Year 10 Physical Science Fundamentals of Chemistry

Chemical Reactions A Chemical Reaction is a process by which a substance is changed into one or more different substances Indications of a chemical reaction include: Heat and light Production of gas Formation of a precipitate Color change

Chemical Equations A chemical equation is a symbolic way of representing a chemical reaction. It shows the formulae all reactants and products and their physical states. The equation may also indicate conditions necessary for the reaction to occur. The states of the reactants and products should be indicated in the equation by: (s) for solid (l) for liquid, (g) for gas and (aq) for a solution of water

Chemical Equations When writing chemical equations, you need to consider: Law of Conservation of Mass “In a chemical reaction, the total mass of the products equals the total mass of the reactants” Law of Constant Proportions “The same compound, however formed, contains the same elements chemically combined, in constant proportions by weight”

Balancing Chemical Equations The total mass of the reactants in a chemical reaction is equal to the total mass of the products. Atoms are not created or destroyed, but are rearranged to form new substances. A balanced chemical equation has the same number of each type of atom on each side of the equation. Balancing is achieved by placing numbers (coefficients) in front of each formula. We cannot change the formula of any substance in the equation, as these are found by experiment.

Rules for Balancing Chemical Equations 1. Write the reactants and products using the correct formula and state for each substance. Count the number of atoms of each element on the LHS of the equation. Do the same for the RHS and compare for each element. If any of these numbers don’t match, the equation is not balanced and you’ll need to complete the following steps:

Rules for Balancing Chemical Equations 3. Balance by placing coefficients in front of the formulae. DO NOT CHANGE THE ACTUAL FORMULA If any substance is present as an element, leave the balancing of it to last. 4. Check all atoms or ions to ensure that they are balanced. 5. Make sure that the coefficients are in their lowest possible ratio.

Combustion Reactions When hydrocarbons burn in a plentiful supply of oxygen, they produce carbon dioxide and water. When a limited air supply is available, carbon monoxide may be formed in preference to carbon dioxide. When octane, C8H18, is burnt in air: 2C8H18 (g) + 25O2 (g)  16CO2 (g) + 18H2O (g) When octane, C8H18, is burnt in an engine: 2C8H18 (g) + 17O2 (g)  16CO (g) + 18H2O (g)

Acid-Base & Neutralisation Reactions When an acid reacts with a base to form a salt and water, a neutralisation reaction occurs: Acid + Base  Salt + Water Hydrochloric acid reacts with the base sodium hydroxide to form sodium chloride and water: HCl (aq) + NaOH (ag)  NaCl (aq) + H2O (l)

Acid-Base & Neutralisation Reactions Other common acid-base reactions are: 1. Acid + Metal  Salt + Hydrogen 2HCl (aq) + Mg (s)  MgCl2 (aq) + H2 (g) Acid + Metal Carbonate  Salt + Carbon Dioxide + Water 2HCl (aq) + Na2CO3 (aq)  2NaCl (aq) + CO2 (g) + H2O (l) 3. Acid + Metal Oxide  Salt + Water 2HCl (aq) + CuO (s)  CuCl2 (aq) + H2O (l) Acid + Metal hydroxide  Salt + Water H2SO4 (aq) + 2NaOH (aq)  Na2SO4 (aq) + H2O

Precipitation Reactions Precipitation occurs when ions in solution “swap partners” to form a new compound of low solubility in water. This low-solubility compound forms as solid particles that eventually settle. It is called a precipitate. To predict whether a precipitate will form, you need to know which substances are soluble in water and which are insoluble. Solutions of sodium chloride and silver nitrate react to form a precipitate of silver chloride. NaCl (aq) + AgNO3 (aq)  AgCl (s) + NaNO3 (aq)

Precipitation Reactions

Barium Sulphate Contrasting in Medicine

Ionic Equations An ionic equation shows the association of the ions that form the precipitate. The other ions that are present but remain in solution are termed spectator ions. Spectator ions are not shown in the ionic equation.

Ionic Equations - Rules Write the balanced chemical equation. Decide which substances are soluble and which will form precipitates. Expand the chemical equation by dissociating all the soluble compounds into their free ions. Cancel all free ions that are unchanged on both sides of the equation (the spectators). Write the net ionic equation.

Ionic Equations - Example Write a balanced ionic equation for the reaction between barium chloride and sodium sulphate: BaCl2 + Na2SO4 → BaSO4 + 2NaCl BaSO4 is insoluble and forms the precipitate. NaCl is soluble and forms the spectator ion. 3. Ba2+ (aq) + 2Cl- (aq) + 2Na+ (aq) + SO42- (aq) → BaSO4 (s) + 2Na+ (aq) + 2Cl- (aq) Hence we get: Ba2+ (aq) + SO42- (aq) → BaSO4 (s)