Balancing equations

Law of Conservation of Mass This law states that matter is never created or destroyed during a chemical reaction but is instead just rearranged during a chemical reaction. Hydrogen + Oxygen  Water H 2 + O 2  H 2 O Atoms retain their identity – All the Hydrogen atoms are still hydrogen atoms. They are just rearranged into different molecules and compounds

Law of conservation of mass But this is not the whole story. In a chemical reaction no matter is created or destroyed. So there must be the same amount of each atom on both sides of the chemical reaction. Hydrogen + Oxygen  Water H 2 + O 2  H 2 O Reactants H = O = Products H = O =

Balancing equations Step by step 1.Write out your equation 2.Write a list of the number of atoms of reactants and products 3.Add coefficients to the front of compounds and molecules in the reaction to balance the equation. 4.As you add each coefficient write changes to the list of reactants and products

Counting Atoms Count the number of atoms of each type in these molecules. Propane H 2 SO 4

Balancing equations practice Magnesium + hydrochloric acid  Hydrogen + Magnesium Chloride Mg + HCl  H 2 + MgCl 2 Mg + 2HCl  H 2 + MgCl 2

Tin dioxide + hydrogen gas  Tin + Water SnO 2 + H 2  Sn + H 2 O SnO 2 + 2H 2  2Sn + H 2 O Balancing equations practice

Sodium Carbonate+ hydrochloric acid  water + carbon dioxide + sodium chloride Na 2 CO 3 + HCl  H 2 O + CO 2 + NaCl Na 2 CO 3 + 2HCl  H 2 O +CO 2 + 2NaCl

Solutions revisited We have discussed solutions as uniform mixtures. This is still true but chemists find it useful to discuss solutions in terms of solute and solvent. Solute – The substance which dissolves in the solution. This substance changes phase. E.g. solid becomes a liquid. Solvent – The substance which the solute dissolves into. This substance does not change phase. E.g a Liquid solvent stays liquid.

Examples if solutions Solvent (water) Solute (coffee) Coffee drink (solution) Solvent (water) Solute (sodium hydroxide) sodium hydroxide solution