A chemical equation is a shorthand description of the changes that occur in a chemical reaction such as;
Methane + Oxygen → Carbon Dioxide + Water
It allows us to determine the quantitative relationships between reactants and products. To be valid, a chemical equation must be:
• Consistent with the experimental facts. Just because one can write an equation, it does not mean that it can really happen.
• Consistent with conservation of mass i.e. the total number of atoms of each element in the reactants must be equal to the total in the products. Atoms cannot disappear.
• Consistent with conservation of electric charge i.e. the net charge, before and aft er, must be the same.
To construct a chemical equation for a reaction:
• Write the names of the reactants on the left and products on the right side of the arrow,
• Replace the names with the correct chemical formulae,
• Balance the equation, and
• Write the state symbols aft er each substance: (s) solid, (g) gas, (l) liquid and (aq) aqueous.
Balancing chemical equations
Write the correct formulae for the reactants and products; reactants on left of arrow, products on right. Choose the most complicated compound that contains the greatest number of atoms, whether it is reactant or product. Start with the element in that compound that has the greatest number of atoms (not H or O or a polyatomic ion such as sulphate, nitrate, carbonate etc.). Balance the number of atoms in this compound with the corresponding atom on the other side by putting the appropriate numbers before the formulae. Repeat for other atoms. Next balance any polyatomic groups (such as SO4, NO3 and CO3) that appear on both sides of the equation. Finally check the numerical coeffi cients (the numbers before the formulae) to ensure they are whole numbers and in the lowest possible ratio. Include state symbols if required, in brackets, aft er the formulae.
E.g. Na(s), H2O (l), N2 (g), Cu2+ (aq)
Remember: it is not always possible to follow the guidelines in the previous slides exactly. Never change the formulae, only the numbers before the formulae (the coeffi cients). See examples below:
