Learning focus
Use chemical formulae and equations accurately, convert measured quantities into moles, apply balanced-equation ratios and present multi-step calculations with units and checks.
Translate names first
Write the correct formula of every reactant and product before attempting to balance. An incorrect formula cannot be repaired by balancing. Use diatomic formulae for elemental H2, N2, O2, F2, Cl2, Br2 and I2.
Use an arrow and plus signs
Separate multiple reactants or products with plus signs and use a single reaction arrow. Conditions may be placed above the arrow. Reversible reactions use a double arrow only when specified.
Balance after formulae are fixed
Adjust coefficients placed before formulae. Never change a subscript because that changes the identity of a substance. Coefficients should be the smallest whole-number set.
Check systematically
Count each element on both sides, beginning with elements appearing in the fewest compounds. Leave hydrogen and oxygen until later in many combustion or acid reactions. Finally check all counts again.
Worked example
For aluminium + oxygen -> aluminium oxide: Al + O2 -> Al2O3 becomes 4Al + 3O2 -> 2Al2O3.
Practical or data skill
Convert a set of word equations into unbalanced symbol equations before balancing them in a separate step.
Examination tip
Correct formulae come before correct coefficients.
Review questions and suggested answers
Question 1
Write the unbalanced equation for sodium reacting with chlorine.
Suggested answer
Na + Cl2 -> NaCl.
Question 2
Why may you not change H2O to H2O2 while balancing?
Suggested answer
It would change water into hydrogen peroxide.
Question 3
What is the smallest balanced set for N2 + H2 -> NH3?
Suggested answer
N2 + 3H2 -> 2NH3.
