A balanced chemical equation is the language of chemistry — it concisely describes what reacts, what is produced, the ratio in which substances react, and the physical states of all species. Mastering equation writing and balancing is essential for all quantitative chemistry.
15.1 Writing and Balancing Equations
Definition
Chemical equation
A chemical equation is a symbolic representation of a chemical reaction that uses the chemical formulae of the reactants (left of the arrow) and products (right of the arrow) to show what occurs.
Definition
Balanced chemical equation
A balanced chemical equation is a chemical equation in which the number of atoms of each element is the same on both sides of the reaction arrow, reflecting the Law of Conservation of Mass.
Definition
Reactants
Reactants are the starting substances in a chemical reaction. They are written on the left-hand side of the reaction arrow.
Definition
Products
Products are the new substances formed during a chemical reaction. They are written on the right-hand side of the reaction arrow.
Definition
State symbols
State symbols indicate the physical state of each substance in a chemical equation: (s) = solid, (l) = pure liquid, (g) = gas, (aq) = aqueous solution (dissolved in water).
Rules for balancing chemical equations
- Write the correct chemical formulae for all reactants and products — NEVER change a formula to balance an equation.
- Count the atoms of each element on both sides of the arrow.
- Add coefficients (whole numbers written in front of formulae) to balance — start with the most complex molecule.
- NEVER change subscripts inside a formula to balance (H₂O and H₂O₂ are completely different substances).
- Check that the number of atoms of each element is identical on both sides.
- Add state symbols: (s), (l), (g) or (aq) for each substance.
Watch Out
NEVER balance an equation by changing subscripts! For example, you may NOT write H₃O instead of H₂O to make hydrogen balance — H₃O is a completely different chemical species (the hydronium ion). You may ONLY add or change the coefficients (the big numbers written in front of formulae).
Worked Example
Balance the following equation and add state symbols: Fe + O₂ → Fe₂O₃
Given
- Unbalanced: Fe + O₂ → Fe₂O₃
Find
Balanced equation with state symbols
Solution
- 1Count atoms: Left: Fe = 1, O = 2. Right: Fe = 2, O = 3.
- 2Balance Fe: put coefficient 4 before Fe on left (will explain why below after balancing O).
- 3Balance O: O₂ gives even numbers; Fe₂O₃ has 3 O (odd). To clear the fractions, use 2Fe₂O₃ on right → 6 O on right. Need 6 O on left → 3O₂.
- 4Now Fe on right: 2Fe₂O₃ → 4 Fe on right. So 4Fe on left.
- 5Balanced: 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)
- 6Check: Fe: 4 = 4 ✓; O: 6 = 6 ✓
Worked Example
Balance the following equation and add state symbols: H₂ + O₂ → H₂O
Given
- Unbalanced: H₂ + O₂ → H₂O
Find
Balanced equation with state symbols
Solution
- 1Count atoms: Left: H = 2, O = 2. Right: H = 2, O = 1.
- 2O is unbalanced: put 2 in front of H₂O on right → 2H₂O (O: 2 = 2 ✓; H: now need 4 on left).
- 3Balance H: put 2 in front of H₂ on left → 2H₂.
- 4Balanced: 2H₂(g) + O₂(g) → 2H₂O(l)
- 5Check: H: 4 = 4 ✓; O: 2 = 2 ✓
Practice Question
Balance the following equations and add state symbols: (a) Na + H₂O → NaOH + H₂ (b) CuO + HCl → CuCl₂ + H₂O
(6 marks)