Chemical Reactions and Equations
This domain tests balancing equations with state symbols and identifying the reaction type; every equation you write must be balanced atom by atom.
Types you must recognise
- Combination: A + B → AB, e.g. CaO + H2O → Ca(OH)2, which is exothermic.
- Decomposition: thermal (CaCO3 → CaO + CO2), electrolytic (2H2O → 2H2 + O2), photolytic (2AgCl → 2Ag + Cl2).
- Displacement: Zn + CuSO4 → ZnSO4 + Cu, because zinc is more reactive than copper.
- Double displacement: AgNO3 + NaCl → AgCl + NaNO3, with AgCl as an insoluble precipitate.
- Oxidation is gain of oxygen or loss of electrons; reduction is loss of oxygen or gain of electrons; together they form a redox reaction.
- Exothermic reactions release heat (respiration, burning); endothermic reactions absorb it (decomposition of limestone).
Worked example
Balance Fe + H2O → Fe3O4 + H2. The product Fe3O4 fixes iron at 3, so write 3Fe. It fixes oxygen at 4, so write 4H2O, giving 8 hydrogen atoms, hence 4H2. Check: Fe 3 = 3, O 4 = 4, H 8 = 8. Final equation: 3Fe + 4H2O → Fe3O4 + 4H2. This is a redox reaction - iron is oxidised and hydrogen is reduced.
Traps
- Never change subscripts to balance an equation; change only the coefficients.
- Insoluble products such as AgCl and BaSO4 are marked with a downward arrow.
- Rusting (4Fe + 3O2 + xH2O → 2Fe2O3·xH2O) and rancidity of fats are oxidation processes, not decomposition.
- State symbols (s), (l), (g) and (aq) often carry marks, so include them when asked.