Chemical Reactions
Chemical Reactions tests how to recognise and classify reactions, write balanced equations, and explain oxidation, reduction, corrosion and rancidity. Expect equation balancing and type identification.
Core ideas
- A chemical reaction rearranges atoms; mass is conserved. A balanced equation has equal numbers of each kind of atom on both sides.
- Combination: two or more reactants form one product. Decomposition: one compound breaks into simpler substances, often with heat, light or electricity.
- Displacement: a more reactive element displaces a less reactive one from its compound. Double displacement: ions exchange, often forming a precipitate.
- Oxidation is loss of electrons or gain of oxygen; reduction is gain of electrons or loss of oxygen. Both occur together in redox reactions.
- Exothermic reactions release heat; endothermic reactions absorb heat.
- Corrosion is attack on metals by air and moisture; rancidity is oxidation of fats and oils.
Worked example
Balance: Fe + H2O -> Fe3O4 + H2. Put 3 before Fe and 4 before H2O: 3Fe + 4H2O -> Fe3O4 + 4H2. Check: Fe 3 = 3; O 4 = 4; H 8 = 8. Reasoning: balance oxygen first through H2O, then hydrogen, then iron; never change the formula Fe3O4.
Traps that cost marks
Balancing means changing coefficients, not subscripts. Do not write H2O as H2O2 to make numbers fit. A precipitate is an insoluble solid formed in solution. Rusting needs both oxygen and moisture. In redox, identify which element's oxidation state changes.