Chemical Reactions
Chemical Reactions tests how to write and balance equations, classify reactions, identify oxidation and reduction, and explain corrosion and rancidity. The exam often gives reactants and asks for the type of reaction or a balanced equation.
Core rules
- A balanced equation has equal numbers of each type of atom on both sides. Coefficients are changed; subscripts in formulas are not.
- Combination: two or more reactants form one product. Decomposition: one reactant breaks into two or more products, often with heat, light or electricity.
- Displacement: a more reactive element displaces a less reactive one from a compound. Double displacement: ions exchange partners, often forming a precipitate.
- Redox: oxidation is loss of electrons or gain of oxygen; reduction is gain of electrons or loss of oxygen. Both occur together.
- Corrosion is slow oxidation of metals, such as rusting of iron. Rancidity is oxidation of fats and oils; antioxidants and airtight packaging slow it.
Worked example
Balance: Fe + H2O -> Fe3O4 + H2. Count oxygen: Fe3O4 needs 4 H2O, giving 8 H. To get 8 H on the right, place 4 before H2. Now Fe: 3 Fe are needed. Balanced equation: 3Fe + 4H2O -> Fe3O4 + 4H2. Check atoms: Fe 3 = 3, H 8 = 8, O 4 = 4. The reasoning is to balance the most complex formula first, then hydrogen and iron.
Traps
Never change a formula's subscript to balance. Include state symbols only when known. A displacement reaction is not the same as double displacement. Oxidation and reduction are not separate reactions. Not every reaction is redox; acid-base neutralisation usually is not.