Chemical Reactions
This domain tests whether you can recognise reaction types, write and balance equations, and use observations to identify products and energy changes.
Core rules
- A chemical equation must obey conservation of mass, so the number of atoms of each element must be equal on both sides. Balance by changing coefficients, never subscripts.
- Combination reactions join substances; decomposition reactions break one substance into simpler ones, usually with heat, light, or electricity; displacement reactions occur when a more reactive element replaces a less reactive one; double displacement reactions exchange ions and often form a precipitate.
- Oxidation is loss of electrons or gain of oxygen; reduction is gain of electrons or loss of oxygen. Exothermic reactions release heat; endothermic reactions absorb heat.
Worked example
Balance: Al + O2 → Al2O3. Count oxygen: the product has 3 O, so use 2Al2O3 to give 6 O. That requires 3O2. Now count aluminium: 2Al2O3 contains 4 Al, so use 4Al. The balanced equation is 4Al + 3O2 → 2Al2O3. Check: Al 4 = 4; O 6 = 6. This is a combination reaction and is exothermic.
Traps that cost marks
Do not change a formula such as Al2O3 to AlO to make counting easier. Do not call every reaction with oxygen combustion; rusting is slow oxidation. In displacement, compare reactivity before predicting whether a reaction occurs. Include state symbols only when they are correct.
Key points
- Balance equations by adjusting coefficients only.
- Learn the four main reaction types and their patterns.
- Oxidation and reduction occur together in redox reactions.
- Exothermic releases heat; endothermic absorbs heat.