Classify chemical reactions as synthesis, decomposition, single displacement, or double displacement — and practise balancing equations using conservation of mass.
A chemical reaction occurs when substances (reactants) are transformed into new substances (products). The key principle is the Law of Conservation of Mass (Lavoisier): matter is neither created nor destroyed — the total mass of reactants equals the total mass of products.
To balance an equation, only change the coefficients (numbers in front of formulas) — never change subscripts inside a formula. Count atoms on each side and adjust until they match.
Two or more reactants combine to form a single product: A + B → AB
A single compound breaks down into two or more simpler substances: AB → A + B. These reactions require energy input — heat, electricity, or light.
A more reactive element displaces a less reactive element from a compound: A + BC → AC + B
The activity series ranks metals from most reactive (K, Na, Ca, Mg, Al, Zn, Fe, Ni, Sn, Pb, H, Cu, Ag, Au) to least reactive. A metal can only displace metals below it in the series.
Two compounds exchange ions to form two new compounds: AB + CD → AD + CB. These often produce a precipitate (insoluble solid), a gas, or water.
| Type | Energy change | Temperature | Examples |
|---|---|---|---|
| Exothermic | Releases energy (heat) | Surroundings get hotter | Combustion, neutralisation, respiration |
| Endothermic | Absorbs energy (heat) | Surroundings get cooler | Photosynthesis, decomposition, dissolving NH₄NO₃ |
Combustion is the reaction of a fuel with oxygen, releasing heat and light. It is a type of redox reaction where carbon compounds are oxidised.
How quickly reactants are converted to products depends on:
| Time (s) | Gas volume — large chips (cm³) | Gas volume — powdered marble (cm³) |
|---|---|---|
| 0 | 0 | 0 |
| 20 | 10 | 35 |
| 40 | 22 | 58 |
| 60 | 33 | 60 |
| 80 | 45 | 60 |
| 100 | 55 | 60 |
| 120 | 60 | 60 |