ZIMSEC O Level · Chemistry · Form 4
Energy changes
- Exothermic reactions release heat (combustion, neutralisation, respiration); temperature rises.
- Endothermic reactions absorb heat (thermal decomposition, photosynthesis); temperature falls.
- Breaking bonds takes in energy; making bonds releases energy. Energy profile diagrams show reactants, products and activation energy.
Rates of reaction
Particles must collide with enough energy (the activation energy). The rate increases with:
- Higher temperature: particles move faster and more collisions have enough energy.
- Higher concentration or pressure: more frequent collisions.
- Smaller particle size: larger surface area.
- A catalyst: provides a pathway with lower activation energy and is not used up.
Measure the rate by gas volume against time, or loss of mass.
Reversible reactions and industry
Some reactions go both ways (⇌) and reach dynamic equilibrium.
- Haber process: N₂ + 3H₂ ⇌ 2NH₃, iron catalyst, about 450 °C and 200 atm. Ammonia is used for fertilisers (Zimbabwe makes ammonium nitrate fertiliser).
- Contact process: 2SO₂ + O₂ ⇌ 2SO₃, vanadium(V) oxide catalyst, about 450 °C, for sulfuric acid.
Exam tip: On a rate graph, the steepest part is the fastest rate; the line levels off when a reactant is used up.
Check yourself
- Why does powdered marble react faster with acid than lumps?
- Give the catalyst in the Haber process.
- Is dissolving ammonium nitrate in water exothermic or endothermic if the water cools?
Show answers
- Larger surface area gives more frequent collisions.
- Iron.
- Endothermic.
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