Topic 5 of 8 · Form 3
Chemical energetics and reactions
Exothermic (ΔH negative) and endothermic (ΔH positive) reactions; reaction pathway diagrams with activation energy; bond breaking is endothermic and bond making exothermic.
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Energy changes
Exothermic (ΔH negative) and endothermic (ΔH positive) reactions; reaction pathway diagrams with activation energy; bond breaking is endothermic and bond making exothermic.
Worked example: Bond energies: H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol. Find ΔH for H₂ + Cl₂ → 2HCl.
Broken: 436 + 242 = 678; made: 2 × 431 = 862 → ΔH = 678 − 862 = −184 kJ/mol
Rates of reaction
Effects of concentration, pressure, surface area, temperature and catalysts, explained by collision theory; measuring rates (gas volume, mass loss); interpreting graphs.
Reversible reactions and equilibrium
Dynamic equilibrium in closed systems; effects of changing conditions; the Haber process (iron catalyst, about 450 °C, about 200 atm) and the Contact process (V₂O₅ catalyst, about 450 °C) – compromise conditions.
Redox
Oxidation and reduction as gain or loss of oxygen and of electrons, and change in oxidation number; oxidising and reducing agents (e.g. acidified potassium manganate(VII) is decolourised by reducing agents).
Check yourself
- Why does a catalyst increase the rate of a reaction?
- Why is a temperature of about 450 °C used in the Haber process when lower temperatures give a higher yield?
- Identify the oxidising agent in Zn + Cu²⁺ → Zn²⁺ + Cu.
Show answers
- It provides a pathway with lower activation energy.
- Lower temperatures would make the rate too slow; 450 °C is a compromise between rate and yield.
- Cu²⁺ (it gains electrons).
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