Energetics, rates and equilibrium
Energy changes in reactions, what affects the rate of a reaction, reversible reactions and equilibrium, and redox.
📌 Key points
- An exothermic reaction transfers thermal energy to the surroundings (temperature rises, ΔH negative). An endothermic reaction takes in thermal energy (temperature falls, ΔH positive).
- The activation energy (Ea) is the minimum energy that colliding particles need in order to react.
- Breaking bonds is endothermic and making bonds is exothermic. ΔH = energy needed to break bonds − energy released when bonds form.
- The rate of reaction increases with higher concentration (or gas pressure), higher temperature, larger surface area (smaller pieces) and a catalyst.
- Collision theory: higher concentration or surface area gives more frequent collisions; higher temperature gives particles more kinetic energy, so collisions are more frequent and more of them have energy at least equal to Ea.
- A catalyst increases the rate by providing an alternative pathway with a lower activation energy, and is unchanged at the end. Enzymes are biological catalysts.
- Rates can be followed by measuring gas volume with a gas syringe, or loss of mass, against time. The gradient of the graph is steepest at the start, when concentration is highest.
- A reversible reaction (⇌) can go both ways, e.g. heating blue hydrated copper(II) sulfate gives white anhydrous copper(II) sulfate; adding water reverses it. In a closed system it reaches dynamic equilibrium: forward and reverse rates are equal and concentrations stay constant.
- Raising the pressure moves the equilibrium to the side with fewer gas molecules; raising the temperature moves it in the endothermic direction. Haber process: N2 + 3H2 ⇌ 2NH3 at 450 °C, 20 000 kPa, iron catalyst. Contact process: 2SO2 + O2 ⇌ 2SO3 at 450 °C, 200 kPa, vanadium(V) oxide catalyst.
- Oxidation is gain of oxygen, loss of electrons or an increase in oxidation number; reduction is the opposite. An oxidising agent is reduced; a reducing agent is oxidised.
📖 Key terms
- Exothermic
- A reaction that transfers thermal energy to the surroundings.
- Endothermic
- A reaction that takes in thermal energy from the surroundings.
- Activation energy
- The minimum energy that colliding particles must have to react.
- Catalyst
- A substance that increases the rate of a reaction and is unchanged at the end.
- Dynamic equilibrium
- Rates of forward and reverse reactions are equal and concentrations do not change.
- Oxidising agent
- A substance that oxidises another substance and is itself reduced.
- Reducing agent
- A substance that reduces another substance and is itself oxidised.
✏️ Worked example
H2 + Cl2 → 2HCl. Bond energies (kJ/mol): H–H 436, Cl–Cl 242, H–Cl 431. Calculate ΔH and say whether the reaction is exothermic or endothermic.
- Energy to break bonds: 436 + 242 = 678 kJ.
- Energy released making bonds: 2 × 431 = 862 kJ.
- ΔH = 678 − 862 = −184 kJ/mol.
- ΔH is negative, so the reaction is exothermic: ΔH = −184 kJ/mol.
When explaining rates, write "more frequent collisions" (more collisions per second), not just "more collisions". For temperature, also mention activation energy.
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