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LearnEdexcel4CH12017 specificationPaper 2C · Paper 2 (all content)Physical chemistry

Paper 2C · Module

Physical chemistry

3 subtopics≈ 135 min in total

Start the module: Rate of reaction Open graphing calculator

Rate of reaction
Particles react only when they collide with at least the activation energy. Rate increases with concentration (or gas pressure) and surface area, because collisions become more frequent; with temperature, because particles collide more often and more collisions have enough energy; and with a catalyst, which provides a route with lower activation energy. Rate is the gradient of a product-against-time graph.
≈ 45 min
Exothermic and endothermic reactions
Exothermic reactions release heat to the surroundings (temperature rises; $\Delta H$ negative), e.g. combustion, neutralisation. Endothermic reactions take in heat (temperature falls; $\Delta H$ positive), e.g. thermal decomposition, photosynthesis. Breaking bonds takes in energy; making bonds releases it. $\Delta H=\sum(\text{bonds broken})-\sum(\text{bonds made})$ . Energy released into water: $Q=mc\Delta T$ .
≈ 45 min
Reversible reactions and equilibrium
A reversible reaction can go both ways ( $\rightleftharpoons$ ). In a closed system it reaches dynamic equilibrium: forward and backward rates are equal, so concentrations stay constant. Changing conditions shifts the equilibrium to oppose the change: higher pressure favours fewer gas moles; higher temperature favours the endothermic direction. The Haber process ( $\mathrm{N_2+3H_2\rightleftharpoons2NH_3}$ ) uses about $450\,^\circ\mathrm{C}$ , $200$ atm and an iron catalyst as a compromise between yield and rate.
≈ 45 min