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LearnEdexcelYCH112018 specificationWCH12 · Unit 2: Energetics, Group Chemistry, Halogenoalkanes and AlcoholsIntroduction to kinetics and equilibria

WCH12 · Module

Introduction to kinetics and equilibria

2 subtopics≈ 95 min in total

Start the module: Equilibrium and Kc Open graphing calculator

Equilibrium and Kc
At dynamic equilibrium in a closed system, forward and reverse rates are equal and concentrations are constant. Le Chatelier's principle: if conditions change, the equilibrium shifts to oppose the change. For $a\mathrm{A}+b\mathrm{B}\rightleftharpoons c\mathrm{C}+d\mathrm{D}$ , $K_c=\frac{[\mathrm{C}]^c[\mathrm{D}]^d}{[\mathrm{A}]^a[\mathrm{B}]^b}$ (products over reactants, powers = coefficients). Use an ICE table to find equilibrium concentrations. Only temperature changes $K_c$ : for exothermic forward reactions, raising $T$ lowers $K_c$ . Concentration and pressure changes shift the position but not $K_c$ ; catalysts change neither.
≈ 50 min
Rates and the Boltzmann distribution
Rate = change in concentration per unit time (gradient of a concentration–time graph). The Boltzmann distribution shows the spread of molecular energies: it starts at the origin, rises to a peak (most probable energy) and tails off without touching the axis. Only molecules with $E\ge E_a$ react. At higher temperature the curve flattens and shifts right; the area beyond $E_a$ grows a lot, so rate increases significantly. A catalyst gives a pathway with lower $E_a$ , so more molecules exceed it. Homogeneous catalysts are in the same phase as reactants; heterogeneous catalysts are in a different phase (adsorption on surfaces).
≈ 45 min