Kinetics and chemical equilibrium
Kinetics and chemical equilibrium studies how fast reactions proceed and where they end up. The exam tests collision theory, rate measurements, equilibrium constants and Le Chatelier's principle, including industrial examples such as the Haber process.
Core rules
- Rate is the change in concentration of a reactant or product per unit time. It depends on concentration, temperature, surface area and catalysts.
- Collision theory: particles must collide with energy at least equal to the activation energy Ea and with correct orientation. A Maxwell-Boltzmann distribution shows that raising temperature increases the fraction of molecules with E ≥ Ea.
- Catalysts provide an alternative pathway with lower Ea; they increase rate but do not change the position of equilibrium or ΔH.
- At equilibrium, forward and reverse rates are equal and concentrations remain constant. The equilibrium constant Kc = [products]^coefficients/[reactants]^coefficients for homogeneous reactions.
- Le Chatelier's principle: a system at equilibrium responds to oppose a change. Increasing temperature favours the endothermic direction and changes K; changing pressure affects only gaseous species; a catalyst changes neither K nor yield.
Worked example
For H2(g) + I2(g) ⇌ 2HI(g), 1.00 mol of each reactant is placed in a 1.00 dm3 vessel. At equilibrium, 1.50 mol of HI is present. Find Kc. Change in HI = +1.50 mol, so H2 and I2 each change by −0.75 mol. Equilibrium amounts: H2 = 0.25 mol, I2 = 0.25 mol, HI = 1.50 mol; in 1.00 dm3 these are the concentrations. Kc = (1.50)2/[(0.25)(0.25)] = 2.25/0.0625 = 36.0. Kc has no units here because Δn(gas) = 0.
Traps that cost marks
- Confusing rate with equilibrium position: a catalyst speeds up both directions equally.
- Thinking that changing concentration changes Kc; only temperature changes K.
- Forgetting to convert amounts to concentrations before calculating Kc.
- Applying pressure changes to solids or liquids, or to reactions with equal gas moles on both sides.
- Quantifying Le Chatelier's shift without checking whether K actually changes.