Edexcel International A Level Chemistry Kinetics Exam Questions

About Edexcel International A Level Chemistry Kinetics

Kinetics appears twice in Edexcel International A Level Chemistry. In Unit 2 it introduces collision theory, activation energy and the Maxwell-Boltzmann distribution to explain the effects of concentration, pressure, temperature, surface area and catalysts on rate. In Unit 4 it becomes quantitative: you determine orders of reaction from experimental data, write rate equations, find the rate constant and its units, use half-lives, link rate equations to mechanisms through the rate-determining step, and use the Arrhenius equation to find activation energy from a graph. Examiners like Maxwell-Boltzmann sketches, initial rates tables, concentration time graphs and the SN1 versus SN2 link to rate equations.

What you need to know

• Collision theory Reactions occur when particles collide with energy at least equal to the activation energy and in the correct orientation. Increasing concentration, pressure or surface area increases the frequency of collisions.
• Maxwell-Boltzmann distribution The curve starts at the origin and never meets the energy axis. At a higher temperature the peak moves right and is lower, and many more molecules have energy greater than or equal to Ea, which is the main reason rate increases.
• Catalysts A catalyst provides an alternative route with a lower activation energy, so more particles have enough energy to react. Heterogeneous catalysts are in a different phase and homogeneous catalysts in the same phase as reactants.
• Measuring rates Methods include collecting gas, measuring mass loss, colorimetry, titrating quenched samples and timing the appearance of a precipitate (clock reactions). Initial rate comes from the gradient of a tangent at t = 0.
• Orders and rate equations Rate = k[A]^m[B]^n. Zero order: rate does not change with concentration. First order: rate doubles when concentration doubles and half-life is constant. Second order: rate quadruples when concentration doubles.
• Rate-determining step The slowest step in a mechanism. Species in the rate equation appear in or before this step, so SN2 (rate = k[RX][OH-]) and SN1 (rate = k[RX]) can be distinguished.
• Arrhenius equation k = Ae^(-Ea/RT), so ln k = -Ea/RT + ln A. A graph of ln k against 1/T has gradient -Ea/R, giving Ea in J mol-1.

Common mistakes in Kinetics

• Drawing a Maxwell-Boltzmann curve that touches the energy axis or starts above zero. It starts at the origin and approaches the axis without touching it.
• Saying higher temperature increases rate mainly because of more collisions. The main reason is that more particles have energy above the activation energy.
• Getting units of k wrong. Rearrange the rate equation and cancel units, for example second order overall gives dm3 mol-1 s-1.
• Using orders from the balanced equation. Orders can only be found by experiment.
• Forgetting to convert Ea to kJ mol-1 or using temperature in Celsius in the Arrhenius equation. T must be in kelvin.

Try three Kinetics exam-style questions