OxfordAQA International A Level Chemistry 3.1.6 Kinetics Exam Questions

About OxfordAQA International A Level Chemistry 3.1.6 Kinetics

Kinetics is a physical chemistry topic examined in Unit 2. You need to explain reaction rates using collision theory and activation energy, draw and interpret Maxwell-Boltzmann distribution curves, and explain the effects of temperature, concentration, pressure and catalysts. Examiners expect precise language: a reaction happens only when particles collide with energy greater than or equal to the activation energy. The topic leads on to equilibria in 3.1.7 and to rate equations and the Arrhenius equation in 3.1.11 at A2. Common questions ask you to sketch a curve at a higher temperature, or to explain why a small temperature rise has a large effect on rate.

What you need to know

• Collision theory Particles must collide with energy equal to or greater than the activation energy and in the correct orientation. Rate depends on the frequency of successful collisions.
• Activation energy Activation energy is the minimum energy particles need to collide successfully and react.
• Maxwell-Boltzmann distribution The curve shows the number of molecules with each energy. It starts at the origin, has a peak at the most probable energy, and does not touch the energy axis at high energy. The area under the curve equals the total number of molecules.
• Temperature At a higher temperature the peak moves to the right and is lower, while the area stays the same. Many more molecules have energy at or above the activation energy, so a small temperature rise causes a large increase in rate.
• Concentration and pressure Higher concentration of a solution, or higher pressure of a gas, puts more particles in the same volume. This increases the frequency of collisions, so more successful collisions happen per unit time.
• Catalysts A catalyst provides an alternative pathway with a lower activation energy and is not used up. On a Maxwell-Boltzmann curve, more molecules have energy above the lower activation energy, but the curve itself does not change.

Common mistakes in 3.1.6 Kinetics

• Drawing the higher temperature curve with a higher peak or touching the axis. The peak is lower and moves right, and the curve never meets the energy axis.
• Saying higher concentration gives more collisions. Say more frequent collisions, or more collisions per unit time.
• Saying a catalyst gives particles more energy. It lowers the activation energy by providing an alternative route.
• Explaining the temperature effect only with collision frequency. The main reason is that many more molecules have energy above the activation energy.

Try three 3.1.6 Kinetics exam-style questions