Edexcel A Level Chemistry Topic 9: Kinetics I Exam Questions

About Edexcel A Level Chemistry Topic 9: Kinetics I

Topic 9 of Edexcel A Level Chemistry, Kinetics I, explains why reactions happen at different rates. You use collision theory to explain the effects of concentration, pressure, surface area and temperature on rate, and the Maxwell-Boltzmann distribution to show why a small temperature rise has a large effect. Catalysts are explained using reaction profiles and the Maxwell-Boltzmann distribution, including their industrial importance. You also need to suggest practical methods for following a reaction, such as measuring gas volume, mass loss or colour change, and to find rate from the gradient of a graph. It is assessed in Paper 2 and extended in Topic 16, Kinetics II.

What you need to know

• Rate of reaction Rate is the change in concentration of a reactant or product per unit time. Find it from the gradient of a tangent to a concentration-time or volume-time graph.
• Collision theory Particles must collide with energy greater than or equal to the activation energy and with the correct orientation. More frequent successful collisions mean a faster rate.
• Maxwell-Boltzmann distribution A plot of number of molecules against energy, starting at the origin and never meeting the energy axis. Area under the curve equals the total number of molecules.
• Effect of temperature At higher temperature the curve flattens and shifts right, with the same area. Many more molecules have energy above Ea, so the rate rises significantly.
• Catalysts A catalyst provides an alternative pathway with lower activation energy and is unchanged chemically at the end. It increases the proportion of molecules with enough energy to react without changing the distribution.
• Measuring rate Methods include collecting gas in a gas syringe, measuring mass loss, colour change with a colorimeter, and timing a precipitate obscuring a cross, as in the sodium thiosulfate and acid reaction.

Common mistakes in Topic 9: Kinetics I

• Drawing the higher temperature Maxwell-Boltzmann curve with a higher peak; the peak is lower and moves to the right.
• Saying a catalyst shifts the Maxwell-Boltzmann curve; the curve stays the same but the Ea line moves to lower energy.
• Explaining a temperature rise only by more frequent collisions; the main factor is more collisions with energy at least Ea.
• Saying a catalyst is not used up because it does not take part in the reaction; it takes part but is regenerated.
• Finding rate from an average over a whole graph when an initial rate tangent is asked for.

Try three Topic 9: Kinetics I exam-style questions