AQA A Level Chemistry 3.1.5 Kinetics Exam Questions

About AQA A Level Chemistry 3.1.5 Kinetics

Kinetics explains why reactions happen at the rates they do. On the AQA A Level course you need collision theory, the meaning of activation energy, the Maxwell-Boltzmann distribution of molecular energies and how it changes with temperature, and how concentration, pressure and catalysts affect rate. Required practical 3 investigates how rate changes with temperature. This topic leads into 3.1.9 Rate Equations, which makes the treatment quantitative, and links to equilibria and industrial compromise conditions. Examiners regularly ask you to sketch and label distribution curves, and to explain rate changes in terms of the proportion of particles with energy at least equal to the activation energy.

What you need to know

• Collision theory Particles must collide with energy greater than or equal to the activation energy, Ea, and usually with the correct orientation. Rate depends on the frequency of successful collisions.
• Maxwell-Boltzmann distribution A curve of number of molecules against energy. It starts at the origin, has a peak at the most probable energy, is not symmetrical, and never touches the energy axis at high energy. The area under the curve is the total number of molecules.
• Effect of temperature At a higher temperature the peak moves right and is lower, and the area stays the same. A much larger proportion of molecules have energy at least Ea, so a small rise in temperature causes a large increase in rate.
• Concentration and pressure Increasing concentration of a solution or pressure of a gas puts more particles in the same volume, so collisions are more frequent and there are more successful collisions per unit time.
• Catalysts A catalyst increases rate without being used up, by providing an alternative reaction route with a lower activation energy. On the distribution, more molecules lie to the right of the new, lower Ea.
• Required practical 3 A common method is sodium thiosulfate with hydrochloric acid, timing how long a cross takes to disappear at different temperatures. Rate is proportional to 1/time.

Common mistakes in 3.1.5 Kinetics

• Explaining the temperature effect mainly by more frequent collisions. The key reason is that a greater proportion of molecules have energy equal to or above Ea.
• Drawing distribution curves that do not start at the origin, that cross or touch the energy axis, or that change in area when temperature changes.
• Saying a catalyst gives particles more energy. A catalyst lowers the activation energy by providing an alternative route.
• Writing 'more collisions' instead of 'more successful collisions per unit time' or 'more frequent collisions'. Rate is about frequency.
• Moving the peak to the left or making it taller at higher temperature. It moves right and gets lower.

Try three 3.1.5 Kinetics exam-style questions