AQA GCSE Chemistry 4.6.1 Rate of Reaction Exam Questions

About AQA GCSE Chemistry 4.6.1 Rate of Reaction

AQA GCSE Chemistry 4.6.1 explains how quickly reactions happen and how to change their speed. You calculate mean rates from experimental data, interpret graphs, and use collision theory to explain the effects of concentration, pressure, surface area, temperature and catalysts. The required practical investigates how concentration affects rate by measuring gas volume or the time for a solution to become cloudy. This topic opens Paper 2 and links to reaction profiles in 4.5 and equilibrium in 4.6.2. Higher tier students also draw tangents to find the rate at a specific time and use units such as mol/s.

What you need to know

• Measuring rate Mean rate = quantity of reactant used / time, or quantity of product formed / time. Units include g/s or cm3/s. Higher tier only: rate in mol/s, and finding the rate at a given time by drawing a tangent to the curve and calculating its gradient.
• Factors affecting rate Increasing concentration of solutions, pressure of gases, surface area of solids (smaller pieces) or temperature increases the rate. Catalysts also increase rate.
• Collision theory Reactions occur only when particles collide with sufficient energy, at least the activation energy. Increasing concentration, pressure or surface area increases the frequency of collisions; increasing temperature increases both the frequency of collisions and the energy of particles, so more collisions are successful.
• Catalysts A catalyst changes the rate without being used up. It provides a different pathway with a lower activation energy. Catalysts are not included in the equation, and enzymes are biological catalysts.
• Required practical Investigate how changing concentration affects rate by measuring the volume of gas produced (for example magnesium and hydrochloric acid) or by a change in colour or turbidity (sodium thiosulfate and hydrochloric acid, timing until a cross disappears).

Common mistakes in 4.6.1 Rate of Reaction

• Saying particles collide 'more' when temperature rises without explaining energy. You must also say more particles have energy equal to or greater than the activation energy, so a greater proportion of collisions are successful.
• Saying catalysts give particles more energy. They provide an alternative pathway with lower activation energy.
• Writing 'more collisions' instead of 'more frequent collisions' or 'more collisions per second'. Examiners want frequency.
• Saying larger pieces have a larger surface area. Smaller pieces (powders) have a larger surface area to volume ratio.
• Drawing a tangent that crosses the curve. A tangent should touch the curve at one point only.

Try three 4.6.1 Rate of Reaction exam-style questions