Edexcel GCSE Chemistry Topic 7: Rates of Reaction and Energy Changes Exam Questions

About Edexcel GCSE Chemistry Topic 7: Rates of Reaction and Energy Changes

Topic 7 of Edexcel GCSE Chemistry has two parts. The first is rates of reaction: how to measure them, how temperature, concentration, pressure, surface area and catalysts change them, and how collision theory explains these effects. A core practical investigates the effect of changing conditions on rate, either by collecting gas or by observing a colour change or cloudiness. The second part covers heat energy changes: exothermic and endothermic reactions, temperature change experiments, reaction profiles and, at Higher tier, bond energy calculations. Topic 7 is assessed on Paper 2. Interpreting rate graphs and calculating energy changes from bond energies are common question types.

What you need to know

• Measuring rate Rate can be followed by measuring the volume of gas produced (gas syringe or upturned measuring cylinder), the loss of mass as gas escapes, or the time for a precipitate to obscure a mark. Mean rate = amount of product formed / time. Gradients of rate graphs show rate; a steeper slope means a faster reaction.
• Collision theory Particles must collide with enough energy (the activation energy) to react. Increasing concentration or pressure increases the frequency of collisions; increasing surface area exposes more particles; increasing temperature makes particles move faster, so collisions are more frequent and more energetic.
• Catalysts and enzymes Catalysts speed up reactions by providing an alternative pathway with lower activation energy and are not used up. Enzymes are biological catalysts, used for example in making alcoholic drinks by fermentation.
• Core practical Investigate the effects of changing conditions on the rate of reaction, for example marble chips with hydrochloric acid (volume of gas) or sodium thiosulfate with hydrochloric acid (time for a cross to disappear).
• Heat energy changes Exothermic reactions release heat energy, raising the temperature of the surroundings; endothermic reactions take in heat energy. Temperature changes can be measured for neutralisation, dissolving salts in water, displacement and precipitation reactions.
• Reaction profiles and bond energies Reaction profiles show the energy of reactants and products, the activation energy and the overall energy change. Breaking bonds is endothermic and making bonds is exothermic. Higher tier only: energy change = energy to break bonds minus energy released forming bonds.

Common mistakes in Topic 7: Rates of Reaction and Energy Changes

• Saying higher temperature increases the number of collisions without mentioning energy. More particles also have at least the activation energy.
• Saying a catalyst lowers the activation energy of the same pathway. It provides an alternative pathway with lower activation energy.
• Reading the rate as the height of a curve. Rate is the gradient (steepness), not the amount of product.
• Doing bond energy sums the wrong way round. Use bonds broken minus bonds made, so an exothermic reaction gives a negative value.
• Thinking endothermic reactions feel hot. The surroundings cool down because energy is taken in.

Try three Topic 7: Rates of Reaction and Energy Changes exam-style questions