CCEA GCSE Chemistry 2.3 Rates of Reaction Exam Questions

About CCEA GCSE Chemistry 2.3 Rates of Reaction

Rates of Reaction is a Unit 2 topic in CCEA GCSE Chemistry with a prescribed practical investigating how changing one variable affects rate. You need to describe methods of following a reaction, such as measuring gas volume, mass loss or the time for a cross to disappear, and interpret graphs of these results. Collision theory is used to explain the effects of temperature, concentration, surface area and catalysts. The topic links to energy changes and activation energy in 2.8 and to equilibrium and the Haber process in 2.4. Examiners frequently ask for explanations using collision theory, average rate calculations and sketching a second curve on a rate graph.

What you need to know

• Measuring rate Rate = change in amount of reactant or product / time. Collect gas in a gas syringe, measure mass loss on a balance as gas escapes, or time how long it takes for a precipitate to hide a cross (sodium thiosulfate with acid).
• Collision theory Particles must collide with enough energy (at least the activation energy) to react. Rate depends on the frequency of successful collisions.
• Temperature Particles move faster, so they collide more often and more collisions have energy equal to or greater than the activation energy.
• Concentration, pressure and surface area Higher concentration (or gas pressure) means more particles in the same volume, so more frequent collisions. Smaller pieces or powder have a larger surface area, exposing more particles for collisions.
• Catalysts A catalyst speeds up a reaction without being used up by providing an alternative pathway with a lower activation energy. Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide.
• Rate graphs The steeper the gradient, the faster the rate. The curve levels off when a reactant is used up. If the same amounts of reactants are used, the final volume of gas is the same even when the rate changes.

Common mistakes in 2.3 Rates of Reaction

• Saying particles collide more at higher temperature without mentioning that more collisions have enough energy to react.
• Writing more collisions instead of more frequent collisions or more collisions per second.
• Drawing a faster reaction curve that ends higher when the amount of reactant is unchanged. It should level off at the same final value.
• Saying a catalyst gives particles more energy. It lowers the activation energy.
• Describing a larger lump as having a larger surface area. Smaller pieces have a larger total surface area.

Try three 2.3 Rates of Reaction exam-style questions