WJEC GCSE Chemistry 1.5 Rate of Chemical Change Exam Questions

About WJEC GCSE Chemistry 1.5 Rate of Chemical Change

This Unit 1 topic explains how fast reactions happen and how to control them. You need to measure rate by following gas volume, loss of mass or the time for a precipitate to obscure a cross, and interpret rate graphs. You must explain how temperature, concentration (or pressure), surface area and catalysts affect rate using collision theory and activation energy. Enzymes are covered as biological catalysts. There are specified practicals using gas collection and the reaction of sodium thiosulfate with acid. Examiners commonly ask you to calculate a mean rate from a graph and to explain an effect in terms of the frequency of successful collisions.

What you need to know

• Measuring rate Rate can be followed by collecting gas in a gas syringe, measuring loss in mass on a balance, or timing how long a precipitate takes to hide a cross. Mean rate = amount of product formed (or reactant used) / time.
• Rate graphs The steepest part of the curve shows the fastest rate, at the start. The line levels off when a reactant is used up. A faster reaction gives a steeper initial gradient; the final amount depends on the quantity of limiting reactant.
• Collision theory Particles must collide with at least the activation energy to react. Higher concentration or gas pressure, and smaller particle size (larger surface area), increase the frequency of collisions.
• Temperature At higher temperature particles move faster, so they collide more often and a greater proportion of collisions have energy equal to or above the activation energy.
• Catalysts and enzymes A catalyst speeds up a reaction without being used up, by providing a route with lower activation energy. Enzymes are biological catalysts that work best at an optimum temperature and pH.
• Specified practicals Investigate rate by collecting gas (e.g. marble chips and hydrochloric acid, or magnesium and acid) and by the sodium thiosulfate and hydrochloric acid reaction, timing the disappearance of a cross at different temperatures or concentrations.

Common mistakes in 1.5 Rate of Chemical Change

• Saying particles collide more at higher temperature so the reaction is faster, and stopping there. Also state that more collisions have energy greater than the activation energy.
• Writing that a catalyst is not involved in the reaction. It takes part but is chemically unchanged at the end, and it lowers the activation energy.
• Stating that larger lumps have a larger surface area. Smaller pieces or powder have a larger surface area to volume ratio, giving more frequent collisions.
• Saying higher concentration makes more product. With the same amount of limiting reactant, the final amount of product is the same; only the rate changes.

Try three 1.5 Rate of Chemical Change exam-style questions