GCSE Chemistry: Rates of Reaction

Rates of reaction is the most practical-flavoured topic in GCSE Chemistry — nearly every question is anchored in an experiment someone could actually run — and it rewards one skill above all: explaining every observation through collision theory. This quiz builds exactly that habit.

The measurement questions use the three standard experimental setups: collecting hydrogen gas from magnesium and acid (60 cm³ in 40 seconds giving a mean rate of 1.5 cm³/s), tracking mass loss as carbon dioxide escapes a flask (4.2 g over two minutes — with the minutes-to-seconds conversion that catches the unwary), and the cloudiness methods for precipitate-forming reactions. A rearrangement question runs the rate formula backwards, finding the time from a volume and a rate.

The factors questions cover concentration, temperature, surface area and catalysts — first as predictions, then as explanations. Collision theory carries the explanatory weight throughout: concentration increasing collision frequency, grinding a solid exposing more surface for collisions, and the crucial two-part temperature explanation (more frequent collisions AND a greater proportion of particles exceeding the activation energy) that distinguishes full-mark answers.

Catalysts get precise treatment: they provide an alternative pathway with lower activation energy, they emerge unchanged, and — the misconception the quiz explicitly targets — they do not increase the yield of product, only the speed of reaching it.

Every calculation carries its units through the working, and every explanation ends at the particle level, mirroring the "explain in terms of particles" phrasing that GCSE mark schemes use for this topic.

• Calculate mean rates from gas volume, mass loss or time data with correct units
• Choose appropriate methods for measuring the rate of a given reaction
• Explain concentration and surface area effects through collision frequency
• Give the two-part collision theory explanation for temperature effects
• State precisely what catalysts do and do not change

Topic scope follows the Department for Education's GCSE combined science subject content, rate and extent of chemical change strand: measuring and calculating rates, factors affecting rate, collision theory and catalysts.