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AS · Practice questions · Rate of reaction

Measure the speed.

Six original Cambridge-style questions on rate of reaction: definitions, methods, gradients and tangents, and collision theory.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Rate is a gradient.

01
Recall
[2 marks]

State the two conditions needed for a collision between particles to result in a reaction.

  • The particles must collide with energy equal to or greater than the activation energy. ✓
  • They must collide with the correct orientation. ✓
02
Explain
[2 marks]

Explain, using collision theory, why increasing the pressure increases the rate of a reaction between gases.

  • The same number of molecules occupy a smaller volume, so the concentration is higher. ✓
  • Collisions are more frequent, so there are more successful collisions per unit time. ✓
03
Recall
[2 marks]

Suggest a method of following the rate of each reaction: (a) Zn + H2SO4 → ZnSO4 + H2; (b) Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2O.

  • (a) Collect the hydrogen in a gas syringe and record the volume at regular times. ✓
  • (b) Time how long it takes for the precipitate of sulfur to hide a cross under the flask. ✓
04
Calculation
[2 marks]

In an experiment 0.150 g of magnesium is lost from a 1.00 g ribbon in 30.0 s. Calculate the mean rate of reaction of magnesium in mol s−1. (Ar: Mg 24.3)

  • n(Mg) = 0.150 ÷ 24.3 = 6.17 × 10−3 mol. ✓
  • Rate = 6.17 × 10−3 ÷ 30.0 = 2.06 × 10−4 mol s−1. ✓
05
Calculation
[2 marks]

A tangent drawn to a volume-time curve at t = 0 passes through (0 s, 0 cm3) and (25 s, 60 cm3). Calculate the initial rate.

  • Gradient = 60 ÷ 25. ✓
  • Initial rate = 2.4 cm3 s−1. ✓
06
Explain
[3 marks]

Explain the shape of the volume-time graph for the reaction of excess marble chips with hydrochloric acid.

  • It is steepest at the start, where the acid concentration is highest and collisions most frequent. ✓
  • The gradient falls as the acid is used up, because collisions become less frequent. ✓
  • It becomes horizontal when all the acid (the limiting reagent) has reacted. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.