Collision theory and rates of reactionEdexcel A-Level Chemistry: Subtopic test

1. 1
   A technician reacts marble chips (calcium carbonate) with dilute hydrochloric acid and wants to understand how the rate of the reaction can be controlled.

   (a)

   The technician doubles the concentration of the acid. Which statement best explains why the rate increases?

   [1 mark]

   • AThe acid particles have more kinetic energy, so a greater proportion collide with enough energy
   • BThe activation energy of the reaction is lowered
   • CThe acid particles move faster, so they collide more often
   • DThere are more acid particles per unit volume, so collisions are more frequent and the frequency of successful collisions increases

   (b)

   The technician replaces the marble chips with the same mass of marble powder. Which statement explains the faster rate?

   [1 mark]

   • AThe powder particles have more kinetic energy, so more collisions exceed the activation energy
   • BThe powder has a larger surface area, so more calcium carbonate is exposed and collisions with acid particles are more frequent
   • CCrushing the marble lowers the activation energy of the reaction
   • DThe concentration of the acid is increased by the powder

   (c)

   Define the term activation energy and explain why most collisions between particles do not lead to a reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A class investigates how temperature affects the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which produces a pale yellow precipitate of sulfur. They time how long it takes for the cloudiness to hide a cross marked on paper under the flask.

   (a)

   The experiment is repeated with the solutions warmed by 10 K. Which statement best explains why the rate increases?

   [1 mark]

   • AA greater proportion of collisions have energy equal to or greater than the activation energy
   • BThe activation energy decreases as temperature rises
   • CCollisions become more frequent only, and the proportion with enough energy is unchanged
   • DThe enthalpy change of the reaction becomes more negative

   (b)

   The activation energy of the reaction at 30 °C compared with 20 °C is

   [1 mark]

   • Agreater, because the particles have more energy to overcome
   • Bsmaller, because the collisions are more energetic
   • Cunchanged, because the activation energy is a property of the reaction pathway
   • Dzero, because the reaction is spontaneous at both temperatures

   (c)

   The cross disappears after 80 s at 20 °C and after 41 s at 30 °C. Calculate the rate of reaction, as 1/time, at each temperature and the factor by which the rate increases.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student reacts an excess of marble chips with 50 cm³ of dilute hydrochloric acid and measures the volume of carbon dioxide collected in a gas syringe at regular intervals, then plots volume against time.

   (a)

   A tangent drawn to the curve at time zero passes through the points (0 s, 0 cm³) and (30 s, 33 cm³). Describe how the initial rate is found from the graph and calculate its value.

   [3 marks]

   (b)

   A tangent drawn at 60 s passes through the points (30 s, 52 cm³) and (90 s, 70 cm³). Calculate the rate at 60 s and explain, using collision theory, why it is lower than the initial rate.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Chemists and engineers use collision theory to explain and control how quickly reactions happen, from industrial plants to laboratory experiments.

   (a)

   Finely powdered flour suspended in the air of a mill can explode, but a sack of flour kept in the same building is hard to burn. Explain this difference using collision theory, and explain why the reaction, once started by a spark, becomes faster and faster.

   [6 marks]

   (b)

   Pieces of magnesium ribbon of equal mass react with an excess of hydrochloric acid. Experiment 1 uses 0.50 mol dm⁻³ acid at 20 °C and the ribbon disappears in 120 s. Experiment 2 uses 1.00 mol dm⁻³ acid at 20 °C and takes 62 s. Experiment 3 uses 0.50 mol dm⁻³ acid at 40 °C and takes 29 s. Calculate the relative rate (1/time) in each experiment and evaluate, using collision theory, the effect of concentration and of temperature on the rate.

   [6 marks]

   Total for question 4: 12 marks

End of questions