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

1. 1
   A student investigates the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which forms a fine yellow precipitate of sulfur. She places the reaction flask on a printed cross and measures the time taken for the cross to disappear when viewed from above.

   (a)

   The student doubles the concentration of hydrochloric acid and the cross disappears sooner. Which statement best explains this?

   [1 mark]

   • AA greater proportion of the particles have energy equal to or greater than the activation energy
   • BThe activation energy of the reaction is lower
   • CThere are more particles in a given volume, so collisions are more frequent
   • DThe particles move faster, so each collision is more energetic

   (b)

   The cross disappears after 40 s. What is the relative rate of the reaction, calculated as 1 ÷ time?

   [1 mark]

   • A0.025 s⁻¹
   • B0.25 s⁻¹
   • C25 s⁻¹
   • D40 s⁻¹

   (c)

   At a higher temperature the cross disappears after 22 s. Calculate the relative rate (1 ÷ time) at each temperature and the factor by which the rate has increased.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Marble chips (calcium carbonate) are added to excess dilute hydrochloric acid in a conical flask on a balance. Carbon dioxide escapes, so the mass of the flask and contents falls as the reaction proceeds.

   (a)

   The same mass of marble is used as powder instead of chips and the reaction is faster. Why?

   [1 mark]

   • AThe activation energy is lower for a powder
   • BThe acid particles move faster
   • CA greater proportion of acid particles have energy above the activation energy
   • DMore calcium carbonate is exposed at the surface, so collisions with acid particles are more frequent

   (b)

   Which statement defines the activation energy?

   [1 mark]

   • AThe energy released when products form
   • BThe minimum energy that colliding particles must have for a reaction to occur
   • CThe mean kinetic energy of the particles in the mixture
   • DThe difference in enthalpy between reactants and products

   (c)

   The reaction is repeated at a higher temperature. Explain, using collision theory, why the rate increases.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student reacts excess magnesium ribbon with 50 cm³ of dilute sulfuric acid and collects the hydrogen in a gas syringe. She plots the volume of gas against time and draws tangents to the curve.

   (a)

   Explain, using collision theory, why the gradient of the graph decreases as the reaction proceeds.

   [3 marks]

   (b)

   A tangent drawn at 60 s passes through the points (20 s, 8 cm³) and (100 s, 56 cm³). A tangent drawn at the start passes through (0 s, 0 cm³) and (30 s, 45 cm³). Calculate the rate of reaction at 60 s and the initial rate, and state how many times faster the initial rate is.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A company makes calcium chloride solution by reacting limestone (calcium carbonate) with excess hydrochloric acid in a large tank. Engineers can vary the acid concentration, the temperature and the particle size of the limestone to change how quickly carbon dioxide is released.

   (a)

   Explain, using collision theory, how an increase in acid concentration, an increase in temperature and a decrease in the particle size of the limestone would each increase the rate of reaction.

   [6 marks]

   (b)

   In three trials 10.0 g of limestone was reacted with excess acid and the time taken to collect 100 cm³ of carbon dioxide was recorded. Trial 1: lumps at 20 °C, 250 s. Trial 2: powder at 20 °C, 62 s. Trial 3: lumps at 40 °C, 105 s. Calculate the mean rate for each trial, compare the effect of crushing with the effect of heating, and evaluate which change the company should choose.

   [6 marks]

   Total for question 4: 12 marks

End of questions