Collision theory, concentration and pressureAQA A-Level Chemistry: Subtopic test

1. 1
   A student investigates the reaction between marble chips (calcium carbonate, in excess) and 50.0 cm³ of hydrochloric acid, CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g), by measuring the volume of carbon dioxide produced using a gas syringe. She repeats the experiment using 2.0 mol dm⁻³ acid in place of 1.0 mol dm⁻³ acid. All other conditions, including the temperature and the mass and size of the marble chips, are kept the same.

   (a)

   Which statement defines activation energy?

   [1 mark]

   • AThe energy released when a reaction takes place
   • BThe average kinetic energy of all the particles in the mixture
   • CThe energy needed to break all of the bonds in the reactants
   • DThe minimum energy that colliding particles must have for a reaction to occur

   (b)

   Why is the reaction faster when 2.0 mol dm⁻³ acid is used?

   [1 mark]

   • AThe acid particles have more kinetic energy
   • BThere are more acid particles per unit volume, so collisions with the marble are more frequent
   • CThe activation energy of the reaction is lower
   • DA greater proportion of the collisions have energy equal to or greater than the activation energy

   (c)

   Explain why most collisions between particles do not lead to a reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Nitrogen monoxide reacts with oxygen in the gas phase at constant temperature: 2NO(g) + O₂(g) → 2NO₂(g). A mixture of the gases is held in a sealed cylinder fitted with a movable piston.

   (a)

   The piston is pushed in until the volume of the gas mixture is halved, with the temperature kept constant. What happens to the concentration of nitrogen monoxide?

   [1 mark]

   • AIt halves
   • BIt stays the same
   • CIt doubles
   • DIt quadruples

   (b)

   Which change would NOT alter the rate of the reaction at constant temperature?

   [1 mark]

   • AAdding argon gas to the cylinder while the piston is held in a fixed position
   • BMoving the piston to halve the volume of the cylinder
   • CAdding more nitrogen monoxide while the volume is constant
   • DDoubling the amount of oxygen in the same volume

   (c)

   Explain, in terms of particles, why halving the volume of the cylinder increases the rate of reaction.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student investigates how concentration affects the rate of the reaction between sodium thiosulfate and hydrochloric acid: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l). In each experiment 5.0 cm³ of 1.0 mol dm⁻³ hydrochloric acid is added to the thiosulfate solution at 20 °C, and the time taken for a cross viewed through the mixture to disappear is measured. Experiment 1 uses 50.0 cm³ of 0.20 mol dm⁻³ sodium thiosulfate solution and takes 36 s. Experiment 2 uses 25.0 cm³ of 0.20 mol dm⁻³ sodium thiosulfate solution mixed with 25.0 cm³ of water, and takes 72 s.

   (a)

   Explain, in terms of collisions, why the cross disappears more slowly in experiment 2.

   [3 marks]

   (b)

   Explain why 1/t can be used as a measure of the rate. Calculate 1/t for each experiment and state what the results show about the effect of concentration.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemical company runs an irreversible reaction between two gases in a sealed reactor at a constant temperature of 600 K. The company can operate the reactor at a pressure of 100 kPa or 400 kPa. Operating at 400 kPa needs compressors and a reactor with thicker walls, and increases the risk of leaks.

   (a)

   Explain, in terms of collisions between particles, why increasing the pressure from 100 kPa to 400 kPa increases the rate of reaction, and why only some collisions lead to a reaction.

   [6 marks]

   (b)

   Evaluate whether the company should operate the reactor at 400 kPa rather than 100 kPa.

   [6 marks]

   Total for question 4: 12 marks

End of questions