1. 1
   A limescale-removal company tests two descaling solutions, one dilute and one concentrated hydrochloric acid, on identical marble chips of calcium carbonate, measuring the mass of carbon dioxide gas released over time.

   (a)

   Which solution would produce carbon dioxide gas at the faster initial rate?

   [1 mark]

   • AThe dilute hydrochloric acid, because it has fewer particles
   • BThe concentrated hydrochloric acid, because it has a greater number of acid particles per unit volume
   • CBoth solutions would react at exactly the same rate
   • DNeither solution would react, since marble does not react with acid

   (b)

   Which practical method could the company use to measure the rate of this reaction?

   [1 mark]

   • AMeasuring the decrease in mass of the reaction flask over time as carbon dioxide gas escapes
   • BMeasuring the colour change of the acid over time
   • CMeasuring the melting point of the marble chips before the reaction
   • DMeasuring the pH of the marble chips before adding acid

   (c)

   Sketch data from the company's experiment showed that the mass of the flask decreased quickly at first, then more slowly, before levelling off completely. Explain, in terms of collision theory, why the rate of mass loss slows down as the reaction proceeds.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A bakery compares how quickly granulated sugar and powdered icing sugar of the same mass dissolve and react with an acidic syrup at the same temperature.

   (a)

   Which sample would react fastest, and why?

   [1 mark]

   • AThe granulated sugar, because larger particles have more mass
   • BThe granulated sugar, because it has fewer particles exposed to the syrup
   • CBoth would react at the same rate, since surface area does not affect rate
   • DThe powdered icing sugar, because it has a greater surface area to volume ratio, allowing more frequent collisions

   (b)

   Which statement, based on collision theory, correctly explains why increasing surface area increases the rate of reaction?

   [1 mark]

   • AIncreasing surface area increases the activation energy of the reaction
   • BIncreasing surface area decreases the number of particles available to react
   • CIncreasing surface area exposes more particles at the surface of the solid, increasing the frequency of collisions between reacting particles
   • DIncreasing surface area only affects reactions involving gases, not solids

   (c)

   Suggest one other change the bakery could make to the granulated sugar sample, other than powdering it, to increase its rate of reaction with the syrup, and explain why it would work.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An industrial chemical plant carries out a reaction at 40 °C and records the time taken for a fixed amount of gas to be produced. The plant then repeats the experiment at 60 °C, keeping all other conditions the same, and finds the reaction is completed in much less time.

   (a)

   Explain, using collision theory, why raising the temperature increases the rate of reaction.

   [3 marks]

   (b)

   The plant also tests the effect of increasing the pressure of a gaseous reaction mixture at constant temperature. Explain why increasing the pressure increases the rate of this gaseous reaction, and evaluate one limitation of using time taken for a fixed amount of gas as the only measurement to compare the two temperature experiments.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A car manufacturer fits catalytic converters containing platinum to reduce harmful emissions, while a detergent company adds biological enzymes to washing powder to break down stains at lower washing temperatures.

   (a)

   Explain, using collision theory and the idea of activation energy, how both the platinum catalyst and the enzyme increase the rate of their respective reactions without being used up.

   [6 marks]

   (b)

   Evaluate the practical methods that could be used to investigate the effect of the platinum catalyst on the rate of the exhaust gas reaction, and discuss why the manufacturer must be careful in choosing suitable practical methods for this investigation.

   [6 marks]

   Total for question 4: 12 marks

End of questions