1. 1
   A student places a flask of sodium thiosulfate solution on a piece of paper marked with a black cross, adds dilute hydrochloric acid, starts a stopwatch, and records the time taken until the cross can no longer be seen through the cloudy mixture, repeating this at several different concentrations of acid.

   (a)

   A student investigates the effect of acid concentration on the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid, timing how long it takes for a cross drawn on paper beneath the flask to disappear from view. What is being measured in this method?

   [1 mark]

   • AThe time taken for the solution to become cloudy enough to block the cross from view
   • BThe final mass of the flask and contents
   • CThe volume of gas produced
   • DThe colour of the sodium thiosulfate solution

   (b)

   To make this a fair test, which variable must the student keep constant between repeats?

   [1 mark]

   • AThe concentration of the acid, since this is being investigated
   • BThe volume of sodium thiosulfate solution used
   • CThe time taken for the cross to disappear
   • DThe cloudiness of the final mixture

   (c)

   The student finds that the cross disappears more quickly when a more concentrated acid is used. Explain how the student could use the times recorded to calculate a value proportional to the rate of reaction for each concentration.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A group of students investigates the effect of temperature on the rate of the reaction between magnesium ribbon and dilute hydrochloric acid, heating separate samples of the acid to a range of set temperatures in a water bath, then adding an equal length of magnesium ribbon to each and measuring the volume of gas collected in a gas syringe every 10 seconds.

   (a)

   A different group investigates the effect of temperature on the rate of reaction between magnesium ribbon and dilute hydrochloric acid, using a gas syringe to collect hydrogen gas. What apparatus should they use to control the temperature of the acid before each run?

   [1 mark]

   • AA stopwatch
   • BA gas syringe
   • CA measuring cylinder
   • DA water bath

   (b)

   Which variable should the students keep the same for every repeat of this investigation to make it a fair test?

   [1 mark]

   • AThe volume of gas collected
   • BThe temperature of the acid
   • CThe length (mass) of magnesium ribbon used
   • DThe colour of the magnesium

   (c)

   Describe how the students could use their results to compare the rate of reaction at each temperature tested.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student is planning the required practical to investigate the factors that affect the rate of a reaction, choosing to focus on how the surface area of a solid reactant affects the rate of reaction between calcium carbonate and dilute hydrochloric acid.

   (a)

   Describe how a student could carry out an investigation into the effect of surface area on the rate of reaction between calcium carbonate and dilute hydrochloric acid, using marble chips of different sizes but the same total mass.

   [3 marks]

   (b)

   Explain why it is important to use an excess of hydrochloric acid in this investigation, and describe one precaution the student should take to ensure the results are reliable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A school science department is preparing a set of instructions for the required practical on investigating factors that affect the rate of reaction, to be used by students across two separate experiments: one using the disappearing cross method with sodium thiosulfate and acid, and one measuring gas volume for magnesium and acid at different temperatures.

   (a)

   Design a full method for the required practical investigating how the concentration of hydrochloric acid affects the rate of its reaction with sodium thiosulfate solution, using the disappearing cross method. Your answer should include the apparatus needed, the method used to control variables, and how the results would be used to reach a conclusion.

   [6 marks]

   (b)

   Explain why measuring the volume of gas produced using a gas syringe, for the reaction of magnesium with hydrochloric acid at different temperatures, might be considered a more objective method than the disappearing cross method used for sodium thiosulfate and acid, and discuss one limitation shared by both methods.

   [6 marks]

   Total for question 4: 12 marks

End of questions