1. 1
   A student investigates the reaction between magnesium ribbon and dilute hydrochloric acid. She adds a 3 cm strip of magnesium ribbon to 50 cm3 of 1.0 mol/dm3 hydrochloric acid in a conical flask and measures the volume of gas produced every 10 seconds using a gas syringe, until the reaction finishes.

   (a)

   Which gas is produced in this reaction?

   [1 mark]

   • AHydrogen
   • BOxygen
   • CCarbon dioxide
   • DChlorine

   (b)

   The student repeats the experiment using the same mass of magnesium powder instead of ribbon, keeping the acid concentration and volume the same. Compared with the ribbon, what effect does using powder have on the initial rate of reaction?

   [1 mark]

   • AThe reaction stops completely because powder does not react with acid
   • BThe initial rate decreases because the powder has a smaller surface area
   • CThe initial rate stays the same because the mass of magnesium is unchanged
   • DThe initial rate increases because the surface area to volume ratio of the magnesium is greater

   (c)

   Using collision theory, explain why increasing the surface area to volume ratio of the magnesium increases the rate of reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A different student reacts marble chips (calcium carbonate) with dilute hydrochloric acid in a conical flask placed on a balance, recording the total mass of the flask and contents every 15 seconds. The mass decreases steadily as the reaction proceeds.

   (a)

   Why does the total mass decrease during this reaction?

   [1 mark]

   • AThe marble chips dissolve completely and disappear
   • BWater evaporates from the acid due to heating
   • CCarbon dioxide gas escapes from the flask into the air
   • DThe balance loses accuracy as the reaction proceeds

   (b)

   The student repeats the experiment using hydrochloric acid at a higher concentration, keeping the mass and size of marble chips the same. Which row correctly describes the effect on the initial rate of reaction and the total mass of gas eventually lost?

   [1 mark]

   • AInitial rate increases; total mass of gas lost increases
   • BInitial rate increases; total mass of gas lost stays the same
   • CInitial rate stays the same; total mass of gas lost increases
   • DInitial rate decreases; total mass of gas lost stays the same

   (c)

   Using collision theory, explain why increasing the concentration of the hydrochloric acid increases the initial rate of reaction with the marble chips.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A technician investigates the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which produces a fine, pale yellow precipitate of sulfur that gradually clouds the solution. She places the flask on top of a piece of paper marked with a black cross and times how long it takes for the cross to become invisible when viewed from above, at four different reaction temperatures. Her results are: at 20C the cross disappears after 240 seconds; at 30C after 120 seconds; at 40C after 60 seconds; at 50C after 30 seconds.

   (a)

   Describe the pattern shown by these results and suggest what this shows about the effect of temperature on the rate of this reaction.

   [3 marks]

   (b)

   Using collision theory, explain why increasing the temperature increases the rate of this reaction between sodium thiosulfate and hydrochloric acid.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A brewery uses an enzyme, a biological catalyst, to speed up the breakdown of starch into sugars during the production of an alcoholic drink, before the sugars are fermented. A quality control chemist compares two batches: Batch 1 uses no enzyme and takes 180 minutes for the starch to fully break down at 35C. Batch 2 uses the enzyme and takes only 20 minutes to fully break down the starch at the same temperature, with the same starting mass of starch and same volume of water.

   (a)

   Explain, in terms of activation energy and particle collisions, how the enzyme increases the rate of starch breakdown, and why a catalyst such as this enzyme is not itself used up or changed by the end of the reaction.

   [6 marks]

   (b)

   The chemist wants to further increase the rate of starch breakdown in Batch 2 without changing the enzyme used. She proposes two changes to test separately: (i) raising the reaction temperature from 35C to 60C, and (ii) increasing the concentration of the enzyme solution used. Evaluate each proposed change, explaining whether it would be expected to increase the rate of reaction and identifying any limitation of using that change with an enzyme-catalysed reaction.

   [6 marks]

   Total for question 4: 12 marks

End of questions