1. 1
   A student carries out the reaction between magnesium ribbon and dilute hydrochloric acid twice: once with the acid at room temperature, and once with the acid heated to 50 degrees Celsius, measuring the time taken for the magnesium to fully disappear each time.

   (a)

   A student repeats a reaction between magnesium and hydrochloric acid, but this time heats the acid to a higher temperature before adding the magnesium. What effect does increasing the temperature have on the rate of reaction?

   [1 mark]

   • AIt increases the rate of reaction
   • BIt decreases the rate of reaction
   • CIt has no effect on the rate of reaction
   • DIt stops the reaction completely

   (b)

   Why does increasing the temperature of the acid have this effect on the rate?

   [1 mark]

   • AThe concentration of the acid decreases
   • BThe particles become larger
   • CThe particles move more slowly
   • DThe particles move faster, so they collide more frequently and with more energy

   (c)

   The magnesium disappeared much faster in the heated acid than in the room-temperature acid. Explain this result in terms of particle collisions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A researcher reacts the same mass of marble chips (calcium carbonate) with excess dilute hydrochloric acid in two separate flasks: one containing large marble chips, and one containing the same mass of marble crushed into a fine powder.

   (a)

   A researcher repeats a reaction between marble chips and hydrochloric acid, using large marble chips in one flask and the same total mass of small marble chips (crushed into powder) in another. Which flask is expected to react fastest?

   [1 mark]

   • ABoth react at exactly the same rate
   • BThe flask with the large marble chips
   • CThe flask with the small, powdered marble chips
   • DNeither flask reacts

   (b)

   What property of the solid reactant explains this difference in rate?

   [1 mark]

   • AColour
   • BSurface area
   • CDensity
   • DMelting point

   (c)

   Explain, in terms of particle collisions, why the powdered marble chips react faster with the acid than the same mass of large marble chips.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemical engineer is optimising an industrial gas-phase reaction between hydrogen and iodine, carried out inside a sealed, rigid reaction vessel, and investigates the separate effects of changing the pressure and changing the concentration of the reacting gases.

   (a)

   A gas reaction between hydrogen and iodine is carried out inside a sealed, rigid container. Explain the effect on the rate of reaction if the pressure of the gases inside the container is increased.

   [3 marks]

   (b)

   The engineer then investigates the effect of increasing the concentration of a reactant in solution instead, for a different reaction. Explain, in terms of particle collisions, why increasing the concentration of a reactant in solution increases the rate of reaction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A food scientist is investigating why a solid antacid tablet dissolves and reacts with stomach acid much faster when it is crushed into a powder and the acid is warmed, compared with using a whole tablet in acid at a lower temperature.

   (a)

   Describe and explain, in terms of particle collisions, how each of the following four factors affects the rate of a chemical reaction: temperature, concentration of a solution, pressure of a gas, and surface area of a solid reactant.

   [6 marks]

   (b)

   Using ideas from collision theory, explain fully why the food scientist observes the antacid reacting fastest when the tablet is crushed AND the acid is warmed, compared with changing only one of these variables at a time, and suggest why a real investigation should only change one variable at a time when comparing rates.

   [6 marks]

   Total for question 4: 12 marks

End of questions