OCR GCSE Combined Science: Chemistry: Monitoring and Controlling Reactions

A student investigates how the rate of reaction between marble chips (calcium carbonate) and dilute hydrochloric acid changes. They carry out the reaction at a higher temperature and, separately, using acid at a higher concentration. In both cases the reaction is faster.

Explain, using collision theory, why increasing the temperature and increasing the concentration both increase the rate of this reaction.

(6 marks)

A student measures the rate of a reaction that produces a gas. They collect the gas and record its volume over time. In the reaction, 48 cm³ of gas is collected in the first 60 s.

(a) Calculate the mean rate of reaction over the first 60 s, in cm³/s. Show your working. (2 marks)

(b) As the reaction continues, the rate gets slower even though the temperature is unchanged. Explain, in terms of particles, why the rate decreases as the reaction proceeds. (2 marks)

Marble chips react with dilute hydrochloric acid. A student repeats the reaction using the same mass of calcium carbonate, but as a fine powder instead of large chips. The powdered form reacts faster.

(a) Explain, in terms of surface area and collisions, why the powder reacts faster than the chips. (2 marks)

(b) State the name given to a substance that speeds up a reaction without being used up itself. (1 mark)

A catalyst can be used to increase the rate of a chemical reaction.

(a) Explain, in terms of activation energy, how a catalyst increases the rate of a reaction. (2 marks)

(b) State one reason why using a catalyst can be an advantage in an industrial process. (1 mark)

Some reactions are reversible. The reaction below can go in both directions and can reach equilibrium in a closed container:

$\text{A} + \text{B} \rightleftharpoons \text{C} + \text{D}$A+B⇌C+D

Explain what is meant by a system being at equilibrium, in terms of the rates of the forward and backward reactions. (2 marks)

For particles to react, they must collide.

State the two conditions that a collision between reacting particles must meet for the reaction to happen. (1 mark)