Controlling Reactions (OCR GCSE Chemistry A (Gateway))

Antacid tablets are used to treat indigestion.

A student investigates two different antacid tablets, X and Y. Both tablets, X and Y, contain calcium carbonate, CaCO₃.

Calcium carbonate reacts with hydrochloric acid. Calcium chloride, CaCl₂, water and carbon dioxide are made.

Write a balanced symbol equation for this reaction.

The diagram shows the apparatus the student uses.

The student reacts tablet X with 100 cm³ of hydrochloric acid. The hydrochloric acid is in excess.

He measures the volume of gas made every minute during the first five minutes.

He does a second experiment using tablet Y and a fresh 100 cm³ sample of the same hydrochloric acid.

The table shows his results.

i) The graph shows the results for tablet X.

What is the volume of gas made by the end of the experiment?

Answer = ...................................... cm³ [1]

ii) Plot the results for tablet Y on the grid. Draw a line of best fit.

[2]

iii) Tablet X contains less calcium carbonate than tablet Y. How do the results show this?

[1]

The rate of reaction between calcium carbonate and hydrochloric acid can be increased by:

• Using a more concentrated solution of hydrochloric acid

• Increasing the temperature of the acid.

Explain how each of these methods increase the rate of the reaction.

Use ideas about collisions between particles.

A student investigates the rate of reaction between dilute hydrochloric acid and magnesium carbonate.

She wants to find out how the concentration of the acid changes the rate of reaction.

Fig. 18.1 shows the apparatus she uses.

Carbon dioxide gas is given off in the reaction.

The student measures the loss in mass every 30 seconds for 5 minutes.

Another student also investigates the rate of reaction between dilute hydrochloric acid and magnesium carbonate.

Fig. 18.2 shows diagrams of some of the apparatus he uses.

The student also wants to find out how the concentration of the acid changes the rate of reaction.

Describe the method he follows using the apparatus in Fig. 18.2.

Include a labelled diagram of the set-up of the apparatus he uses.

The students now investigate the reaction between sodium thiosulfate and dilute hydrochloric acid.

They want to find out how temperature changes the rate of reaction.

Fig. 18.3 is a diagram of their experiment.

Add dilute hydrochloric acid and start timing

A yellow solid is made during the reaction.

The students time how long it takes for the cross to disappear. This is the reaction time.

Look at their results.

i) Plot a graph of the results on the grid. Draw a line of best fit.

[3]

ii) What i s the reaction time at 25 °C?

Reaction time = ....................................................... s [1]

iii) What happens to the rate of reaction as the temperature increases?

[1]

iv) Explain your answer to (b)(iii). Include ideas about collisions between particles in your answer.

[2]

A student investigates the reaction between magnesium and dilute hydrochloric acid, HCl.

The student adds magnesium ribbon to hydrochloric acid in a beaker, as shown in the diagram.

Magnesium chloride, MgCl₂, and hydrogen gas are made.

Write the balanced symbol equation for this reaction.

The student measures the time it takes for all the magnesium to react. This is the reaction time.

The student does five experiments.

This is the student’s prediction:

“The smaller the volume of acid and the smaller the mass of magnesium, the shorter the reaction time.”

Look at the student’s results.

Describe and explain whether the student’s results support his prediction.

Include ideas about the reacting particle model in your answer.

The student repeats experiment 1. This time he uses acid at a lower temperature.

Explain, using the reacting particle model, what happens to the rate of reaction and predict the reaction time for this reaction.

During chemical reactions, reactants are used up and the rate of reaction decreases.

Explain, in terms of particles, why the rate of reaction decreases.

The student repeats experiment 1. This time he uses acid at a higher temperature.

Explain, using the reacting particle model, what happens to the rate of reaction and predict the reaction time for this reaction.

Another student investigates the reaction between marble chips and hydrochloric acid.

She times how long it takes for all the marble chips to react.

Look at her results.

Look at the student’s results for experiment 2.

Calculate the mean rate of reaction in experiment 2.

Give your answer to 3 significant figures and in standard form.

Mean rate of reaction = ................................................... g/s

A student investigates the reaction between marble chips, CaCO₃, and hydrochloric acid.

Calcium chloride, CaCl₂, carbon dioxide and water are made.

Write a balanced symbol equation for the reaction.

The student does three experiments, A, B and C.

In each experiment she uses a different size of marble chip. She uses the same mass of marble in each experiment. She also uses the same concentration of acid. Look at the graph of her results.

i) Look at the line for experiment B on the graph.

When is the rate of reaction greatest?

Choose your answer from the list.

0 – 30 seconds

30 – 60 seconds

60 – 90 seconds

90 – 120 seconds

Answer = ..................................... seconds

[1]

ii) Look at the line for experiment C.

Calculate the rate of reaction during the first 45 seconds.    

Give your answer to 2 significant figures.

Answer = ..................................... cm³ / s

[3]

The rate of reaction between marble and hydrochloric acid can be decreased by:

• Using a more dilute solution of hydrochloric acid

• Cooling the acid.

Explain how each of these methods make the reaction slower.

Use ideas about collisions between particles.

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

Look at the energy profile diagram for a reaction without a catalyst.

Complete the energy profile diagram to show

i) The reaction profile for the reaction with a catalyst. 

[1]

ii) Label the activation energy for the reaction with a catalyst. 

[1]