How do catalysts affect reaction rates? | 16–18 years

In this session, students explore the story behind the sinking of the Kursk submarine. They respond to text and background information before carrying out an experiment to compare the effects of a range of catalysts on the rate of decomposition of hydrogen peroxide.

The investigation can be planned with different degrees of open-endedness, providing a setting for students develop coherent thinking about experimental design and set up.

Learning objectives

Students will know that:

• Catalysts increase the rate of reactions.
• Heterogeneous catalysts are in a different phase from the reactants.
• Controlling the ways catalysts work is essential to their use.

Sequence of activities

Introduction

The first activity provides its own introduction, using a news story, the sinking of the Kursk submarine in August, 2000:

1. Give each student the ‘What sank the Kursk submarine?’ worksheet.
2. Ask them to work through the questions and prepare their feedback.

Plenary 1

In a plenary:

1. Review the answers to the questions.
2. Ensure that students realise that hydrogen peroxide was involved in the torpedoes that exploded.
3. Introduce the next activity (to find out what makes hydrogen peroxide decompose at a faster rate) and relate this to the learning objectives.

Investigation

Give each student a copy of Which catalyst?, the Equipment list for the investigation and an Apparatus diagram.

Arrange the students into groups of no more than three. Supervise the groups as they:

1. Carry out the investigation to find out which is the best catalyst for hydrogen peroxide decomposition.
2. Prepare their feedback.
3. Select a spokesperson.

Plenary 2

Draw the students into a plenary, and:

1. Review each group’s results through feedback.
2. Pose the following questions:
   • What would happen if highly concentrated hydrogen peroxide was used with the most effective catalyst?
   • What might have happened to create the conditions that caused the Kursk to sink?
   • Why might the Russian navy have tried to avoid admitting this in the first place?
   • What safeguards could be put in place to ensure that torpedoes using hydrogen peroxide don’t explode without warning?

Feedback

Use students’ reports of the experiment as a vehicle for providing written feedback on their understanding of the investigation and the role of heterogeneous catalysts in increasing rate of reaction.

Commentary

The initial work sets a strong context for students to see the relevance of the learning objectives.

The investigation provides a setting for students to develop coherent thinking about experimental design and set up, depending on the degree of open-endedness permitted. By this process, they informally assess their ideas about experimental procedure. During the plenary there can be some further peer assessment. Using questions in the closing plenary permits the teacher to analyse class learning and application of knowledge.

Written feedback on the accounts of the experiments permits analysis of individual strengths and weaknesses in understanding.

Practical notes

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.

Teaching notes

The degree of open-endedness of the investigation can be varied depending on the amount of time available. For example, in a more closed version that requires less time, groups of students may be given the procedure and two or three potential catalysts to test. A more open approach would be to show students the technique then to leave the remainder of the experimental design to them. More open still would be simply to give students the question for the investigation, leaving all aspects of experimental design to them. This requires some time for research.

If time is really pressing, the investigation may be replaced with a demonstration illustrating the potential power generated by the catalysed decomposition of hydrogen peroxide. 

Answers

What sank the Kursk?

1. In the Barents Sea, north of Norway.
2. The second one.
3. The events occurred 135 seconds apart.
4. They have the same shape.
5. This suggests they were caused by the same type of event.
6. A collision would have produced one signal only.
7. An underwater explosion was responsible.
8. This means the Kursk could not have been in collision.
9. 1.5 and 3.5
10. The strong structure could absorb or modify the explosions. They could therefore have been larger than the data suggest.
11. Key points: leakage of hydrogen peroxide caused a torpedo to explode within the submarine, triggering a larger explosion as the front of the submarine blew off.

Which catalyst? Decomposing hydrogen peroxide

1. Manganese(IV) oxide will probably be the fastest.
2. Maximum concentration hydrogen peroxide would produce oxygen at a much faster rate.
3. If the hydrogen peroxide leaks on to a catalyst, gas would be produced in a confined space creating an explosion within the torpedo storage area, or within a torpedo head itself. This would create highly dangerous circumstances in which all the torpedoes would be at risk of explosion.