Interpreting rate of reaction graphs | 14-16 years

In this activity, pre‑drawn graphs are chosen by students to match the most likely outcomes from different reaction conditions for the reaction between calcium carbonate and hydrochloric acid. The graphs are a prop to help students check and reinforce what they have learnt about factors affecting rates of reaction, including concentration, temperature and surface area of solid reactants.

The resource would be particularly useful for checking on learning towards the end of teaching this topic or when revising for an external assessment.

Learning objectives

Students will be able to:

• Interpret graphs which show how the rate of reaction is affected by temperature and by the concentration and surface area of reactants.

Sequence of activities

Demonstration and introduction

1. Add some marble chips to hydrochloric acid in a conical flask with a side arm and draw attention to the gas evolved.
2. Issue mini whiteboards.
3. Set up a discussion, first asking the students to write down the factors which can affect the rate of reaction. Ensure that the effect of concentration of reactant, temperature and surface area of a solid reactant are identified.
4. Continue the discussion by asking them to write down what determines how much product is produced during a reaction. Ensure that the amount of the reactant that is used up is identified.
5. After discussing this, share the learning objectives with students.

Review apparatus

Now ask the students to draw the apparatus they would use to collect and record the volume of gas produced over a period of time.

As they share their answers, ensure that they discuss methods of collecting and measuring the gas evolved using a syringe or an inverted burette in a trough of water.

Activity: stage 1

Explain that they are going to:

1. Be given details about the conditions for six different experiments in which marble chips were added to hydrochloric acid and some graphs showing the volume of gas produced over a period of time.
2. Select which graph is most likely to be produced in the different conditions.

Note that:

• Graph A is produced when 1 g of small lumps of calcium carbonate is added to 100 cm³ of 1 mol dm^-3 hydrochloric acid at 20 °C.
• The same graph might be produced by more than one set of conditions.

Activity: stage 2

Give each student a ’Rate of reaction graphs student sheet’ and an ‘Information sheet’ and allow time for them to explain their choices on their student sheet. Circulate and support students:

• Allowing them time to explain their choices.
• Prompting those who need help to focus on the amount of gas produced and the rate at which it is produced.

Activity: stage 3

Arrange students into groups of three to:

1. Share their answers.
2. Discuss similarities and differences in their choices.
3. Review their ideas in the light of discussions with their peers.
4. Record changes in their ideas.

Plenary

Bring students together in a plenary and:

1. Select groups to explain the choices that they have made.
2. Encourage other groups to challenge or to add to these explanations.
3. Allow time for the students to reflect again on what they have written on their student sheet.

Feedback

Take in the ‘Rate of reaction graphs student sheet’. Write comments or questions to highlight the achievement that has been demonstrated. Suggest which aspects of the topic require further attention and where the student can find resources that will help them.

Commentary

To make the learning objectives meaningful there is an extended initial stage, with a passive demonstration but also with a discussion where everyone contributes.

After matching the graphs, students are given several opportunities to review their ideas and organise their knowledge.

With the written feedback, students are helped so that they can recognise what they need to do, to deal with any shortcomings in their understanding of how these graphs represent changes.

Practical notes

Equipment

For the demonstration:

• Eye protection
• Conical flask with side arm
• Marble chips
• Hydrochloric acid (IRRITANT)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection.
• It is the responsibility of the teacher to carry out an appropriate risk assessment for the demonstration.

Answers and notes

• Graph C is the graph which is most likely to be produced from experiment 2.
  • The concentration of the acid is the same as in experiment 1 so the rate or reaction will be the same. There is, however, twice as much calcium carbonate used so the volume of carbon dioxide collected will be twice that collected in experiment 1.
• Graph A is the graph which is most likely to be produced from experiment 3.
  • The only difference in the conditions compared with experiment 1 is that there is twice as much acid. This will not make any difference to the graph as it is the marble chips that are used up, not the acid, so the graph will be the same.
• Graph D is the graph which is most likely to be produced from experiment 4.
  • The acid is twice as concentrated compared with experiment 1 so the rate of reaction will be faster. The amount of calcium carbonate used up is the same so the volume of carbon dioxide produced will be the same.
• Graph D is the graph which is most likely to be produced from experiment 5.
  • The temperature is higher than it is in experiment 1 so the rate of reaction will be higher. The amount of calcium carbonate is the same so that the volume of carbon dioxide produced will be the same.
• Graph B is the graph which is most likely to be produced from experiment 6.
  • The concentration of the acid is twice what it is in experiment 1 so the rate of reaction will be higher. The amount of calcium carbonate is twice what it is in experiment 1 so that the volume of carbon dioxide will also be twice that produced in experiment 1.