Where does energy come from? | 16-18 years

In this activity, students reinforce their understanding of how energy is released when chemical bonds are formed. A diagnostic task probes their initial ideas, while a simple experiment shows how to measure an energy change that cannot be measured directly, introducing enthalpy change of combustion and Hess’s law.

Learning objectives

Students will understand that:

• Energy is released when chemical bonds are formed.
• Energy is released in exothermic chemical reactions.

Sequence of activities

Introduction

1. Using the question, ‘Where does energy come from?’, ask students for examples of energy sources. Make a list of their suggestions.
2. Introduce exothermic chemical reactions as sources of energy, using combustion as a specific example, if this has not been mentioned earlier. Share the learning objectives.

Diagnostic task: stage 1

Give each student a copy of ‘Where does energy come from when methane burns?’. Ask each student to complete this diagnostic task that will expose thinking about answers to this question.

Diagnostic task: stage 2

Organise students into groups of three or four. Circulate and support as they:

1. Share their individual ideas.
2. Agree responses.
3. Prepare to feedback to the class.

Diagnostic task: stage 3

Gather the students to a plenary and:

1. Review the responses from the diagnostic task.
2. Ask students to write the chemical equation for the reaction between methane and oxygen.
3. Use molecular models to demonstrate that energy needs to be supplied to break bonds – pull the model molecules apart.
4. Reform the models into carbon dioxide and water; show that the reverse process, energy release, occurs when new chemical bonds are formed.

Practical

1. Tell students about the next activity, to compare the energy released when different fuels react with oxygen.
2. Introduce the term ‘enthalpy change of combustion’ as a value for the energy released when one mole of fuel is completely burned in oxygen.
3. Give each student a copy of the worksheet ‘How much energy comes from burning fuels?’.
4. Arrange the students to work in pairs.
5. Supervise the students as they work through the practical activity, and have molecular models available.

Plenary

In a plenary, ask students to:

1. Share their values to get a complete set across the class.
2. Review the data. (Note: the data can be improved using a correction method, described below in ‘Practical notes’.)
3. Draw the graph of the enthalpy of combustion values for the primary alcohol series.

Use questions to develop understanding:

• Why are all the values low compared with the ‘databook’ values?
• Why are enthalpy of combustion values always negative?
• Explain why the graph is a straight line.
• Does the graph go through the origin? If so, why?
• What is the name of the compound with no carbon atoms indicated on the graph? Does this compound have an enthalpy of combustion value?
• What would the value for pentan-1-ol be?
• What would the value for propan-2-ol be?
• Review the original question – where does energy come from?

Ensure students realise that in the case of fuels, energy comes from the fuel-oxygen system.

Commentary

The diagnostic activity provides a framework for individual reflection. The group discussion is a time for refining and developing coherent thinking. During the review of the responses, misunderstandings can be clarified.

The class experiment followed by teacher questioning will help reinforce learning about energy release.

The review of learning that each student writes gives a further opportunity for the teacher to check learning and to highlight strengths and weaknesses.

Practical notes

Apparatus

For each pair of students:

• Eye protection
• Spirit burner containing an alcohol (see ‘Chemicals’ for options)
• Splints
• Copper calorimeter (this can be an empty, clean food can)
• Heat shield (this can be a large, empty, clean food can cut down the side)
• Measuring cylinder, 50 cm³ or 100 cm³, or pipette, 50 cm³
• Protective mat
• Thermometer reading 0–110 °C in 0.1 °C increments
• Retort stand, boss and clamp
• Access to a balance weighing to 0.01 g

For the whole group of students:

• A range of spirit burners with different alcohols, preferably in sequence (see ‘Chemicals’)
• Access to molecular modelling kits

Chemicals

• A range of different alcohols, such as (in sequence):
  • Methanol (FLAMMABLE)
  • Ethanol (FLAMMABLE)
  • Propan-1-ol (FLAMMABLE)
  • Butan-1-ol (FLAMMABLE)
• Water, 100–200 cm³ for each pair of students

Health, safety and technical notes

1. Read our standard health and safety guidance.
2. Wear eye protection throughout.
3. It is the responsibility of the teacher to carry out an appropriate risk assessment.
4. The procedure must NOT be attempted with petrol or any highly volatile, low flash-point hydrocarbons.
5. Use a safe spirit lamp with the wick fitting tightly in the holder and the holder fitting tightly in the neck of the lamp. Reduce the capacity of large burners by filling with cotton wool or epoxy resin.

Principal hazard

• Flammable liquids.

Correcting the data

This experiment produces notoriously low values compared to the standard values. To help students get a better set of results, a correction method can be applied.

In carrying out the calculation, students should use a ‘perfect’ value for the amount of energy transferred to the water.

To get this ‘perfect’ value, students must measure the number of moles of one of the alcohols burned and multiply it by the standard enthalpy of combustion value for that specific alcohol. The result will be the maximum theoretical amount of energy that could have been transferred to the water.

The ‘perfect’ value can then be used for every alcohol, as the same volume of water and the same temperature rise should have been recorded for all of them. The enthalpies of combustion for the other alcohols used in the experiment can be found by dividing this amount of energy by the number of moles of alcohol used in heating the water.

Answers

• Where does the energy come from when methane burns?
  • Answer: Energy comes from making bonds in carbon dioxide and water.