||Where does energy come from?
In this activity, students explore their understanding
that energy is released when chemical bonds form. A diagnostic task probes
their initial ideas, while a simple experiment shows how to measure an
energy change that cannot be measured directly, introducing Hess’s law.
Students will understand that:
- energy is released when chemical bonds are formed
- energy is released in exothermic chemical reactions.
Sequence of activities
||Using the question, ‘Where does energy come from?’
ask students for examples of energy sources. Make a list of their
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.
||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.
||Organise students into groups of three or four.
Circulate and support as they:
- share their individual ideas
- agree responses
- prepare to feedback to the class.
||Gather the students to a plenary.
- Review the responses from the diagnostic task.
- Ask students to write the chemical equation for the reaction
between methane and oxygen.
- Use molecular models to demonstrate that energy needs to be
supplied to break bonds – pull the model molecules apart.
- Reform the models into carbon dioxide and water; show that the
reverse process, energy release, occurs when new chemical bonds are
||Tell students about the next activity, to compare the
energy released when different fuels react with oxygen.
- Introduce the term enthalpy change of combustion as a
value for the energy released when one mole of fuel is completely
burned in oxygen.
- Give each student a copy of the worksheet How much energy
comes from burning fuels?
- Arrange the students to work in pairs.
- Have molecular models available.
Supervise the students as they work through the practical activity.
||In a plenary, ask students to:
- share their values to get a complete set across the class
- review the data
- draw the graph of the enthalpy of combustion values for the
primary alcohol series.
Note: the data can be improved using a correction method,
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.
Assessment for learning 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
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
For each student
||Where does energy come from when methane burns?
||How much energy comes from burning fuels?
- Graph paper (or access to pc graphing facilities)
- Access to a spreadsheet facility.
For each pair of students
- Spirit burner containing an alcohol, (one of methanol, ethanol,
propan-1-ol or butan-1-ol (all Flammable))
- Copper calorimeter (this can be an empty, clean food can)
- Heat shield (this can be a large, empty, clean food can cut down the
- 100‑200 cm3 water
- 50 cm3 or 100 cm3 measuring cylinder or 50 cm3
- Protective mat
- Eye protection
- Thermometer reading 0‑110 °C in 0.1 °C increments
- Access to a balance weighing to 0.01g
- Retort stand, boss and clamp.
For the whole group of students
- A range of spirit burners with different alcohols, preferably in
sequence ‑ methanol, ethanol, propan-1-ol or butan-1-ol
- Access to molecular modelling kits.
It is the responsibility of the teacher to carry out an appropriate
- The procedure must not be attempted with petrol or any highly
volatile, low flash-point hydrocarbons.
- 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.
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
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.
Where does the energy come from when methane burns?
Energy comes from making bonds in carbon dioxide and water.
V. Barker, Thesis, A longitudinal study of 16-18 year olds’
understanding of basic chemical ideas. York: University of York, 1994.
G. Burton et al, Salters Advanced Chemistry Activities
Pack. Oxford: Heinemann, 1994.