Calorimetry
Measuring energy transfers
Energy can be released in chemical reactions as light, sound or electrical energy. But it is most often released as heat energy. This allows us to easily measure the amount of energy transferred.
Calorimetry
Measuring heat transfers is called calorimetry. The diagram shows a simple calorimetry experiment to measure the heat energy released from burning fuel. You should be able to recognise and label apparatus like that on the right.
In a typical calorimetry experiment:
- cold water is measured into a copper calorimeter - a small metal can
- the starting temperature of the water is recorded
- the water is heated using the flame from the burning fuel
- the final temperature of the water is recorded
The spirit burner containing the fuel is usually weighed before and after the experiment. In this way, the mass of the fuel burned can be found.
Fair testing
When comparing different fuels, it is important to carry out a fair test. Several variables should be kept constant, including:
- the mass - or volume - of water used
- the starting temperature of the water
- the temperature increase
- the distance of the flame from the calorimeter
More reliable results can be obtained by repeating the experiment many times. The biggest source of error in calorimetry is usually unwanted heat loss to the surroundings. This can be reduced by insulating the sides of the calorimeter and adding a lid.
Calculating energy transfers
The amount of energy transferred from the burning fuel to the water in the calorimeter can be calculated if you know:
- the mass of water heated
- the temperature rise
This is the equation you need:
energy transferred (joules, J) = mass of water heated (grams, g) × 4.2 × temperature rise (ºC)
For a given amount of water heated up, the greater the temperature rise, the greater the amount of heat energy transferred to the water. For example, twice as much energy is transferred to the water to achieve a temperature increase of 20ºC compared with 10ºC.
Comparing fuels
You can compare fuels by measuring the mass of fuel burned in the experiment. The best fuel is likely to release the most energy per gram of fuel. This is worked out using:
energy released (J/g of fuel) = energy transferred to water (J) ÷ mass of fuel burned (g)
- bonds in the reactants are broken
- new bonds are made in the products
Energy is absorbed to break bonds. Bond-breaking is an endothermic process.
Energy is released when new bonds form. Bond-making is an exothermic process.
Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when new bonds form.
Calculating energy transferred (you would be given the equation)
What is the energy transferred to 100cm 3 of water to raise its temperature by 20ºC?
It is useful to remember that 1cm 3 of water has a mass of 1g. So 100cm 3 of water has a mass of 100g.
energy transferred = mass of water heated × 4.2 × temperature rise
= 100 × 4.2 × 20 = 8,400J
It is also useful to remember that 1 kilojoule, 1kJ, equals 1,000J. So the energy transferred is 8.4kJ.
If 0.5g of fuel was used, the energy output of the fuel would be:
8.4 ÷ 0.5 = 16.8kJ/g