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Energy can be released in chemical reactions as light, sound or electrical energy. But it is most often released as heat energy. Measuring heat transfers is called calorimetry.

The diagram shows a simple calorimetry experiment to measure the heat energy released from burning fuel:

Calorimetry method

1. Cold water is measured into a copper calorimeter – a small metal can.
2. The starting temperature of the water is recorded.
3. The water is heated using the flame from the burning fuel.
4. The final temperature of the water is recorded.

The spirit burner containing the fuel is usually weighed before and after the experiment so that 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. They include:

• the 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.

Worked example – calculating energy per gram of fuel

3.5 g of a fuel is burned to heat 50 cm³ of water. The temperature of the water increased from 22°C to 71°C. Calculate the energy released per gram of fuel using the following equation.

Energy given out = mass of water × 4.2 × temperature change

Energy measured in joules, J

Mass of water measured in grams (1 cm³ = 1 g)

4.2 is the specific heat capacity of water, J/g°C

Temperature change = temperature of water after heating – temperature of water before heating

Energy given out per gram = energy given out ÷ mass of fuel

Temperature change = 71 – 22 = 49°C

Energy given out = 50 × 4.2 × 49 = 10,290 J

Energy per gram = 10,290 ÷ 3.5 = 2,940 J/g