5.9 - Energy Changes from Temperature
Energy released per gram of a fuel
- Different fuels don't release the same amount of energy when they burn.
- To compare them fairly, compare the energy each releases for the same mass of fuel: the energy released per gram.
Stage 1 - Energy transferred to the liquid
Where:
- = Energy transferred to the liquid, in joules (J)
- = Mass of liquid heated, in grams (g); for water, 1 cm3 has a mass of 1 g
- = Specific heat capacity of the liquid, in joules per gram per degree Celsius (J/g/°C); for water, about 4.2 J/g/°C, but always use the value supplied with the question or on the data sheet, which may be given as 4.18
- = Temperature rise of the liquid, in degrees Celsius (°C)
Stage 2 - Energy per gram of fuel
Worked example - Energy per gram of a fuel
Ethanol is burned to heat 175 g of water. The temperature of the water rises from 19.5 °C to 45.5 °C, and 1.15 g of ethanol burns. The specific heat capacity of water is 4.2 J/g/°C. Calculate the energy released per gram of ethanol.
Step 1 - Find the temperature rise
ΔT = 45.5 − 19.5 = 26.0 °C
Step 2 - Find the mass of the liquid in grams
m = 175 g
Step 3 - Multiply m × c × ΔT
Q = 175 × 4.2 × 26.0 = 19,110 J
Step 4 - Divide the energy by the mass of fuel burned
19,110 ÷ 1.15 = 16,617.39 J per gram
Step 5 - Round the answer
The mass of water, the mass of ethanol and the two temperatures are all given to three significant figures, so:
Energy released per gram of ethanol = 16,600 J/g, which is 16.6 kJ/g