Energy from bonds

Section 3 of 4

A fuel is a substance that burns to give out useful energy. In lesson 8 you saw where that energy comes from: the new bonds that form as it burns.

When methane burns, the C=O and O–⁠H bonds made are stronger than the C–⁠H and O=O bonds broken. The figure shows by how much.

Burning methane: energy in to break bonds, energy out to make them Two bars. The top bar is the energy taken in to break 4 C–H bonds in methane and 2 O=O bonds in oxygen. The bottom bar, about a third longer, is the energy given out when 2 C=O bonds in carbon dioxide and 4 O–H bonds in water form. The part of the bottom bar that reaches past the top bar is highlighted: that extra energy comes out as heat. breaking 4 C–H and 2 O=O energy taken in making 2 C=O and 4 O–H energy given out the extra comes out as heat
Burning one methane molecule: the bonds it makes give out more energy than the bonds it breaks take in. Bar lengths follow average bond energies.

Every hydrocarbon fuel works the same way. Its carbon atoms end up in carbon dioxide and its hydrogen atoms end up in water, and the strong bonds in those two small molecules give out the energy.

Energy per gram

Energy is measured in joules, and a kilojoule, kJ, is 1,000 joules. A gram of fat, as you saw earlier, gives your body about 37 kJ. Burning a gram of each of these fuels gives out a different amount:

Energy given out by burning 1 gram of each fuel A bar chart of the energy given out when 1 gram of each fuel burns: hydrogen 142 kJ, methane 56 kJ, petrol 46 kJ, ethanol 30 kJ, air-dried wood 16 kJ. Hydrogen gives out about three times as much as petrol. hydrogen 142 kJ methane 56 kJ petrol 46 kJ ethanol 30 kJ air-dried wood 16 kJ energy given out by burning 1 gram
Energy given out by burning 1 gram of each fuel completely, in kilojoules. Values from heat of combustion tables, rounded.

Hydrogen gives out the most per gram, because its atoms are so light, and it burns to water alone. Methane and petrol are all carbon and hydrogen, so they come next. Ethanol gives out less: each molecule already holds an oxygen atom, so it is partly burned before it starts. Air-dried wood is largely cellulose, chains of glucose with oxygen all along them, and it still holds some water, so it gives out less again.

Quick check

Burning 1 gram of ethanol gives out about 30 kJ, and burning 1 gram of petrol gives out about 46 kJ. Why does ethanol give out less?