If you increase the number of atoms in fuels why do we get more heat energy HELP PLEWatch
I think this is right, but I'm not that good at chemistry... I'd check with a chemistry teacher.
If you meant more molecules, each mole of the fuel contributes heat energy equal to the modulus of the enthalpy change of combustion. If you increase the number of moles, then more energy is contributed.
Most of what Cyanide said was ok, but for a reaction that goes to completion, there is nothing you can do to change the composition of equilibrium. The reaction will always go to completion.
I dont think you need an awful lot of detail for GCSE though, so don't worry.