Explain why the complete combustion of ethanol releases more energy than the complete combustion of methanol, in terms of the bonds broken and formed during each reaction.

The combustion of a fuel is exothermic because the energy released when new bonds are formed in the products is greater than the energy required to break the bonds in the reactants.

Ethanol (C₂H₅OH) contains more bonds than methanol (CH₃OH) – it has an additional C–C bond and two extra C–H bonds. Therefore more energy is needed to break the bonds in ethanol.

During complete combustion both fuels form CO₂ and H₂O, but one mole of ethanol produces two moles of CO₂ and three moles of H₂O, whereas one mole of methanol produces only one mole of CO₂ and two moles of H₂O. Thus more bonds are formed in the products from ethanol combustion.

Because the total energy released in forming the CO₂ and H₂O bonds from ethanol exceeds the larger energy input for breaking its bonds, the net enthalpy change is more negative for ethanol than for methanol, so ethanol releases more energy.