Standard Enthalpy Change of Combustion

The enthalpy change at 100 kPa and 298K when one mole of a substance is completely burned in oxygen, with all reactants and products in their standard states. It is given the symbol Δ_cH^θ.Three parts of the definition matter:One mole of the substance is burned, so the equation is balanced for 1 mol of fuel, even if that needs a fraction in front of O₂.
Complete combustion, so all the carbon becomes CO₂ and all the hydrogen becomes H₂O.
Standard states, so water is shown as a liquid, H₂O(l), because that is its state at 298 K.For example, for ethane:C₂H₆(g) + 3½O₂(g) → 2CO₂(g) + 3H₂O(l)Δ_cH^θ[C₂H₆(g)] = -1560 kJ mol^-1Combustion is always exothermic, so every Δ_cH^θ value is negative.Writing the equation as 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O describes two moles of ethane. Its enthalpy change (-3120 kJ) is therefore not the enthalpy change of combustion.Enthalpy changes of combustion are useful because they can be measured fairly directly by burning a fuel, and they can be used to calculate other enthalpy changes that cannot be measured directly.