Define and explain the enthalpy of combustion `(Delta_(c)H^(theta))`.

Explanation

It is the enthalpy change accompanying the complete combustion of one mole of a substance in excess of oxygen or air.
For example, the enthalpy of combustion of carbon is represented as
`C_(s)+O_(2(g))toCO_(2(g)), DeltaH=-393.5 kJ`
Combustion reactions are always accompanied by the evolution of heat, therefore. the value of `Delta_(c)H` is always negative.