Module 1: Thermodynamics

Hess’s law states that “In a reaction, the overall enthalpy change is equal to the sum of the enthalpy changes of all the individual steps taking place in a process”.The enthalpy change (∆H1) for the combustion reaction of methane to form carbon di-oxide and liquid water is equal to -890 kJ.The above reaction takes place in two steps.In the first step, methane is combusted to give carbon di-oxide and water vapour.The enthalpy change for this reaction is ∆H2= -802 kJ.In the second step, water vapour condenses from the gas to the liquid phase.The enthalpy change for this reaction is ∆H3 = -88 kJ.On combining these two values, the enthalpy change is equal to ∆H = -890 kJ.Thus, the enthalpy change of a reaction is the same whether the reaction takes place in single step or several steps.