Hess’s law of constant heat Summation

Hess’s law of constant heat Summation

We have already seen that the heat changes in chemical reactions are equal to the difference in internal energy (∆U) or heat content (∆H) of the products and reactants, depending upon whether the reaction is studied at constant volume or constant pressure. Since ∆U and ∆H are functions of the state of the system, the heat evolved or absorbed in a given reaction depends only on the initial state and final state of the system and not on the path or the steps by which the change takes place.

This generalisation is known as Hess’s law and stated as:

The enthalpy change of a reaction either at constant volume or constant pressure is the same whether it takes place in a single or multiple steps provided the initial and final states are same.

∆H_r = ∆H₁ + ∆H₂ + ∆H₃

Application of Hess's Law: 

Hess's law can be applied to calculate enthalpies of reactions that are difficult to measure. For example, it is very difficult to measure the heat of combustion of graphite to give pure CO. However, enthalpy for the oxidation of graphite to CO₂ and CO to CO₂ can easily be measured. For these conversions, the heat of combustion values are –393.5 kJ and – 283.5 kJ respectively.

From these data the enthalpy of combustion of graphite to CO can be calculated by applying Hess's law.

The reactions involved in this process can be expressed as follows

According to Hess law,

ΔH₁ = ΔH₂ + ΔH₃

–393.5 kJ= X – 283.5 kJ

X= -110.5 kJ