G.H. Hess, a Russian chemist, gave a law about the heat of reaction. It States that the total amount of heat evolved or absorbed in a reaction is same whether the reaction takes place in one step or in a number of steps i.e, amount of heat evolved or absorbed (enthalpy change) of a reaction is independent of the path of the reaction.
Let us suppose that A can be changed to Z directly
A -----------> Z+ Q¹ ∆H=-Q¹
Where Q¹ is the amount of heat evolved.
Let us suppose that the same Change is brought about by a series of steps.
A -----------> B +q¹ ∆H¹=-q¹
B -----------> C +q² ∆H²=-q²
C -----------> Z +q³ ∆H³=-q³
: . Total heat evolved Q²=q¹+q²+q³
According to this law Q¹=Q²,
This law can be illustrated by considering the formation of CO2 from carbon. Carbon can be converted to CO2 by the following two different ways.
In path I , carbon is directly converted into CO2 by burning. Carbon is excess of air. The enthalpy change is found to be -94.0 kcal
C(s) + O2(g) ----------> CO2 (g) ∆H=-94.0 Kcal.
In path II , carbon is burnt in a limited supply of air to get carbon monoxide which is then oxidised to CO2.
C(s) + 1/2 O2(g) ----------> CO(g) ∆H=-26.0Kcal
CO(g) + 1/2 O2(g) -----------> CO2(g) ∆H=-68.0Kcal
: . Total enthalpy change in Path II=(-26.0-68.0)kcal=-94.0kcal
Total enthalpy in path I and path II are equal. Hence the results illustrates the Hess's law of constant heat summation.
Hess's law of constant heat summation is very helpful to determine the heat of many reaction as follows.
a) Heat of reaction for chemical reactions, which can not be measured directly.
b) Heat of formation of compounds.
c) Bond energy etc.