Question

Why does Hess's law hold true?

Answer 1

Hess's Law:

1. Hess's law has the properties of state function and the state function does not depend on the path taken for the formation or dissociation. It only depends on the state at the moment.
2. Only existence in the reactant and the final product's state is possible in the chemical system.
3. These two states of the same system (formed by the same kind and a number of atoms) give two specific enthalpy values.
4. All steps need to be continued at the same temperature and it is necessary to balance out the calculations for the respective steps.
5. Thus the transition of the system from one state to another state requires a change in enthalpy.
6. It is the change between the product's state enthalpy minus the reactant state enthalpy.
7. As a further illustration of the law, consider the formation of carbon dioxide. There are two ways in which $C{O}_2$ can be formed.
8. In the first step, burning carbon in excess of oxygen. $C\left(s\right)+O_2\left(g\right)\underset{}{\to }C{O}_2\left(g\right);{\Delta }_{r}H=-393.5kJ$
9. In the second step, by burning carbon in a limited supply of oxygen to form CO and then CO is converted to CO₂.$C\left(s\right)+12O_2\left(g\right)\underset{}{\to }CO\left(g\right);{\Delta }_{r}H=-110.54kJ$$CO\left(g\right)+12O_2\left(g\right)\stackrel{}{\to }C{O}_2\left(g\right);{\Delta }_{r}H=-393.5kJ$
10. On adding, we get $C\left(s\right)+O_2\left(g\right)\stackrel{}{\to }C{O}_2\left(g\right);{\Delta }_{r}H=-393.5kJ$
11. Thus, in both cases, ${\Delta }_{r}H$ is the same. This proves the law.
12. Hence Hess's law is true.