Hess’s Law - Practice Questions & MCQ

Hess's Law

This rule is a consequence of the Enthalpy or Internal energy being a state function.

According to Hess's Law, "The enthalpy change for a process is independent of path or way of a process. The enthalpy change ($\Delta$H or $\Delta$E for any physical or chemical process remains the same whether the process is carried out in one step or in many steps. "

Let us consider the following example

$\begin{aligned} & \mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2 ; \Delta \mathrm{H}_1=-94.3 \mathrm{kcal} \\ & \mathrm{C}+\frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{CO} ; \Delta \mathrm{H}_2=-68.3 \mathrm{kcal} \\ & \mathrm{CO}+\frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{CO}_2 ; \Delta \mathrm{H}_3=-26 \mathrm{kcal}\end{aligned}$

It can be seen that $\Delta \mathrm{H}_1=\Delta \mathrm{H}_2+\Delta \mathrm{H}_3$

Hence it proves that

• $\Delta$H or $\Delta$E is not proportional to path or way of reaction
• $\Delta$H or $\Delta$E is a state function that depends only on the initial and final state (state function).

Applications of Hess Law :-

It has the following applications:

1. It helps in finding enthalpy changes for those reactions also which are experimentally not possible.

2. It also helps in finding $\Delta \mathrm{H}_{\text {formation }}, \Delta \mathrm{H}_{\text {combustion }}$ etc.

3. For the determination of resonance energy and lattice energy. 

4. For the determination of enthalpies of reactions occurring very slowly.