Hess’s Law MCQ - Practice Questions & Answers

Solve by difficulty

EASY

The enthalpy change for a reaction does not depend upon the

physical states of reactants and products

use of different reactants for the same product

nature of intermediate reaction steps

difference in initial or final temperatures of involved substances.

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Concepts Covered - 1

Hess’s Law

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

$\\\mathrm{C + O _{2} \rightarrow CO _{2} ; \Delta H_1 =-94.3 kcal } \\ \mathrm{C +\frac{1}{2} O _{2} \rightarrow CO ; \Delta H_2 =-68.3 kcal } \\ \mathrm{CO +\frac{1}{2} O _{2} \rightarrow CO _{2} ; \Delta H_3 =-26 kcal}$

$\mathrm{It\ can\ be\ seen\ that\ \Delta H_1 =\Delta H_2 + \Delta H_3}$

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.