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Hess’s Law MCQ - Practice Questions with Answers

Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

Quick Facts

Hess’s Law is considered one of the most asked concept.
4 Questions around this concept.

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.

View Solution

Three thermochemical equations are given below

(i) $C_{\left ( graphite \right )}+O_{2}\left ( g \right )\rightarrow CO_{2}\left ( g \right );$

$\Delta _{r} H^{o}=xkJmol^{-1}$

(ii) $C_{\left ( graphite \right )}+\frac{1}{2}O_{2}\left ( g \right )\rightarrow CO\left ( g \right );$

$\Delta _{r} H^{o}=ykJmol^{-1}$

(iii) $CO\left ( g \right )\frac{1}{2}O_{2}\left ( g \right )\rightarrow CO_{2}\left ( g \right );$

$\Delta _{r} H^{o}=zkJmol^{-1}$

Based on the above equations, find out which of the relationship given below is correct?

x=y-z

z=x+y

x=y+z

y=2z-x

View Solution

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 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 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.