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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

The enthalpy change for a reaction does not depend upon the

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?

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 (\DeltaH or \DeltaE 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

  • \DeltaH or \DeltaE is not proportional to path or way of reaction
  • \DeltaH or \DeltaE 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.

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

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

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Reference Books

Hess’s Law

Chemistry Part I Textbook for Class XI

Page No. : 175

Line : 45

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