   Chapter 18, Problem 22PS

Chapter
Section
Textbook Problem

Calculate the standard entropy change for the formation of 1.0 mol of the following compounds from the elements at 25 °C. (a) H2S(g) (b) MgCO3(s)

(a)

Interpretation Introduction

Interpretation:

The standard entropy change for formation of one mole H2S(g) should be calculated.

Concept introduction:

Entropy is a measure of the randomness of the system. It is a thermodynamic quantity and an extensive property. It is represented by the symbol S. It can also be defined as the degree of energy dispersal. More the dispersal in energy, more is the value if entropy.

The standard entropy change for any reaction is the sum of standard molar entropies of product, subtracted from the sum of standard molar entropies of reactants. The standard molar entropies are multiplied by the stoichiometric coefficient which is as per the balanced equation.

ΔrS°=nS°(products)nS°(reactants)

Explanation

The standard entropy change for the formation of 1mol of H2S(g) is calculated as follows,

Given: 1mol of H2S(g)

The standard entropy of H2S(g) is 205.79 J/Kmol

The standard entropy of H2(g) is 130.7 J/Kmol

The standard entropy of S(g) is 167.83 J/Kmol

The balanced chemical equation for the evaporation of ethanol is H2(g)+S(g)H2S(g)

The expression for the standard entropy change is as below,

ΔrS°=nS°(products)nS°(reactants)=[(1 mol H2S(g)/mol-rxn)S°[H2S(g)]

(b)

Interpretation Introduction

Interpretation:

The standard entropy change for formation of one mole MgCO3(s) should be calculated.

Concept introduction:

Entropy is a measure of the randomness of the system. It is a thermodynamic quantity and an extensive property. It is represented by the symbol S. It can also be defined as the degree of energy dispersal. More the dispersal in energy, more is the value if entropy.

The standard entropy change for any reaction is the sum of standard molar entropies of product, subtracted from the sum of standard molar entropies of reactants. The standard molar entropies are multiplied by the stoichiometric coefficient which is as per the balanced equation.

ΔrS°=nS°(products)nS°(reactants)

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