   Chapter 18, Problem 21PS

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) HCl(g) (b) Ca(OH)2(s)

(a)

Interpretation Introduction

Interpretation:

The standard entropy changes for formation of one mole of the HCl at 25oC 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° = ΔnS°(products)ΔnS°(reactants)

Explanation

The standard entropy change for the formation of 1mol of HCl is calculated as follows,

Given:

The standard entropy of HCl(g) is 186.2 J/Kmol

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

The standard entropy of Cl2(g) is 223.08 J/Kmol

The balanced chemical equation for the formation of HCl is 12H2(g)+12Cl2(g)HCl(g).

The stoichiometric coefficient of H2(g) is 12, Cl2(g) is 12 and HCl(g) is 1.

The expression for the standard entropy change is,

ΔrS°=ΔnS°(products)ΔnS°(reactants)=(1 mol HCl(g)/mol-rxn)S°[HCl(g)]- [ (0

(b)

Interpretation Introduction

Interpretation:

The standard entropy changes for formation of one mole of the Ca(OH)2(s) at 25oC 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° = ΔnS°(products)ΔnS°(reactants)

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