   Chapter 18, Problem 20PS

Chapter
Section
Textbook Problem

Use S° values to calculate the standard entropy change, ΔrS°, for each of the following processes, and comment on the sign of the change. (a) NH4C1(s) → NH4Cl(aq) (b) CH3OH(ℓ) → CH3OH(g) (c) CCl4(g) → CCl4(ℓ) (d) NaCl(s) → NaCl(g)

(a)

Interpretation Introduction

Interpretation:

The standard entropy change for given NH4Cl(s)NH4Cl(aq) process should be calculated using So values.

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 reaction NH4Cl(s)NH4Cl(aq) is calculated below.

Given: NH4Cl(s)NH4Cl(aq)

The Appendix L referred for values of standard entropies and entered as below,

The standard entropy of NH4Cl(s) is 94.85 J/Kmol.

The standard entropy of NH4Cl(aq) is 169.9 J/Kmol.

The balanced chemical equation is,

NH4Cl(s)NH4Cl(aq)

The expression for the standard entropy change is,

ΔrS°=nS°(products)nS°(reactants)=[[(1 mol NH4Cl(aq)/mol-rxn)S°[NH4Cl(aq)]]

(b)

Interpretation Introduction

Interpretation:

The standard entropy change for given CH3OH(l)CH3OH(g) process should be calculated using So values.

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)

(c)

Interpretation Introduction

Interpretation:

The standard entropy change for given CCl4(g)CCl4(l) process should be calculated using So values.

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)

(d)

Interpretation Introduction

Interpretation:

The standard entropy change for given NaCl(s)NaCl(g) process should be calculated using So values.

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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