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Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

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BuyFindarrow_forward

Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

Calculate the standard entropy changes for the following processes using the entropy values in Appendix L. Are the signs of the calculated values of ΔrS° in accord with predictions?

  1. (a) Dissolving 1 mol of NH4Cl(s) in water: NH4Cl(s) → NH4Cl(aq)
  2. (b) Oxidation of ethanol: C2H5OH(g) + 3 O2(g) → 2 CO2(g) + 3 H2O(g)

(a)

Interpretation Introduction

Interpretation:

The standard entropy changes for the following given NH4Cl(s)NH4Cl(aq) process should be calculated using the entropy values in the appendix L. It should be identifed that the sign of calculated ΔrSo is in accordance with the predicted.

Concept introduction:

Entropy: 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 of 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 dissolving 1 mol of NH4Cl(s) in water is calculated as follows,

Given: NH4Cl(s)NH4Cl(aq)

The Appendix L was referred for the values of entropies.

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

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

The given balanced chemical equation is: NH4Cl(s)NH4Cl(aq)

The stoichiometric coefficient of NH4Cl(s) and NH4Cl(aq) is 1:1.

The expression for the standard entropy change is,

  ΔrS°nS°(products)nS°(reactants)=[(1 mol NH4Cl(

(b)

Interpretation Introduction

Interpretation:

The standard entropy changes for the given C2H5OH(g)+3O2(g)2CO2(g)+3H2O(g) process should be calculated using the entropy values in the appendix L. It should be identifed that the sign of calculated ΔrSo is in accordance with the predicted.

Concept introduction:

Entropy: 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 of 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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