   Chapter 18, Problem 34PS

Chapter
Section
Textbook Problem

Using values of ΔfH° and S°, calculate the standard molar free energy of formation, ΔfG°, for each of the following: (a) Ca(OH)2(s) (b) Cl(g) (c) Na2CO3(s) Compare your calculated values of ΔfG° with those listed in Appendix L. Which of these formation reactions are predicted to be product-favored at equilibrium at 25 °C?

(a)

Interpretation Introduction

Interpretation:

The the standard molar free energy for formation of Ca(OH)2(s) should be calculated and compared with the values placed in appendix L. It should be identified that whether the reaction is product favored at equilibrium.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and enthalpy by the following expression,

ΔGo=ΔHo-TΔSo

The sign of ΔGo should be positive for a product-favored reaction. Thus, spontaneous reactions are referred to those that have negative free energy formation.

Explanation

The standard molar energy of formation for Ca(OH)2(s) is calculated below.

Given:

The Appendix L referred for the values of standard entropies and enthalpies.

Ca(s)+H2(g)+O2(g)Ca(OH)2(s)ΔfH°(kJ/mol)000-986.09So(J/K×mol)41.59130.7205.0783.39

The enthalpy change is expressed as,

ΔrH°fH°(products)fH°(reactants)[(1 mol Ca(OH)2(s)/mol-rxn)ΔfH°[Ca(OH)2(s)]-[(1 mol H2(g)/mol-rxn)ΔfH°[H2(g)]+(1 mol O2(g)/mol-rxn)ΔfH°[O2(g)]+(1 mol Ca(s)/mol-rxn)ΔfH°[Ca(s)]] ]

Substituting the values,

ΔrH°[(1 mol Ca(OH)2(s)/mol-rxn)(-986.09 kJ/mol)[(1 mol H2(g)/mol-rxn)(0 kJ/mol)+(1 mol O2(g)/mol-rxn)(0 kJ/mol)+(1 mol Na(s)/mol-rxn)(0 kJ/mol)] ]=-986

(b)

Interpretation Introduction

Interpretation:

The the standard molar free energy for formation of Cl(g) should be calculated and compared with the values placed in appendix L. It should be identified that whether the reaction is product favored at equilibrium.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and enthalpy by the following expression,

ΔGo=ΔHo-TΔSo

The sign of ΔGo should be positive for a product-favored reaction. Thus, spontaneous reactions are referred to those that have negative free energy formation.

(c)

Interpretation Introduction

Interpretation:

The the standard molar free energy for formation of Na2CO3(s) should be calculated and compared with the values placed in appendix L. It should be identified that whether the reaction is product favored at equilibrium.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and enthalpy by the following expression,

ΔGo=ΔHo-TΔSo

The sign of ΔGo should be positive for a product-favored reaction. Thus, spontaneous reactions are referred to those that have negative free energy formation.

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