   Chapter 18, Problem 31PS

Chapter
Section
Textbook Problem

Using values of ΔfH° and S°, calculate ΔrG° for each of the following reactions at 25 °C. (a) 2 Pb(s) + O2(g) → 2 PbO(s) (b) NH3(g) + HNO3(aq) → NH4NO3(aq) Which of these reactions is (are) predicted to be product-favored at equilibrium? Are the reactions enthalpy- or entropy-driven?

(a)

Interpretation Introduction

Interpretation:

The standard free energy change ΔG for the given reaction should be calcualted using SoandΔfHo values.

Concept introduction:

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

ΔG=ΔHTΔS

Here, ΔH is the change in enthalpy and ΔS is the change in entropy.

Explanation

The standard free energy change for the reaction of lead and oxygen at 298 K is calculated below.

Given:

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

2Pb(s)+O2(g)2PbO(s)ΔfH°(kJ/mol)00219S( J/Kmol)+64.81+205.07+66.5

ΔrH°=nΔfH°(products)nΔfH°(reactants)=[(2 mol PbO(s)/mol-rxn)ΔfH°[PbO(s)][(2 mol Pb(s)/mol-rxn)ΔfH°[Pb(s)]+(1 mol O2(g)/mol-rxn)ΔfH°[O2(g)]] ]

Substituting the enthalpy values

ΔrH°=[(2 mol PbO(s)/mol-rxn)(219 kJ/mol)[(2 mol Pb(s)/mol-rxn)(0)+(1 mol O2(g)/mol-rxn)(0)] ] =438 kJ/mol-rxn

<

(b)

Interpretation Introduction

Interpretation:

The standard free energy change ΔG for the given reaction should be calcualted using SoandΔfHo values.

Concept introduction:

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

ΔG=ΔHTΔS

Here, ΔH is the change in enthalpy and ΔS is the change in entropy.

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