   Chapter 18, Problem 94SCQ

Chapter
Section
Textbook Problem

For each of the following processes, predict the algebraic sign of ΔrH°, ΔrS°, and ΔrG°. No calculations are necessary; use your common sense. (a) The decomposition of liquid water to give gaseous oxygen and hydrogen, a process that requires a considerable amount of energy. (b) Dynamite is a mixture of nitroglycerin, C3H5N3O9, and diatomaceous earth. The explosive decomposition of nitroglycerin gives gaseous products such as water, CO2, and others; much heat is evolved. (c) The combustion of gasoline in the engine of your car, as exemplified by the combustion of octane. 2 C8H18(g) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)

(a)

Interpretation Introduction

Interpretation:

The algebraic sign of ΔrHoΔrSo and ΔrGo for the given reaction should be predicted without using calculations.

Concept introduction:

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

The sign of ΔSo should be positive for an entropy-favored reaction and the sign of ΔH should be negative for an enthalpy favored-reaction.

ΔrHo is negative for an exothermic reaction and positive for endothermic reactions. The entropy change is positive whenever number of moles of gases is increasing in any reaction.

Explanation

The decomposition of liquid water to give gaseous oxygen and hydrogen requires a considerable amount of energy. When energy is being absorbed during any reaction then such reactions are endothermic in nature. ΔrH° is positive for endothermic reactions.

The decomposition of liquid water tends to increase the number of moles of gases. In the gaseous state, the particles are more random. In solids, the particles have fixed positions and entropy is less. In liquids, there are constraints due to forces between the particles. Thus, the sign of ΔrS° for the process of decomposition of liquid water is positive.

The Gibbs free energy is related to entropy and entropy by the following expression,

ΔGo= ΔHo

(b)

Interpretation Introduction

Interpretation:

The algebraic sign of ΔrHoΔrSo and ΔrGo for the given reaction should be predicted without using calculations.

Concept introduction:

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

The sign of ΔSo should be positive for an entropy-favoured reaction and the sign of ΔH should be negative for an enthalpy favoured-reaction.

ΔrHo is negative for an exothermic reaction and positive for endothermic reactions. The entropy change is positive whenever number of moles of gases is increasing in any reaction.

(c)

Interpretation Introduction

Interpretation:

The algebraic sign of ΔrHoΔrSo and ΔrGo for the given reaction should be predicted without using calculations.

Concept introduction:

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

The sign of ΔSo should be positive for an entropy-favoured reaction and the sign of ΔH should be negative for an enthalpy favoured-reaction.

ΔrHo is negative for an exothermic reaction and positive for endothermic reactions. The entropy change is positive whenever number of moles of gases is increasing in any reaction.

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