   Chapter 18, Problem 29PS

Chapter
Section
Textbook Problem

Classify each of the reactions according to one of the four reaction types summarized in Table 18.1. (a) Fe2O3(s) + 2 Al(s) → 2 Fe(s) + Al2O3(s) ΔrH° = −851.5 kj/mol-rxn ΔrS° = −375.2 J/K · mol-rxn (b) N2(g) + 2 O2(g) → 2 NO2(g) ΔrH° = 66.2 kJ/mol-rxn ΔrS° = −121.6 J/K · mol-rxn TABLE 18.1 Predicting Whether a Reaction Will Be Spontaneous Under Standard Conditions (a)

Interpretation Introduction

Interpretation: The reaction Fe2O3(s)+2Al(s)2Fe(s)+Al2O3(s) should be classified based on reaction types discussed in Table 18.1 on the basis of given ΔS and ΔH values.

Concept introduction: The universe consists of two parts, systems and surroundings. The entropy change for the universe is the sum of entropy change for the system and for surroundings.

ΔS(universe)= ΔS(system)+ΔS(surroundings)

The ΔS(universe) should be greater than zero for a spontaneous process.

The  ΔS(system) can be calculated by the following expression,

ΔS(system)=ΔrS°=nS°(products)nS°(reactants)

The ΔS(surroundings) can be calculated by the following expression,

ΔS(surroundings)=ΔrHT

Here, ΔrH is the enthalpy change for the reaction.

Thus, a reaction can be categorized as spontaneous or non-spontaneous on the basis of ΔS and ΔH values.

Explanation

Referring Table 18.1 the given reaction classified as follows,

The given reaction is,

Fe2O3(s) + 2Al(s)2Fe(s) + Al2O3(s)

The value of ΔrH is 851.5 kJ/mol- rxn

(b)

Interpretation Introduction

Interpretation: The reaction N2(g) + 2O2(g)2NO2(g) should be classified based on reaction types discussed in Table 18.1 on the basis of given ΔS and ΔH values.

Concept introduction: The universe consists of two parts, systems and surroundings. The entropy change for the universe is the sum of entropy change for the system and for surroundings.

ΔS(universe)= ΔS(system)+ΔS(surroundings)

The ΔS(universe) should be greater than zero for a spontaneous process.

The  ΔS(system) can be calculated by the following expression,

ΔS(system)=ΔrS°=nS°(products)nS°(reactants)

The ΔS(surroundings) can be calculated by the following expression,

ΔS(surroundings)=ΔrHT

Here, ΔrH is the enthalpy change for the reaction.

Thus, a reaction can be categorized as spontaneous or non-spontaneous on the basis of ΔS and ΔH values.

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