   Chapter 18, Problem 30PS

Chapter
Section
Textbook Problem

Classify each of the reactions according to one of the four reaction types summarized in Table 18.1. (a) C6H12O6(s) + 6 O2(g) → 6 CO2(g) + 6 H2O(ℓ) ΔrH° = −673 kj/mol-rxn ΔrS° = 60.4 j/K · mol-rxn (b) MgO(s) + C(graphite) → Mg(s) + CO(g) ΔrH° = 490.7 kJ/mol-rxn ΔrS° = 197.9 J/K · mol-rxn TABLE 18.1 Predicting Whether a Reaction Will Be Spontaneous Under Standard Conditions (a)

Interpretation Introduction

Interpretation:

The reaction C6H12O6(s) + 6O2(g)6CO2(g) + 6H2O(l) 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 the ΔS and ΔH values.

Explanation

The given reaction is,

C6H12O6(s) + 6O2(g)6CO2(g) + 6H2O(l)

The value of ΔrH is 673 kJ/mol- rxn

(b)

Interpretation Introduction

Interpretation:

The reaction MgO(s)+C(graphite)Mg(s)+CO(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 the ΔS and ΔH values.

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