   Chapter 18, Problem 52GQ

Chapter
Section
Textbook Problem

Using standard entropy values, calculate ΔrS° for the formation of 1.0 mol of NH3(g) from N2(g) and H2(g) at 25 °C.

Interpretation Introduction

Interpretation:

The standard entropy changes for the formation of ammonia should be calculated.

Concept introduction:

Entropy is a measure of the randomness of the system. It is a thermodynamic quantity and an extensive property. It is represented by the symbol S. It can also be defined as the degree of energy dispersal. More the dispersal in energy, more is the value if entropy.

The standard entropy change for any reaction is the sum of standard molar entropies of product, subtracted from the sum of standard molar entropies of reactants. The standard molar entropies are multiplied by the stoichiometric coefficient which is as per the balanced equation.

ΔrS°=nS°(products)nS°(reactants)

Explanation

The standard entropy change for the formation of ammonia is calculated below.

Given:

The standard entropy of NH3(g) is 192.77 J/Kmol.

The standard entropy of N2(g) is 191.56 J/Kmol.

The standard entropy of H2(g) is 130.7 J/Kmol.

The balanced chemical equation for the formation of ammonia is:

12N2(g) + 32H2(g)NH3(g)

The expression for the standard entropy change is,

ΔrS°=nS°(products)nS°(reactants)=[[(1 mol NH3(g)/mol-rxn)S°[ NH3(g)]][(0

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