   Chapter 18, Problem 61GQ

Chapter
Section
Textbook Problem

Calculate the entropy change, ΔrS°, for the vaporization of ethanol, C2H5OH, at its normal boiling point, 78.0 °C. The enthalpy of vaporization of ethanol is 39.3 kJ/mol.

Interpretation Introduction

Interpretation:

The ΔrS for a vaporization of ethanol should be calculated under given conditions.

Concept introduction:

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

ΔrG=ΔrHTΔrS

Here, ΔrH is the change in enthalpy and ΔrS is the change in entropy.

The value of ΔrG is zero at the boiling point of the liquid. Thus the entropy change can be written as,

ΔrS=ΔrHT

Explanation

The ΔrS for a vaporization of ethanol at its boiling point is calculated below.

The given reaction is,

C2H5OH(l)C2H5OH(g)

The boiling point is 78°C.

The enthalpy of vaporization of ethanol is 39.3 kJ/mol.

The expression for free energy change is,

ΔrG=ΔrHTΔrS

At the boiling point the value of ΔrG is zero

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