   Chapter 18, Problem 96SCQ

Chapter
Section
Textbook Problem

Use values of ΔfG° for solid and gaseous iodine at 25 °C (Appendix L) to calculate the equilibrium vapor pressure of iodine at this temperature.

Interpretation Introduction

Interpretation:

The equilibrium vapor pressure of iodine at 25 °C should be calculated using ΔfGo.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔrGo. It can be calculated in a similar manner as entropy and enthalpy. The expression for the free energy change is:

ΔrG°fG°(products)fG°(reactants)

ΔGo is related to the equilibrium constant K by the equation, ΔrGo= - RT lnKp

Explanation

The equilibrium vapor pressure of iodine at equilibrium is calculated as follows,

The Appendix L was referred for the values of standard free energy values.

The given reaction is, I2(s)I2(g)

The standard entropy change for I2(s) is zero, thus ΔrGo for the reaction is equal to the ΔfGo of iodine gas.

Thus, the ΔrGo value is 19.327 kJ/mol-rxn.

ΔGo is related to the equilibrium constant K by the equation,

ΔrGo= -RT lnKp

Here, lnKp=pI2(g)pI2(s)

Substituting the respective values,

ΔrGo= - (0.008314 kJ/K×mol)(298 K)ln(pI2(g)pI2(s))

The value of pI2(s) is zero

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