Chapter 16, Problem 74QAP

Interpretation Introduction

(a)

Interpretation:

The standard Gibbs free energy ${\text{ΔG}}^{\text{0}}$ for the following reaction needs to be deduced at T = 25 °C

$\text{ZnS(s)}\to \text{Zn(s) + S(s)}$. From the calculated value of the free energy, the non-feasibility of the above reaction needs to be demonstrated.

Concept introduction:

1. The change in the Gibbs free energy, ΔG is a thermodynamic function which governs the spontaneity of a chemical reaction. The negative value of ΔG represents that the reaction is spontaneous, whereas, the positive value represents that the reaction is non-spontaneous, and if ΔG = 0, then it represents that the reaction is at equilibrium.
2. The standard Gibbs free energy, ${\text{ΔG}}^{\text{0}}$ is the value measured under standard conditions i.e. Pressure = 1 atm and Temperature = 25° C
3. The standard Gibbs free energy ${\text{ΔG}}^{\text{0}}$ is given in terms of the difference in the standard Gibbs Free energy of formation of the products and that of reactants.

$\Delta {\text{G}}^0\text{ = }\sum {\text{n}}_{\text{p}}{\text{ΔG}}_{\text{f}}^{\text{0}}\text{(products) - }\sum {\text{n}}_{\text{r}}{\text{ΔG}}_{\text{f}}^{\text{0}}\text{(reactants) ------(1)}$

Where n_p and n_r are the number of moles of the products and reactants.

Interpretation Introduction

(b)

Interpretation:

The standard Gibbs free energy ${\text{ΔG}}^{\text{0}}$ for the overall reaction needs to be deduced from the following two contributing reactions. It needs to be demonstrated that the overall reaction is feasible at T = 25 °C.

$\text{ZnS(s)}\to \text{Zn(s) + S(s)}$

${\text{S(s) + O}}_{\text{2}}\text{(g)}\to {\text{ SO}}_{\text{2}}\text{(g) }$

Concept introduction:

Free energy changes for reactions are additive. If a net reaction is a composite of two reactions with free energy represented as, ΔG₁ and ΔG₂ respectively, then the free energy change for the net reaction would be:

${\text{ΔG = ΔG}}_{\text{1}}{\text{ + ΔG}}_{\text{2}}\text{ ------(2)}$