   Chapter 21, Problem 99GQ

Chapter
Section
Textbook Problem

Metals react with hydrogen halides (such as HCl) to give the metal halide and hydrogen: M(s) +   n  HX(g)  →  MX n ( s ) +   1 2 n  H 2 (g) The free energy change for the reaction is ΔrG° = ΔfG°(MXn) – nΔfG°[HX(g)].(a) ΔfG° for HCl(g) is −95.1 kJ/mol. What must be the value for ΔfG° for MXn for the reaction to be product-favored at equilibrium?(b) Which of the following metals is (are) predicted to have product-favored reactions with HCl(g): Ba, Pb, Hg, Ti?

(a)

Interpretation Introduction

Interpretation:

The value for ΔfG° for MXn for the reaction to be product favored at equilibrium is to be stated.

Concept introduction:

The change of Gibbs free energy to form one mole of a substance from its constituent elements when all the substances in the standard form is known as standard Gibbs free energy of formation (ΔfG°).

The standard Gibbs free energy of reaction (ΔrG) in terms of standard Gibbs free energy of formation (ΔfG°) is written as,

ΔrG=ΔfGproductsΔfGreactants

If the value of standard Gibbs free energy of reaction (ΔrG) is positive then the reaction is reactant favored. If the value of standard Gibbs free energy of reaction (ΔrG) is negative then the reaction is product favoured and spontaneous.

Explanation

Any metal M reacts with hydrogen chloride to give metal halide and hydrogen. The chemical equation of any metal M with hydrogen chloride is written as,

M(s)+nHCl(g)MCln(s)+12nH2(g) (1)

The standard Gibbs free energy of reaction (ΔrG) in terms of standard Gibbs free energy of formation (ΔfG°) is written as,

ΔrG=ΔfG(MCln)nΔfG(HCl) (2)

The standard Gibbs free energy of formation (ΔfG°) for HCl is 95.1kJmol1

(b)

Interpretation Introduction

Interpretation:

Among the metals, Ba, Pb, Hg and Ti which is/are predicted to have a product-favored reaction with HCl is to be stated.

Concept introduction:

The change of Gibbs free energy to form one mole of a substance from its constituent elements when all the substances in the standard form is known as standard Gibbs free energy of formation (ΔfG°).

The standard Gibbs free energy of reaction (ΔrG) in terms of standard Gibbs free energy of formation (ΔfG°) is written as,

ΔrG=ΔfGproductsΔfGreactants

If the value of standard Gibbs free energy of reaction (ΔrG) is positive then the reaction is reactant favoured. If the value of standard Gibbs free energy of reaction (ΔrG) is negative then the reaction is product favoured and spontaneous.

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