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J. W. Hill, R. H. Petrucci, T. W. McCreary, & S. S. Perry General Chemistry 4th edition

Problem 18.53

Write the equilibrium constant expression for each of the following reactions, and determine the numerical value of Keq at 25 oC.

(a) PbO2(s) + 4 H+(aq) + 2 Cl(aq)Pb2+(aq) + 2 H2O(l) + Cl2(g)

(b) 3 O2(g) + 2 Br(aq)2 BrO3(aq) (basic solution)




Answer:
(a) PbO2(s) + 4 H+(aq) + 2 Cl(aq)Pb2+(aq) + 2 H2O(l) + Cl2(g)

The reaction is already balanced so the mass action expression for the equilibirum constant is easily found by inspection. Since a thermodynamic equilibrium constant is required, aqueous components are given as concentrations and gases are given as partial pressures (pure liquids and pure solids are ignored):

To find the value of this equilibrium constant, use the Table of Standard Reduction Potentials to find the cell potential and then use this value to find the equilibrium constant.

PbO2(s) + 4 H+(aq) + 2 ePb2+(aq) + 2 H2O(l)

Eored = 1.455 V

2 Cl(aq) Cl2(g) + 2 e

Eoox = –1.358 V

Eocell = 1.455 – 1.358 = 0.097 V

 

Keq = exp(nFEocell/RT)

n = 2
F = 96485 coul/mol
R = 8.314 J/mol•K
T = 298 K
 

Keq = exp[(2)(96485)(0.097)/(8.314)(298)] = 1900

(b) 3 O2(g) + 2 Br(aq)2 BrO3(aq) (basic solution)

The reaction is already balanced so the mass action expression for the equilibirum constant is easily found by inspection. Since a thermodynamic equilibrium constant is required, aqueous components are given as concentrations and gases are given as partial pressures (pure liquids and pure solids are ignored):

To find the value of this equilibrium constant, use the Table of Standard Reduction Potentials to find the cell potential and then use this value to find the equilibrium constant.

O2(g) + 2 H2O(l) + 4 e4 OH(aq)

Eored = 0.401 V

Br(aq) + 6 OH(aq)BrO3(aq) + 3 H2O(l) + 6 e

Eoox = –0.584 V

Eocell = 0.401 – 0.584 = –0.183 V

 

Keq = exp(nFEocell/RT)

n = 12
F = 96485 coul/mol
R = 8.314 J/mol•K
T = 298 K
 

Keq = exp[(12)(96485)(–0.183)/(8.314)(298)] = 7×10–38