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Question

For the galvanic cell at 25oC,
Ag|AgCl(s),KCl(0.2 M)||KBr(0.001 M),AgBr(s)|Ag

Which of the following(s) is/are true for the given cell?
Given:
Ksp(AgCl)=2.8×1010
Ksp(AgBr)=3.3×1013

A
At 25oC, Ecell=0.122 V, non-spontaneous cell reaction
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B
At 25oC, Ecell=0.037 V, non-spontaneous cell reaction
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C
At 25oC, Ecell=0.037 V, spontaneous cell reaction
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D
At 25oC, Ecell=0.122 V, spontaneous cell reaction
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Solution

The correct option is B At 25oC, Ecell=0.037 V, non-spontaneous cell reaction
From cell representation,
At anode,
AgL.H.SAg+L.H.S

At cathode,
Ag+R.H.S+eAgR.H.S

The cell reaction is
AgL.H.S+Ag+R.H.SAg+L.H.S+AgR.H.S

Nernst equation for the given concentration cell is
Ecell=E0cell0.0591 log Ag+L.H.SAg+R.H.S
For concentration cell,
E0cell=0

Ecell=0.0591 log[Ag+]RHS[Ag+]LHS.....eqn(1)

For
AgClAg++Cl
Ksp(AgCl)=2.8×1010
Ksp(AgCl)=[Ag+][Cl]
[Ag+]=Ksp(AgCl)[Cl]
Concentration of KCl is 0.2 M

[Cl]=0.2 M
[Ag+]=2.8×10100.2

[Ag+]LHS=14×1010 M

For
AgBrAg++Br
Ksp(AgBr)=3.3×1013
Ksp(AgBr)=[Ag+][Br]
[Ag+]=Ksp(AgBr)[Br]
Concentration of KBr is 0.001 M

[Br]=0.001 M
[Ag+]=3.3×10130.001

[Ag+]RHS=3.3×1010 M

Substituting Ag+ concentrations in equation (1) gives,

Ecell=0.0591 log3.3×101014×1010

Ecell=0.0591 log 0.235

Ecell0.037 V

The emf of cell is negative, hence, the cell reaction is non-spontaneous.

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