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1

### AIPMT 2007

MCQ (Single Correct Answer)
The efficiency of a fuel cell is given by
A
$$\Delta$$G/$$\Delta$$S
B
$$\Delta$$G/$$\Delta$$H
C
$$\Delta$$S/$$\Delta$$G
D
$$\Delta$$H/$$\Delta$$G

## Explanation

Efficiency of a fuel cell ($$\phi$$) = $${{\Delta G} \over {\Delta H}} \times 100$$

Generally, fuel cells are expected to have an efficiency of 100 percent.
2

### AIPMT 2007

MCQ (Single Correct Answer)
The equilibrium constant of the reaction:
Cu(s) + 2Ag+(aq) $$\to$$ Cu2+(aq) + 2Ag(s);
Eo = 0.46 V at 298 K is
A
2.0 $$\times$$ 1010
B
4.0 $$\times$$ 1010
C
4.0 $$\times$$ 1015
D
2.4 $$\times$$ 1010

## Explanation

RT ln K = nFE°

ln K = $${{nFE^\circ } \over {RT}}$$

= $${{2 \times 0.46} \over {0.0591}}$$

$$\Rightarrow$$ K = 4 $$\times$$ 1015
3

### AIPMT 2006

MCQ (Single Correct Answer)
A hypothetical electrochemical cell is shown below.

$$A\left| {{A^ + }\left( {xM} \right)} \right|\left| {{B^ + }\left( {yM} \right)} \right|B$$

The emf measured is + 0.20 V. The cell reaction is
A
A + B+ $$\to$$ A+ + B
B
A+ + B $$\to$$ A + B+
C
A+ + e$$-$$ $$\to$$ A;  B+ + e$$-$$ $$\to$$ B
D
the cell reaction cannot be predicted.

## Explanation

From the given expression:

At anode : A $$\to$$ A+ + e

At cathode : B+ + e $$\to$$ B

Overall reaction is : A + B+ $$\to$$ A+ + B
4

### AIPMT 2006

MCQ (Single Correct Answer)
EoFe2+/Fe = $$-$$ 0.441 V and EoFe3+/Fe2+ = 0.771 V, the standard EMF of the reaction Fe + 2Fe3+ $$\to$$ 3Fe2+ will be
A
0.111 V
B
0.330 V
C
1.653 V
D
1.212 V

## Explanation

At anode : Fe $$\to$$ Fe2+ + 2e-; Eo = -0.441 V

At cathode : Fe3+ + e $$\to$$ Fe2+; Eo = 0.771 V

Fe + 2Fe3+ $$\to$$ 3Fe2+; Eo = ?

To get the above equation, (ii) × 2 – (i)

2Fe3+ + 2e $$\to$$ 2Fe2+; Eo = 0.771 V

Fe $$\to$$ Fe2+ + 2e-; Eo = - 0.441 V
------------------------------------------------
Fe + 2Fe3+ $$\to$$ 3Fe2+

Eo = 0.771 + 0.441 = 1.212 V

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Class 12