Select the correct alternative from the choices given :
The cell reaction is spontaneous or feasible when e.m.f. of the cell is :

Options

A. negative
B. positive
C. zero
D. either positive or negative

Explanation

To determine when the cell reaction is spontaneous or feasible based on the e.m.f. (electromotive force) of the cell, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the relationship between Gibbs free energy and e.m.f.:**
The Gibbs free energy change (ΔG) for a cell reaction is related to the e.m.f. (E°) of the cell by the equation:
\[ \Delta G = -nFE° \]
where:
- \( n \) = number of moles of electrons transferred in the reaction (always positive),
- \( F \) = Faraday's constant (approximately 96500 C/mol, also positive),
- \( E° \) = e.m.f. of the cell.

2. **Determine the condition for spontaneity:**
For a reaction to be spontaneous, the change in Gibbs free energy (ΔG) must be negative:
\[ \Delta G < 0 \]
From the equation, this means:
\[ -nFE° < 0 \]
Since \( n \) and \( F \) are both positive, the only way for the product to be negative is if \( E° \) is positive:
\[ E° > 0 \]

3. **Evaluate the options:**
- **Negative:** If \( E° \) is negative, then ΔG would be positive, indicating a non-spontaneous reaction.
- **Positive:** If \( E° \) is positive, then ΔG would be negative, indicating a spontaneous reaction.
- **Zero:** If \( E° \) is zero, ΔG would also be zero, which means the reaction is at equilibrium and not spontaneous.
- **Either positive or negative:** This option is incorrect because only a positive e.m.f. leads to spontaneity.

4. **Conclusion:**
The correct alternative is that the cell reaction is spontaneous when the e.m.f. of the cell is **positive**.

### Final Answer:
The correct alternative is **positive**.

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