For the feasibility of a redox reaction in a cell, the emf should be.

Options

A. Positive
B. Fixed
C. Zero
D. Negative

Answer

To determine the feasibility of a redox reaction in a cell, we need to analyze the relationship between the electromotive force (EMF) and the Gibbs free energy change (ΔG) for the reaction. Here’s a step-by-step solution:

### Step 1: Understand the Relationship Between ΔG and EMF
A redox reaction is spontaneous if the change in Gibbs free energy (ΔG) is negative. The relationship between ΔG and the EMF (E) of the cell can be expressed by the equation:

\[ \Delta G = \Delta G^\circ - nFE \]

Where:
- ΔG is the Gibbs free energy change.
- ΔG° is the standard Gibbs free energy change.
- n is the number of moles of electrons transferred in the reaction.
- F is Faraday's constant (approximately 96485 C/mol).
- E is the EMF of the cell.

### Step 2: Analyze the Conditions for Spontaneity
For the reaction to be spontaneous (ΔG < 0), we can rearrange the equation:

\[ \Delta G^\circ - nFE < 0 \]

This implies:

\[ \Delta G^\circ < nFE \]

### Step 3: Determine the Sign of EMF
If we want ΔG to be negative, it follows that:

\[ E > \frac{\Delta G^\circ}{nF} \]

If ΔG° is negative (which is often the case for spontaneous reactions), then for the entire expression to hold true, the EMF (E) must be positive.

### Conclusion
Thus, for a redox reaction to be feasible, the EMF of the cell must be positive.

### Final Answer
The EMF should be **positive**.