Identify the incorrect statement among the following ?

Options

A. A salt bridge is used to eliminate liquid junction potential in a galvanic cell
B. The Gibbs free energy change , `DeltaG` is related with electromotive force (E), as `DeltaG=-nFE`
C. Nernst equation for single electrode potential is given by `E=E^@-(RT)/(nF)In[M^(n+)]`
D. The maximum theoretical efficiency of a hydrogen oxygen fuel cell is 33%

Answer

To solve the question of identifying the incorrect statement among the given options, we will analyze each statement related to galvanic cells and fuel cells step by step.

### Step 1: Understand Liquid Junction Potential
- **Definition**: Liquid junction potential (LJP) occurs when two solutions of electrolytes with different concentrations are in contact, leading to a potential difference due to the movement of ions.
- **Relevance**: In galvanic cells, LJP can interfere with the accurate measurement of electromotive force (EMF).

**Hint**: Remember that LJP arises from concentration differences and affects measurements in electrochemical cells.

### Step 2: Role of Salt Bridge
- **Function**: A salt bridge is used to eliminate liquid junction potential by allowing ions to flow between two half-cells, maintaining electrical neutrality and enabling current flow.
- **Composition**: Commonly, a saturated solution of potassium chloride (KCl) is used as the salt bridge.

**Hint**: A salt bridge helps maintain charge balance in galvanic cells and prevents LJP.

### Step 3: Gibbs Free Energy and EMF Relationship
- **Formula**: The relationship between Gibbs free energy (ΔG) and electromotive force (E) is given by the equation: ΔG = -nFE, where n is the number of moles of electrons transferred and F is Faraday's constant.
- **Interpretation**: A positive E indicates a spontaneous reaction (ΔG < 0).

**Hint**: The sign of ΔG is crucial in determining the feasibility of a reaction.

### Step 4: Nernst Equation
- **Nernst Equation**: The Nernst equation relates the electrode potential to the concentrations of reactants and products. For a half-reaction, it is expressed as:
\[ E = E^\circ - \frac{RT}{nF} \ln \left(\frac{[\text{products}]}{[\text{reactants}]}\right) \]
- **Application**: This equation is essential for calculating the potential of electrochemical cells under non-standard conditions.

**Hint**: Understand how concentration affects the potential of an electrochemical cell.

### Step 5: Efficiency of Hydrogen-Oxygen Fuel Cell
- **Theoretical Efficiency**: The theoretical maximum efficiency of a hydrogen-oxygen fuel cell is nearly 100%. However, in practice, the actual efficiency is lower, around 70%.
- **Key Point**: The statement claiming that the theoretical efficiency is not nearly 100% is incorrect.

**Hint**: Differentiate between theoretical and actual efficiencies in fuel cells.

### Conclusion
After analyzing all the statements, the incorrect statement is related to the theoretical efficiency of a hydrogen-oxygen fuel cell, which is stated to be nearly 100%. This is indeed correct, but the confusion arises when comparing it to actual efficiency, which is lower.

### Final Answer
The incorrect statement is option 4, which incorrectly implies that the theoretical efficiency of a hydrogen-oxygen fuel cell is less than 100%.