Which of the following reaction does not occur spontaneously?

Options

A. `Fe^(+3) +Cd rarrCd^(+2)+Fe^(+2)`
B. `Fe^(+3)+I^(-)rarrFe^(+2)+I_(2)`
C. `Fe^(+2)+MnO_(4)^(-) overset(H^(+))rarr Fe^(+3)+Mn^(+2)`
D. `Fe^(+3)+Ce^(+3)rarr Fe^(+2)+Ce^(+4)`

Answer

To determine which of the given reactions does not occur spontaneously, we need to analyze the standard reduction potentials of the species involved in each reaction. Here’s a step-by-step solution:

### Step 1: Identify the Species and Their Reduction Potentials
We need to write down the standard reduction potential values for the ions involved in the reactions. The relevant ions are:
- Fe³⁺ / Fe²⁺
- Cd²⁺ / Cd
- I⁻ / I₂
- MnO₄⁻ / Mn²⁺
- Ce³⁺ / Ce⁴⁺

### Step 2: List the Standard Reduction Potentials
The standard reduction potentials (E°) for these half-reactions (values may vary slightly based on different sources) are approximately:
- E°(Fe³⁺/Fe²⁺) = +0.77 V
- E°(Cd²⁺/Cd) = -0.40 V
- E°(I⁻/I₂) = +0.54 V
- E°(MnO₄⁻/Mn²⁺) = +1.51 V
- E°(Ce⁴⁺/Ce³⁺) = +1.61 V

### Step 3: Analyze Each Reaction
1. **Cd + Fe³⁺ → Cd²⁺ + Fe²⁺**
- Cd is a strong reducing agent (E° = -0.40 V), and Fe³⁺ is a stronger oxidizing agent (E° = +0.77 V).
- This reaction is spontaneous.

2. **I⁻ + Fe³⁺ → I₂ + Fe²⁺**
- I⁻ (E° = +0.54 V) can reduce Fe³⁺ (E° = +0.77 V) to Fe²⁺.
- This reaction is spontaneous.

3. **Fe²⁺ + MnO₄⁻ → Fe³⁺ + Mn²⁺**
- Fe²⁺ (E° = +0.77 V) can reduce MnO₄⁻ (E° = +1.51 V) to Mn²⁺.
- This reaction is spontaneous.

4. **Fe³⁺ + Ce³⁺ → Fe²⁺ + Ce⁴⁺**
- Ce³⁺ (E° = +1.61 V) is a weaker reducing agent compared to Fe³⁺ (E° = +0.77 V).
- Ce³⁺ cannot reduce Fe³⁺, making this reaction non-spontaneous.

### Conclusion
The reaction that does not occur spontaneously is:
**Fe³⁺ + Ce³⁺ → Fe²⁺ + Ce⁴⁺**