The reducing power of a metal depends on various factors. Suggest the factor which makes Li, the strongest reducing agent in aqueous solution.

Options

A. Sublimation enthalpy
B. Ionisation enthalpy
C. Hydration enthalpy
D. Electron-gain enthalpy

Explanation

To determine why lithium (Li) is the strongest reducing agent in aqueous solution, we need to analyze the factors that influence the reducing power of metals. The reducing power is primarily affected by three key factors: sublimation enthalpy, ionization enthalpy, and hydration enthalpy. Let's break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding Reducing Power**:
- The reducing power of a metal refers to its ability to donate electrons. A stronger reducing agent can more readily lose electrons and thus reduce other substances.

2. **Identifying Relevant Factors**:
- The three main factors affecting reducing power are:
- **Sublimation Enthalpy**: Energy required to convert a solid into a gas.
- **Ionization Enthalpy**: Energy required to remove an electron from a gaseous atom.
- **Hydration Enthalpy**: Energy released when gaseous ions are solvated by water.

3. **Analyzing Sublimation Enthalpy**:
- For alkali metals, the sublimation enthalpy is relatively similar across the group. Thus, this factor does not significantly differentiate lithium from other alkali metals in terms of reducing power.

4. **Analyzing Ionization Enthalpy**:
- Lithium has a high ionization enthalpy because it is a small atom, and its outermost electron is held tightly by the nucleus. This means that it requires a considerable amount of energy to remove this electron, making it less favorable for lithium to act as a reducing agent based solely on ionization enthalpy.

5. **Analyzing Hydration Enthalpy**:
- Lithium has a very high hydration enthalpy. When lithium ions (Li⁺) are formed, they release a significant amount of energy when they are solvated by water. This hydration process is highly exothermic and more than compensates for the energy required for ionization.

6. **Conclusion**:
- The high hydration enthalpy of lithium is the key factor that makes it the strongest reducing agent in aqueous solution. The energy released during hydration is so substantial that it overcomes the energy required for ionization, allowing lithium to effectively donate electrons.

### Final Answer:
The factor that makes lithium the strongest reducing agent in aqueous solution is its **high hydration enthalpy**, which is highly exothermic and compensates for the endothermic ionization enthalpy.

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