Given exothermic reaction
`CoCl_4^(2-)(aq)+6H_2O(l) hArr [Co(H_2O)_6]^(2+) +4Cl^-`
Which one of the following will decrease the equilibrium concentration of `CoCl_4^(2-)` ?

Options

A. Addition of HCI
B. Addition of `Co(NO_3)_2`
C. The solution is diluted with water
D. The temperature is increased

Answer

To solve the problem, we need to analyze the given exothermic reaction and determine which action will decrease the equilibrium concentration of `CoCl_4^(2-)`. The reaction is:

\[ \text{CoCl}_4^{2-} (aq) + 6 \text{H}_2\text{O} (l) \rightleftharpoons [\text{Co}(\text{H}_2\text{O})_6]^{2+} + 4 \text{Cl}^- \]

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- The reaction involves the conversion of `CoCl_4^(2-)` and water into a complex ion `[Co(H_2O)_6]^(2+)` and chloride ions `Cl^-`.
- Since the reaction is exothermic, heat is released when the reaction proceeds to the right (towards products).

2. **Le Chatelier's Principle**:
- According to Le Chatelier's Principle, if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the equilibrium will shift in a direction that counteracts the change.

3. **Analyzing Each Option**:
- **Option 1: Addition of HCl**:
- Adding HCl increases the concentration of `Cl^-` ions. According to Le Chatelier's Principle, the equilibrium will shift to the left to reduce the concentration of `Cl^-`, resulting in an increase in `CoCl_4^(2-)`. Thus, this does not decrease the concentration of `CoCl_4^(2-)`.

- **Option 2: Addition of Co(NO3)2**:
- Adding cobalt ions (`Co^(2+)`) does not affect the equilibrium concentration of `CoCl_4^(2-)` because it is present on both sides of the reaction. Therefore, this will not decrease the concentration of `CoCl_4^(2-)`.

- **Option 3: Addition of Water**:
- Adding water dilutes the reactants. This increases the concentration of the reactants, which will shift the equilibrium to the right to produce more products. As a result, the concentration of `CoCl_4^(2-)` will decrease. This is the correct answer.

- **Option 4: Raising the Temperature**:
- Since the reaction is exothermic, increasing the temperature will shift the equilibrium to the left (towards the reactants) to absorb the added heat. This will increase the concentration of `CoCl_4^(2-)`, not decrease it.

4. **Conclusion**:
- The action that will decrease the equilibrium concentration of `CoCl_4^(2-)` is the addition of water.

### Final Answer:
The addition of water will decrease the equilibrium concentration of `CoCl_4^(2-)`.