When hydrochloric aicd is addded to cobalt and nitrate solution at room temperautre, the following reaction takes place and the reaction mixture becomes blue. On cooling the mixture it becomes pink. On the basis of this information mark the corect ansewer.
`underset("pink")[[Co(H_(2)O)_(6)]]^(3+)(aq)+4CI^(-)hArr` `underset("blue")[CoCI_(4)]^(2-)(aq)+6H_(2)O(l)`

Options

A. `DeltaHgt0` for the reaction
B. `Delta Hlt0` for the reaction
C. `Delta H=0` for the reaction
D. The sign of `DeltaH` cannot be predicted on the basis of this information

Explanation

To solve the problem, we need to analyze the given chemical reaction and the observations made during the experiment. Let's break it down step by step.

### Step 1: Understand the Reaction
The reaction provided is:
\[ \text{[Co(H}_2\text{O)}_6]^{3+}(aq) + 4 \text{Cl}^-(aq) \rightleftharpoons \text{[CoCl}_4]^{2-}(aq) + 6 \text{H}_2\text{O}(l) \]
At room temperature, the mixture is blue, indicating the formation of the complex \(\text{[CoCl}_4]^{2-}\).

### Step 2: Identify the Color Change
When hydrochloric acid is added, the cobalt complex changes from pink to blue. The pink color corresponds to the \(\text{[Co(H}_2\text{O)}_6]^{3+}\) complex, while the blue color corresponds to the \(\text{[CoCl}_4]^{2-}\) complex.

### Step 3: Analyze the Effect of Cooling
Upon cooling the reaction mixture, the color changes back to pink. This suggests that the reaction is shifting back to the left, favoring the formation of the pink complex \(\text{[Co(H}_2\text{O)}_6]^{3+}\).

### Step 4: Determine the Nature of the Reaction
Since cooling the mixture causes the reaction to shift to the left (endothermic direction), we can conclude that the forward reaction (formation of \(\text{[CoCl}_4]^{2-}\)) is endothermic. This means that heat is absorbed in the forward reaction.

### Step 5: Conclusion
Based on the observations and analysis, we conclude that the reaction is endothermic, as the forward reaction absorbs heat. Therefore, the correct answer is that the reaction is endothermic.

### Final Answer
The reaction is endothermic.

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