Question

When hydrochloric acid is added to cobalt nitrate solution at room temperature, 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 correct answer.

[Co(H₂O)₆]³⁺(aq) +4Cl⁻(aq) ⇋ [CoCl₄]²⁻(aq)+ 6H₂O(l)

A. ∆H > 0 for the reaction
B. ∆H < 0 for the reaction
C. ∆H = 0 for the reaction
D. the sign of ∆H cannot be predicated on the basic of this information

Answer 1

The equilibrium between the pink and blue cobalt complexes indicates that the reaction is exothermic. Therefore, the correct answer is B. ΔH < 0 for the reaction, meaning heat is released as the system shifts towards the pink complex upon cooling. Option number a is correct.

Step-by-step explanation:

When hydrochloric acid is added to cobalt nitrate solution, two different complexes are in equilibrium: the pink [Co(H2O)6]3+ and the blue [CoCl4]2-. As the solution cools and turns pink, it indicates that the equilibrium shifts towards the [Co(H2O)6]3+ complex, suggesting an exothermic reaction where ΔH is less than zero (ΔH < 0).

Therefore, the correct answer is B. ΔH < 0 for the reaction, because the system absorbs heat upon cooling, which shifts the equilibrium to favor the pink complex.