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.

$\left[\mathrm{CO}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}_3\right(\mathrm{aq})+4{\mathrm{Cl}}^{-}(\mathrm{aq})\leftrightharpoons {{\left[{\mathrm{COCl}}_4\right]}^2}^{-}(\mathrm{aq})+6{\mathrm{H}}_2\mathrm{O}(\mathrm{l}):$

• A. $△\mathrm{H}<0\mathrm{for}\mathrm{the}\mathrm{reaction}$
• B. $△\mathrm{H}>0\mathrm{for}\mathrm{the}\mathrm{reaction}$
• C. $△\mathrm{H}=0\mathrm{for}\mathrm{the}\mathrm{reaction}$
• D. None of these

Answer 1

The correct option is A

$△\mathrm{H}<0\mathrm{for}\mathrm{the}\mathrm{reaction}$

Explanation for correct option:

(a) $△\mathrm{H}<0\mathrm{for}\mathrm{the}\mathrm{reaction}$

Step 1:

• We have to write the color of each compound.

$$\begin{gathered} \left[\mathrm{CO}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6{]}_3\right(\mathrm{aq})+4{\mathrm{Cl}}^{-}(\mathrm{aq})\leftrightharpoons {{\left[{\mathrm{COCl}}_4\right]}^2}^{-}(\mathrm{aq})+6{\mathrm{H}}_2\mathrm{O}(\mathrm{l}) \\ (\mathrm{pink})(\mathrm{blue}) \end{gathered}$$

Step 2:

• As it is given that on cooling, the reaction shifts to the backward direction.
• This means that the backward reaction is an exothermic reaction.
• Therefore, the forward reaction is an endothermic reaction

Step 3:

• In an endothermic, a system of reactants takes heat from its environment and has a positive $∆\mathrm{H}$
• Hence, $△\mathrm{H}<0\mathrm{for}\mathrm{the}\mathrm{reaction}$
• Hence this option is correct

Explanation for incorrect options:

(b) $△\mathrm{H}>0\mathrm{for}\mathrm{the}\mathrm{reaction}$

• For a reaction $△\mathrm{H}<0$if it is an exothermic reaction
• Since the option is endothermic
• Hence this option is not correct

(c) $△\mathrm{H}=0\mathrm{for}\mathrm{the}\mathrm{reaction}$

• For a reaction $△\mathrm{H}=0$ if has no change in heat
• Since the option is endothermic
• Hence this option is not correct

(d) None of these

• this option is not applicable, since the option is endothermic
• Hence this option is not correct

So the correct option is (a) $△\mathrm{H}<0\mathrm{for}\mathrm{the}\mathrm{reaction}$