Question

Consider following reactions:

I. $\left[Co\right(N{H}_3{)}_6{]}^{3+}\left(aq\right)+6H_3{O}^{+}\left(aq\right)\rightleftharpoons \left[Co\right(H_{2}O{)}_6{]}^{3+}\left(aq\right)+6N{H}_4^{+}\left(aq\right)$

The products are favoured in the above reaction.

II. $\left[Co\right(N{H}_3{)}_6{]}^{2+}\left(aq\right)+6H_3{O}^{+}\left(aq\right)\to \left[Co\right(H_{2}O{)}_6{]}^{2+}\left(aq\right)+6N{H}_4^{+}\left(aq\right)$

The reactions (II) takes place with $H_{2}O$ in a matter of seconds.

Select correct statement:

• A. $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ is thermodynamically and kinetically stable relative to $\left[Co\right(H_{2}O{)}_6{]}^{3+}$
• B. $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ is thermodynamically unstable but kinetically stable relative to $\left[Co\right(H_{2}O{)}_6{]}^{3+}$
• C. $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ is thermodynamically and kinetically unstable relative to $\left[Co\right(H_{2}O{)}_6{]}^{3+}$
• D. $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ is thermodynamically stable but kinetically unstable relative to $\left[Co\right(H_{2}O{)}_6{]}^{3+}$

Answer 1

The correct option is B $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ is thermodynamically unstable but kinetically stable relative to $\left[Co\right(H_{2}O{)}_6{]}^{3+}$

Option (B) is correct. $\left[Co\right(N{H}_3{)}_6{]}^{3+}$ is thermodynamically unstable but kinetically stable relative to $\left[Co\right(H_{2}O{)}_6{]}^{3+}$.

The thermodynamic stability of complex depends on its formation constant ($\beta$); higher is its formation constant, greater the stability. The kinetic nature of complex depends on nature of other reactants, temperature, solvent etc.

$\left[Co\right(N{H}_3{)}_6{]}^{3+}$ is thermodynamically unstable as it has low $\beta$ value. This is possible because its reaction with $H_{2}O$ in acidic medium is slow. Thus, it is inert and kinetically stable relative to $\left[Co\right(H_{2}O{)}_6{]}^{3+}$.