Question

Which reaction is most spontaneous?

• A. $G{a}^{3+}+2e^{-}\to G{a}^{+}$
• B. $I{n}^{3+}+2e^{-}\to I{n}^{+}$
• C. $T{l}^{3+}+2e^{-}\to T{l}^{+}$
• D. Cannot be predicted

Answer 1

Answer: (C)

$T{l}^{3+}+2e^{-}\to T{l}^{+}$

The inertness of s subshell electrons towards the bond formation is called inert pair effect or it can be said as the inactiveness of electrons present in outermost shell (i.e. $n{s}^2$) to get unpaired and involve in bond formation is called inert pair effect.

for example:

1) In 13th group, thallium can exhibit +1 and+3 oxidation states but it is stable in +1 oxidation state only due to inert pair effect

2) In 14th group , lead shows both +2 and +4 oxidation states but it is stable in +2 oxidation state due to inert pair effect.

Due to this inert pair effect,

Stability of $G{a}^{+}<I{n}^{+}<T{l}^{+}$
and $G{a}^{3+}>I{n}^{3+}>T{l}^{3+}$
and $T{I}^{+}>T{l}^{3+}$
Thus, $T{l}^{3+}$ salts are easily reduced to $T{l}^{+}$.

So most spontaneous reaction is
$T{l}^{3+}++2e^{-}\to T{l}^{+}$