Question

Which of the following 3d-metal ion will give the lowest enthalpy of hydration $\left({\Delta }_{\text{hyd}}H\right)$ when dissolved in water?

Answer 1

	Ion	$\Delta H_{\text{Hyd}}^{\circ }.\left(kJ/mole\right)$
(i)	$C{r}^{2+}$	$-1925$
(ii)	$M{n}^{2+}$	$-1862$
(i)	$F{e}^{2+}$	$-1998$
(i)	$C{o}^{2+}$	$-2079$

Hydration enthalpy (ꕔH_Hyd) is the change in enthalpy when one mole of gaseous ion under a standard condition of 1 bar pressure dissolves in a sufficient amount of water to form an infinitely dilute solution.
ꕔH_Hyd is inversely proportional to the size of the ion, hence out of the given options $M{n}^{2+}$ has the highest ionic radii. Therefore it will have the lowest hydration enthalpy. (as evident from the data)