Hydration enthalpy is the energy released when one mole of gaseous ions is dissolved in water to form hydrated ions. For alkali metal ions, the order of hydration enthalpy is determined by their size and charge density.

Key points:

• Hydration enthalpy depends on charge density (charge/size ratio).
• Smaller ions have higher charge density, so they attract water molecules more strongly.
• Among alkali metals, ionic size increases down the group: ${\mathrm{Li}}_{+}^{}<{\mathrm{Na}}_{+}^{}<K_{+}^{}<{\mathrm{Rb}}_{+}^{}<{\mathrm{Cs}}_{+}^{}$
• Thus, charge density decreases down the group.
• Higher charge density leads to greater hydration enthalpy.

Therefore, the correct order is: ${\mathrm{Li}}_{+}^{}>{\mathrm{Na}}_{+}^{}>K_{+}^{}>{\mathrm{Rb}}_{+}^{}>{\mathrm{Cs}}_{+}^{}$

So, the correct option is: Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺

Related Topics

Hydration enthalpy is related to properties like solubility, ionic mobility, and lattice energy. It is also influenced by the nature of the ion and solvent.

Formulae

Hydration enthalpy (ΔH_hyd) is often calculated using Born equation:

where Z is charge, r is ionic radius, ε is dielectric constant of solvent.