Question 1

Why does Li⁺ have the highest hydration enthalpy among alkali metal ions?

Accepted Answer

Li⁺ is the smallest alkali metal cation (ionic radius ≈ 76 pm), so its positive charge is concentrated in a tiny volume — very high charge density. This pulls water dipoles in strongly, forming many ion–dipole bonds and releasing the most energy. Its hydration enthalpy is about −506 kJ/mol, the most negative of the group, decreasing down to Cs⁺ at about −276 kJ/mol.

Question 2

Why is the hydrated radius of Li⁺ the largest even though it is the smallest ion?

Accepted Answer

This is the classic inversion trap. Bare ionic radius increases Li⁺ → Cs⁺, but the hydrated radius does the opposite. Because Li⁺ has the highest charge density, it grabs the thickest shell of water molecules around it — its hydrated radius is about 340 pm versus a bare radius of only 76 pm. Cs⁺ is large and weakly hydrating, so its hydrated radius (≈ 226 pm) is actually the smallest.

Question 3

Why does Li⁺ have the lowest ionic mobility in water?

Accepted Answer

Mobility depends on the hydrated size, not the bare size. Li⁺ carries the largest hydration shell, so the effective moving particle is the biggest and slowest — it has to drag its whole entourage of water along. Cs⁺ has the thinnest shell, so it moves fastest. The mobility order is therefore Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺.

Question 4

Is hydration enthalpy always negative?

Accepted Answer

Yes. Hydration is the formation of ion–dipole attractions between the ion and surrounding water molecules, which is an energy-releasing (exothermic) process, so ΔH_hyd is always negative. The magnitude depends on charge density: higher charge and smaller size give a more negative value, which is why Mg²⁺ hydrates far more strongly than Na⁺.

Hydration Enthalpy — Chhota Ion, Bada Entourage

Practice

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