JEE Main 2023 · 6 April · evening · Single correct · +4 / −1
Ion having highest hydration enthalpy among the given alkaline earth metal ions is :
- (a) Sr²⁺
- (b) Ca²⁺
- (c) Be²⁺
- (d) Ba²⁺
Answer and solution
Answer (c) Be²⁺
Solution
Chemistry ma’am · 3 steps
All four ions carry +2, so the smallest ion attracts water most strongly. That is Be²⁺.
- What decides it. Hydration enthalpy is the energy released when water molecules cluster around a gaseous ion. It grows with the ion's charge density, that is, its charge divided by its size.
- Same charge here. Every ion in the options is +2, so size alone decides. Ionic radius increases down the group, so hydration enthalpy decreases down the group.
Ion Ionic radius Hydration enthalpy Be²⁺ 31 pm -2494 kJ mol⁻¹ Ca²⁺ 100 pm -1577 kJ mol⁻¹ Sr²⁺ 118 pm -1443 kJ mol⁻¹ Ba²⁺ 135 pm -1305 kJ mol⁻¹ - So. Be²⁺ is by far the smallest ion, so it has the highest charge density and releases the most energy on hydration. The answer is (c).
The idea to keep. For ions of the same charge, the smaller ion has the higher hydration enthalpy. Down a group, ions get bigger, so hydration enthalpy falls: Be > Mg > Ca > Sr > Ba.
Answer from the NTA final answer key. Solution by PrepWiser.