Discussion
The enthalpies of hydration of Ca2+, Mn2+ and Zn2+ follow the order
Answer: D
The order of hydration enthalpy (in magnitude) is Zn2+ > Mn2+ > Ca2+.
Hydration enthalpy grows in magnitude as the cation becomes smaller and its effective nuclear charge rises. All three ions are dipositive. Ca2+ is the largest (about 100 pm) and is hydrated least strongly. Mn2+ is smaller, and high-spin d5 gives it no crystal field stabilisation. Zn2+ is smaller again (about 74 pm) with a higher effective nuclear charge across the 3d series, so it is hydrated most strongly. The values are roughly -1580, -1840 and -2050 kJ mol-1 for Ca, Mn and Zn.
Any order placing Ca2+ above a 3d ion ignores its larger radius, and putting Mn2+ above Zn2+ ignores the contraction across the period.
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