Question

Which statement defines the enthalpy of hydration?
the change in enthalpy when 1 mol of a gaseous ion becomes fully hydrated
the change in enthalpy when gas ions condense to form 1 mol of an ionic solid
the change in enthalpy as heat is lost or gained in a reaction
the change in enthalpy as temperature rises in a liquid

Answer 1

"the change in enthalpy when 1 mol of a gaseous ion becomes fully hydrated"

Enthalpy of hydration, Hhyd, of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinite dilute solution in the process,
Mz+(g) + mH2O ->Mz+(aq)
where Mz+(aq) represents ions surrounded by water molecules and dispersed in the solution. The approximate hydration energies of some typical ions are listed here. The table illustrates the point that as the atomic numbers increases, so do the ionic size, leading to a decrease in absolute values of enthalpy of hydration.
http://www.science.uwaterloo.ca/~cchieh/cact/applychem/hydration.html