78 ENERGETICSlattice energy. A/ihyd(M+) and A/ihyd(X ) are the hydration enthalpies
of the ions M+ and X™. These require further consideration.Hydration enthalpiesWhen an ion is solvated the resulting solvated ion is more stable
than the original free ion. Consequently all hydration enthalpies
are negative; hydration is an exothermic process. Since we can
measure the enthalpy of solution and calculate the lattice energy,
we can determine the total hydration enthalpy of the ions. How-
ever, since we are unable to measure hydration enthalpies for
isolated ions, it is necessary to divide this enthalpy to give individual
values. This problem can be resolved by giving an arbitrary value to
the hydration enthalpy of one ionic species so that the others can be
obtained by difference. There are good grounds for using the proton
as the standard giving A/ihyd(H^) the value of — 1091 kJ mol l. On
this basis some hydration enthalpies are given below (kJmoP^1 ,
298 K):H+ -1091 Fe2+ -1946 Tl+ -326 Pb2+ -1481
Li+ -519 Co2+ -1996 Be^2 + -2494 A13+ -4665
Na+ -406 Ni2+ -2105 Mg2+ -1921 Fe3+ -4430
K+ -322 Cu2+ -2100 Ca2+ -1577 F~ -515
Rb+ -293 Zn2+ -2046 Sr2+ -1443 CP -381
Cs+ -264 Hg2+ -1824 Ba2+ -1305 Br^ -347
Ag+ -473 Sn^2 + -1552 Cr2+ -1904 r -305It will be noted that hydration enthalpy decreases with increasing
ionic radius and increases very sharply with increase in ionic
charge, these results being what we should expect for an electro-
state interaction between a charged ion and the dipole of a water
molecule (p. 44).Enthalpies of solution
The enthalpy of solution is quite small for many simple ionic
compounds and can be either positive or negative. It is the difference
between two large quantities, the sum of the hydration enthalpies
and the lattice energy.
Let us consider the halides of sodium and silver. The details of