Hydration
Proton affinities illustrate the role of hydration in aqueous-phase Brønsted acidity. Hydrofluoric acid is a weak acid in aqueous solution (pKa = 3.15) but a very weak acid in the gas phase (Epa (F−) = 1554 kJ/mol): the fluoride ion is as strong a base as SiH3− in the gas phase, but its basicity is reduced in aqueous solution because it is strongly hydrated, and therefore stabilized. The contrast is even more marked for the hydroxide ion (Epa = 1635 kJ/mol), one of the strongest known proton acceptors in the gas phase. Suspensions of potassium hydroxide in dimethyl sulfoxide (which does not solvate the hydroxide ion as strongly as water) are markedly more basic than aqueous solutions, and are capable of deprotonating such weak acids as triphenylmethane (pKa = ca. 30).
To a first approximation, the proton affinity of a base in the gas phase can be seen as offsetting (usually only partially) the extremely favorable hydration energy of the gaseous proton (ΔE = −1530 kJ/mol), as can be seen in the following estimates of aqueous acidity:
| Proton affinity | HHe+(g) | → | H+(g) | + He(g) | +178 kJ/mol | HF(g) | → | H+(g) | + F−(g) | +1554 kJ/mol | H2(g) | → | H+(g) | + H−(g) | +1675 kJ/mol | |||||
| Hydration of acid | HHe+(aq) | → | HHe+(g) | +973 kJ/mol | HF(aq) | → | HF(g) | +23 kJ/mol | H2(aq) | → | H2(g) | −18 kJ/mol | ||||||||
| Hydration of proton | H+(g) | → | H+(aq) | −1530 kJ/mol | H+(g) | → | H+(aq) | −1530 kJ/mol | H+(g) | → | H+(aq) | −1530 kJ/mol | ||||||||
| Hydration of base | He(g) | → | He(aq) | +19 kJ/mol | F−(g) | → | F−(aq) | −13 kJ/mol | H−(g) | → | H−(aq) | +79 kJ/mol | ||||||||
| Dissociation equilibrium | HHe+(aq) | → | H+(aq) | + He(aq) | −360 kJ/mol | HF(aq) | → | H+(aq) | + F−(aq) | +34 kJ/mol | H2(aq) | → | H+(aq) | + H−(aq) | +206 kJ/mol | |||||
| Estimated pKa | −63 | +6 | +36 | |||||||||||||||||
These estimates suffer from the fact the free energy change of dissociation is in effect the small difference of two large numbers. However, hydrofluoric acid is correctly predicted to be a weak acid in aqueous solution and the estimated value for the pKa of dihydrogen is in agreement with the behaviour of saline hydrides (e.g., sodium hydride) when used in organic synthesis.
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