Proton affinity
The proton affinity (PA, Epa) of an
These reactions are always
terms in contrast to the proton affinity.Acid/base chemistry
The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase. The (reportedly) strongest known base is the
Hydration
Proton affinities illustrate the role of
An editor has expressed concern that this table does not seem to have correct values and some of them seem to be heat of formation instead of enthalpy. may have a number of irrelevant and questionable citations. (May 2022) |
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 | [6] | HF(g) | → | H+(g) | + F−(g) | +1554 kJ/mol | [4] | H2(g) | → | H+(g) | + H−(g) | +1675 kJ/mol | [4] | ||
Hydration of acid | HHe+(aq) | → | HHe+(g) | +973 kJ/mol | [10] | HF(aq) | → | HF(g) | +23 kJ/mol | [7] | H2(aq) | → | H2(g) | −18 kJ/mol | [11] | |||||
Hydration of proton | H+(g) | → | H+(aq) | −1530 kJ/mol | [7] | H+(g) | → | H+(aq) | −1530 kJ/mol | [7] | H+(g) | → | H+(aq) | −1530 kJ/mol | [7] | |||||
Hydration of base | He(g) | → | He(aq) | +19 kJ/mol | [11] | F−(g) | → | F−(aq) | −13 kJ/mol | [7] | H−(g) | → | H−(aq) | +79 kJ/mol | [7] | |||||
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.
Difference from pKa
Both proton affinity and
A second difference arises in noting that pKa reflects a thermal free energy for the proton transfer process, in which both enthalpic and entropic terms are considered together. Therefore, pKa is influenced both by the stability of the molecular anion, as well as the entropy associated of forming and mixing new species. Proton affinity, on the other hand, is not a measure of free energy.
List of compound affinities
Proton affinities are quoted in kJ/mol, in increasing order of gas-phase basicity of the base.
References
- Compendium of Chemical Terminology.
- Compendium of Chemical Terminology.
- PMID 30034765.
- ^ .
- Brønsted acid.
- ^ a b Lias, S. G.; Liebman, J. F.; Levin, R. D. (1984). Title J. Phys. Chem. Ref. Data. 13':695.
- ^ ISBN 0-07-112651-1.
- .
- ISBN 9780470132425.
- ^ Estimated to be the same as for Li+(aq) → Li+(g).
- ^ a b Estimated from solubility data.
- ISSN 1522-2675.
- ISSN 0021-9584.
- ISBN 0-07-112651-1
- ^ "Proton affinity of SO3".