The proton affinity, E_pa, of a anion or of a neutral atom or molecule is a measure of its gas-phase basicity. It is the energy released in the following reactions:^[1] An anion is an ion with negative charge. ... Properties In chemistry and physics, an atom (Greek ἄτομος or átomos meaning indivisible) is the smallest particle still characterizing a chemical element. ... In science, a molecule is a group of atoms in a definite arrangement held together by chemical bonds. ...

A⁻ + H⁺ → HA

B + H⁺ → BH⁺

These reactions are always exergonic in the gas phase, i.e. energy is released when the reaction advances in the direction shown. However, proton affinities are conventionally quoted with the opposite sign convention from most other thermodynamic properties, a positive E_pa indicating a release of energy by the system. This is the same sign convention as is used for electron affinity. Look up exergonic in Wiktionary, the free dictionary. ... In physics, a sign convention is a choice of the signs (plus or minus) of a set of quantities, in a case where the choice of sign is arbitrary. ... The electron affinity, Eea, of an atom or molecule is the energy required to detach an electron from a singly charged negative ion, that is the energy change for the process X- → X + e- An equivalent definition is the energy released (Einitial âˆ’ Efinal) when an electron is attached to a...

The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase. The strongest known base is the methanide anion (E_pa = 1743 kJ/mol), slightly stronger than the hydride ion (E_pa = 1675 kJ/mol),^[2] making methane the weakest proton acid^[3] in the gas phase, followed by dihydrogen. The weakest known base is the helium atom (E_pa = 177.8 kJ/mol),^[4] making the hydrohelium(1+) ion the strongest known proton acid. A hydride is a compound of hydrogen with more electropositive elements. ... Methane is a chemical compound with the molecular formula CH4. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Standard atomic weight 4. ...

Proton affinities illustrate the role of hydration is aqueous-phase Brønsted acidity. Hydrofluoric acid acid is a weak acid in aqueous solution (pK_a = 3.15)^[5] but a very weak acid in the gas phase (E_pa (F⁻) = 1554 kJ/mol):^[2] the fluoride ion is a strong a base as SiH₃⁻ in the gas phase, but its basicity is reduced in aqueous solution because it is strong hydrated, and therefore stabilized. The contrast is even more marked for the hydroxide ion (E_pa = 1635 kJ/mol),^[2] 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 (pK_a = ca. 30).^[6] In chemistry, hydration is the condition of being combined with water. ... A Brønsted-Lowry acid (sometimes shortened to Brønsted acid) is an acid that donates a hydrogen ion to another compound, called a Brønsted-Lowry base. ... Hydrofluoric acid is a solution of hydrogen fluoride in water. ... Fluoride is the ionic form of fluorine. ... Hydroxide is a polyatomic ion consisting of oxygen and hydrogen: OH− It has a charge of −1. ... The chemical compound potassium hydroxide, (KOH) sometimes known as caustic potash, potassa, potash lye, and potassium hydrate, is a metallic base. ... Dimethyl sulfoxide (DMSO) is the chemical compound (CH3)2SO. This colorless liquid is an important dipolar aprotic solvent. ... french: triphényl méthane english: triphenyl methane other: methane triphenyl Boiling point: 78,2 C (from CRC Handbook) Categories: Chemistry stubs ...

To a first approximation, the proton affinity of a base in the gas phase can be seen as offsetting (usually only partially) the extermely favorable hydration energy of the gaseous proton (ΔE = −1530 kJ/mol), as can be seen in the following estimates of aqueous acidity:

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 pK_a of dihydrogen is in agreement with the behaviour of saline hydrides (e.g., sodium hydride) when used in organic synthesis. Sodium hydride is a highly flammable, and corrosive chemical compound with formula NaH and CAS number 7646-69-7. ... Organic synthesis is the construction of organic molecules via chemical processes. ...

See also

• Proton affinity (data page)

References

1. ^ "Proton affinity." Compendium of Chemical Terminology.
2. ^ ^{a b c d e} Bartmess, J. E.; Scott, J. A.; McIver, R. T. (1979). J. Am. Chem. Soc. 101:6046.
3. ^ The term "proton acid" is used to distinguish these acids from Lewis acids. It is the gas-phase equivalent of the term Brønsted acid.
4. ^ ^{a b} Lias, S. G.; Liebman, J. F.; Levin, R. D. (1984). J. Phys. Chem. Ref. Data. 13':695.
5. ^ ^{a b c d e f g} Jolly, William L. (1991). Modern Inorganic Chemistry (2nd Edn.). New York: McGraw-Hill. ISBN 0-07-112651-1.
6. ^ Jolly, William L. (1967). J. Chem. Educ. 44:304. Jolly, William L. (1968). Inorg. Synth. 11:113.
7. ^ Estimated to be the same as for Li⁺(aq) → Li⁺(g).
8. ^ ^{a b} Estimated from solubility data.