The concepts contained in this article sound complex: acid base proton transfer. But they are a lot simpler than they sound.
If a hydrogen atom Hº is ionized by stripping off its one electron, what remains is a proton. This small positive particle, symbol H⁺, is the heart of acid base proton transfer reactions.
But That’s Too Simple
In water, the H⁺ ion does not exist as a naked proton. Rather, it is united with a molecule of water in the form of a stabilized hydronium ion (H₃O⁺). So in aqueous solutions, hydronium ions are the proton donors.
Acid Base Proton Transfer
Proton transfer acceptors, rather than being proton poor, are electron rich. Still, since acids are proton donors, it logically follows that bases must be proton acceptors. Consider the reaction of hydrochloric acid and sodium hydroxide.
The reaction of these two compounds yields a large quantity of energy. In a sense, hydrochloric acid is the proton transfer donor and sodium hydroxide is the proton transfer acceptor. Over simplistically and unrealistically, the reaction is written,
HCl + NaOH → NaCl + H₂O
In aqueous media, the reaction can be written as a sequence of steps. These steps reveal more detail behind as to what is occurring:
HCl + H₂O ↔ H₃O⁺ + Cl⁻
NaOH + Water → Na⁺ + OH⁻
Na⁺ + Cl⁻ → NaCl
H₃O⁺ + OH⁻ → 2 H₂O
The first reaction generates the hydronium ion. In the second and third steps, no proton transfer occurs. It is the final reaction between hydronium and hydroxide ions, that involves both the proton donation and the proton acceptance. Yes, these processes occur, not at the molecular level, but at the ionic level.
Note: You might also enjoy What is a Hydronium Ion?