Ammonia molecules are added to a solution of acetic acid. The acetic acid molecules transfer a hydrogen ion; that is, a proton, to the lone pair on the ammonia molecules. The products of the transfer are the ammonium ion and the acetate ion.
In the first part of this animation, gaseous hydrogen chloride is added to water. The hydrogen chloride molecules very rapidly transfer a hydrogen ion to water molecules. The extent of this reaction is also high--most of the hydrogen chloride molecules transfer a proton to water. In the second part, acetic acid molecules are added to water. Even though this reaction is also rapid, it does not occur to a very large extent. The equilibrium between hydronium ions and acetic acid molecules involves transfer of protons both from acetic acid to water and from hydronium ions to acetate ions. At equilibrium there are more acetic acid molecules than hydronium ions.
The diprotic acid hydrogen sulfide is added to water. The transfer of hydrogen ions from the hydrogen sulfide to water occurs to form hydronium ions and hydrogen sulfide ions. Some of the hydrogen sulfide ions then lose hydrogen ions to form the sulfide ion. At equilibrium, there is mainly hydrogen sulfide, with some hydrogen sulfide ions, and an even smaller number of sulfide ions.
This animation is designed to illustrate the amphiprotic character of the hydrogen carbonate ion. The ion can function as both an acid and as a base. It can release a hydrogen ion to water, leaving the carbonate ion and the hydronium ion. It can also accept hydrogen ions from water to form carbonic acid, which then decomposes to carbon dioxide and water.