Proton donors and acceptors
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The definition of an acid as proton donor and a base as a proton acceptor was proposed each by Johannes Brønsted and Thomas Lowry, independently in 1923. This theory is appropriate for most school studies of acids and bases chemistry. The Brønsted Lowry definition is still in line with the Arrhenius or classical definitions of an acid. However, it is broader and extends to include many more ions and molecules that were previously excluded from the Arrehnius definition.
Consider some examples:
HCl(g) + H2O(l) H3O+ + Cl-(aq)
Identify the proton donor and proton acceptor in the reaction above.
HCl is the proton donor because it releases a proton to water. It is the Brønsted Lowry acid. Water accepts the proton from HCl. It is the Brønsted Lowry base.
Identify the proton donor and proton acceptor here: CH3CO2H + H2O(l) CH3CO2–(aq) + H3O+(aq)
CH3CO2H is the Brønsted Lowry acid, dontating a proton to water. Water is a Brønsted Lowry base, accepting a proton from the acid.
Identify the Brønsted Lowry acid and Brønsted Lowry base here: NH3(g) + H2O(l) NH4+(aq) + OH–(aq)
Ammonia is the Brønsted Lowry base, accepting a proton from water. Water is the Brønsted Lowry acid, releasing a proton to water.
Water can act as a Brønsted Lowry acid or a Brønsted Lowry base, either donating or accepting a proton, depending upon its environment.