Historical Development of Acids

Oxygen theory - All acids contained oxygen. Advantages - correlated to known acids at the time - e.g. H2SO4, and Lavoisier showed that many non–metal oxides, when dissolved in water, formed acids e.g. SO2. His theory was flawed as many non-metal oxides are basic, e.g. CaO.

Humphrey Davy in 1810 showed hydrohalic acids e.g. HCl contained no oxygen. He then proposed all acids contained hydrogen, however CH4 is neutral and NH3 is basic so his theory was wrong. His theory was later extended by a different scientist [who?] to say that acids contained replaceable hydrogens, e.g. hydrogen in HCl + NaOH -> H2O + NaCl replaced by Na. However this did not successfully explain

In 1884 Arrhenius postulated that acids dissolved in solution into ions. This however overlooks the role of the solvent since ionisation is a reaction between the acid molecule and the solvent. Additionally, acid-base reactions often don’t involve the acid ionising at all, such as hydrochloric acid when dissolved in benzene reacts with ammonia to form ammonium chloride but it does not ionise.

Brönsted and Lowry individually proposed in 1923 the definition which is widely used today, commonly known as the Brönsted-Lowry acid-base theory. Acids and bases are viewed as a pair where the acid is a proton (H+) donor and the base is the acceptor. This leads to the concept of acid-base conjugate pairs, where the conjugate acid is the base with a proton added, and the conjugate base is the acid with a proton removed. Hence, in
HCl(aq) + H2O(l) → Cl-(aq) + H3O+(aq)
The proton moves from hydrochloric to the water which is acting as a base, where the chlorine ion is the conjugate base and the hydronium ion is the conjugate acid. On the other hand, in
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
Ammonia, a base, takes a proton from water which now acts as an acid, forming the conjugate acid ammonium and hydroxide ion as the conjugate base.