An acid-base reaction, also known as a neutralization reaction, is a proton transfer reaction between an acid (a proton donor) and a base (a proton acceptor). In general, acid-base reactions are typically exothermic and they produce salt and water.
In a majority of cases, the acid and base regents will dissociate to form ions which take part in the reaction mechanism and those which form the salt. For instance, in the aqueous reaction between hydrochloric acid and sodium hydroxide:
- HCl H+ + Cl-
- NaOH Na+ + OH-
Some of these ions will then partake in the proton transfer reaction, such that:
- H+ + OH- H2O
The remaining ions will form a salt, such that:
- Na+ + Cl- NaCl(aq)
Thus the overall reaction is:
- HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)
In this case the salt will remain in aqueous (dissolved) form, however if it was insoluble it would precipitate out of the water.
Not all neutralisation reactions involve the reactants dissociating. For example, ammonia (NH3) is a base which will accept a proton to form ammonium, and hence will undergo an acid-base reaction in its pure form, for example:
- NH3(aq) + H+(aq) NH4+(aq)
Acidity of the Salt
The salt which forms may have a pH of its own due due to its components being conjugate acids and conjugate bases. The conjugate acid of a strong base and the conjugate base of a strong acid are weak, and hence will not dissociate. However, the conjugates of weak acids/bases are strong bases/acids, and hence will undergo a hydrolysis reaction with water to form and acid/base. The general rule is:
- A strong acid and a strong base will form a neutral salt
- A strong acid and a weak base will form an acidic salt (will react with water to form an acid)
- A strong base and a weak acid will form a basic salt (will react with water to form a base)
For instance, a strong acid and weak base reaction may proceed as such:
- 2HCl + Fe(OH)2 2H2O + FeCl2
The iron component of the iron chloride may then undergo a hydrolysis reaction with water, such that:
- Fe2+ + H2O Fe(OH)2 + H+
Since the iron hydroxide is a weak base, it will not dissociate to any great extent and hence will not raise the pH of the water greatly. However, this hydrolysis reaction causes an excess of hydrogen ions, which result in the pH of the water being lowered, hence causing it to be acidic. As the chloride ion is a weak base it will not react with water, producing an acidic salt.
The same process applies for basic salts, however the products will contain an excess of OH- and a weak acid, causing the water to become basic.
There are several uses of acid-base neutralisation reactions.
- Titration: a volumetric analysis of acids and bases to determine an unknown concentration. As the acids/bases neutralise they will cause an ongoing change in pH of the solution producing a characteristic titration curve. This can be used to determine when equal moles of acid and base were added to the reaction vessel, which can in turn be used to determine the unknown concentration.
- Chemical spills: strong acids and bases can be harmful to the environment and should be neutralised as soon as possible. The most commonly used neutralising agent is sodium bicarbonate (NaHCO3) as it is amphiprotic (it can act as either an acid or a base).