What is an acid?

We can think of an acid as a substance that neutralises a base to form a salt and water and also a base as a substance that neutralises an acid. For example:

2HCl(aq) + MgO(s) → MgCl2(aq) + H2O(l)

However, if we analyse the formulae of a number of acids shown below we can see that:

  • they all contain hydrogen atoms.
  • they all ionise to produce H+ ions when dissolved in water.
Name of the acidFormula of the acidIons formed when dissolved in water
Hydrochloric acidHClH+ and Cl
Nitric acidHNO3H+ and NO3
Sulphuric acidH2SO42H+ and SO42–
Ethanoic acidCH3COOHH+ and CH3COO
Benzoic acidC6H5COOHH+ and C6H5COO

From the information in the table above, we can define of an acid as follows:

An acid a substance that releases hydrogen ions as the only positively charged ions when dissolved in water.

What are bases and alkalis?

Metal oxides and hydroxides are called bases. Some bases are soluble in water and they dissolve to form hydroxide ions (OH) in solution. They are called alkalis.

An alkali is a base that is soluble in water.

One example of an alkali is sodium hydroxide.

NaOH(s) → Na+(aq) + OH(aq)

Ammonia gas is a base though it is not an oxide or hydroxide of a metal. When dissolved in water, some of the ammonia molecules react with water molecules to release hydroxide ions. In the process, ammonia accepts a hydrogen ion to become an ammonium ion NH4+.

NH3(g) + H2O(l) ⇌ NH4+(aq) + OH(aq)

Therefore aqueous ammonia is therefore an alkali. Another fact highlighted by the reaction of ammonia with water is that:

A base is a substance that accepts hydrogen ions.

The Brønsted–Lowry definition of acids and bases

The Danish chemist J. Brønsted and the English chemist T. Lowry
suggested another definition of acids and bases that does not involve their respective reactions with water.

According to the Brønsted–Lowry definition of acids and bases:

  • An acid is a proton donor.
  • A base is a proton acceptor.

A proton is a hydrogen ion (H+).


Water as a Brønsted–Lowry base

When hydrogen chloride gas dissolves in water to form hydrochloric acid, it reacts with water to form hydroxonium ions, H3O+, and chloride ions, Cl.

HCl(g) + H2O(l) → H3O+(aq) + Cl(aq)

Hydrochloric acid is an acid because it donates a proton to water and this also means that water is acting as a Brønsted–Lowry base because it is accepting a proton.

Water as a Brønsted–Lowry acid

Water can also act as an acid. For example, in the reaction of ammonia with water, ammonia accepts a proton from the water and becomes an NH4+ ion.

NH3(g) + H2O(l) ⇌ NH4+(aq) + OH(aq)

In the above reaction, water is acting as a Brønsted–Lowry acid because it is donating a proton.

Substances like water, which can act as either acids or bases, are called amphoteric.

Conjugate acids and conjugate bases

In an equilibrium reaction at equilibrium, the products are converted to reactants at the same rate as reactants are converted to products. We can apply the Brønsted–Lowry theory of acids and bases to the reversible reaction between ammonia and water.

NH3(g) + H2O(l) ⇌ NH4+(aq) + OH(aq)

In this reaction, ammonia accepts a proton from the water and becomes an NH4+ ion, with means:

  • NH3 is acting as a base.
  • H2O is acting as an acid.

Now if we consider the reverse reaction:

NH4+(aq) + OH(aq) ⇌ NH3(g) + H2O(l)

the NH4+ ion donates a proton to the OH ion, which means:

  • NH4+ is acting as an acid.
  • OH is acting as a base.

If a reactant is linked to a product by the transfer of a proton we call this pair a conjugate pair, and in a conjugate pair, the acid has one proton more.

Therefore, in the reaction of ammonia and water:

  • NH4+ is a conjugate acid of the base NH3.
  • OH is a conjugate base of acid H2O.

Let us consider the reaction between hydrogen chloride and water as another example:

HCl(g) + H2O(l) → H3O+(aq) + Cl(aq)

For the forward reaction:

  • HCl donates a proton to form Cl
    • which means Cl is the conjugate base of the acid HCl.
  • H2O accepts a proton to form H3O+
    • which means H3O+ is the conjugate acid of the base H2O.

For the reverse reaction:

  • Cl accepts a proton to form HCl
    • which means HCl is the conjugate acid of the base Cl.
  • H3O+ donates a proton to form H2O
    • which means H2O is the conjugate base of the acid H3O+.