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Facts, Summary & Definition

  • The Brønsted-Lowry theory defines an acid as a proton donor
  • Compounds which can act as both an acid and a base are called amphiprotic
  • The strength of an acid is measured using the pH scale
  • The Lewis theory defined an acid as an electron pair acceptor
  • Strong acids completely dissociate in water, and therefore have a large Ka value

What is a Brønsted-Lowry acid?

The Brønsted-Lowry theory defines an acid as a proton (H+) donor, and therefore, bases are proton acceptors. This is the most useful current theory of acids and bases. For a compound to be able to act as a Brønsted-Lowry acid, it must have a hydrogen atom which it can lose but still remain somewhat stable.

This definition can be expressed as an equilibrium equation, where HA is an acid:

HA + B ⇌ A- + HB+

The equilibrium sign is used because the reaction can happen both forward and backwards.

Some compounds can act as both acids and bases – such as water, because it can either lose a hydrogen atom to form OH- or gain one to form H3O+. These compounds are called amphiprotic.

The strength of a Brønsted-Lowry acid is measured using the pH scale.

What is a Lewis acid?

Sometimes there are conditions where the Brønsted-Lowry definitions of an acid doesn’t quite fit – usually in solids and gases. So, in 1923, G.N. Lewis proposed his alternate theory of acids based on structure and bonding.

He described an acid as a species that can accept an electron pair donated from a base.

When a Lewis acid and base react, they form a bond called a covalent bond – this is a type of chemical bond where one species donates a pair of electrons to the other. In this case, the base donates its electrons to the acid.

Lewis acids are called electrophilic because they attract electrons. Many species can act as Lewis acids, such as:

  • All cations because they can accept electrons (e.g. Cu2+ and Fe3+)
  • At atom, molecule, or ion with an incomplete set of electrons (e.g. BF3)

Other, more complicated species can act as Lewis acids too, such as molecules with multiple bonds between two atoms which have different charges.

Acid Strength

The strength of an acid is discussed in terms of its ability or tendency to lose a proton. Strong acids completely dissociate in water – this means that one mole of a strong acid can dissolve in water to form one mole of protons and one mole of the conjugate base. None of the conjugated acid remains.

In contrast, weak acids only partially dissociate in water and, when the reaction is in equilibrium, both the acid and the base are present in the solution.

Stronger acids have a larger Ka than weak acids. Therefore, Ka is used to measure the strength of an acid.

It’s important to note that ‘strong’ and ‘weak’ in terms of acidity does not refer to a solution being concentrated or dilute!


Further Reading