Bronsted-Lowry Acids and Bases

The Bronsted-Lowry definition

Acids and bases are defined in a number of ways. The most useful one, however, is considered the to be the Bronsted-Lowry definition:

• an acid is a substance which can behave as a proton donor
• an alkali is a substance which can behave as a proton acceptor

It is possible to represent a proton as a hydrogen ion, H⁺. Therefore, a substance which is hydrogen bonded to an element containing more electronegativity can behave like an acid. For example, HCl ? H⁺ + Cl^–.

Whereas a species containing a lone pair of electrons can behave like a base. For example, NH₃ + H⁺ ? NH₄⁺.

Conjugate acid-base pairs

Reactions in which acids and bases form are reversible. Therefore, a species formed from an acid donating a proton can accept a proton and behave like a base. Likewise, a species formed from a base accepting a proton can donate a proton and behave like an acid. So:

• Cl^– + H⁺ ? HCl
• NH₄+ ? NH₃ + H⁺

Therefore, acids and bases come in pairs: all acids can lose a proton to form a base and all bases can accept a proton to form an acid. Two species related in this way are called conjugate acid-base pairs. So:

• Cl^– is the conjugate base of HCl and vice versa
• NH₃ is the conjugate base of NH₄⁺ and vice versa

However, not all acids are equally good proton donors and not all bases are equally good proton acceptors. In fact, the more easily an acid can lose protons, the worse its conjugate base will be at accepting them and the more easily a base can accept protons, the worse its conjugate acid will be at donating them. Therefore:

• a strong acid or base has a weak conjugate base
• a weak acid or base has a strong conjugate base

It is possible to rank acids and bases in order of their strength along with the conjugate base. Some species can behave as both an acid and a base and are said to be amphoteric.

Acid-base reactions

Unless there is a base available to accept the protons an acid will not give them away. Likewise, unless an acid is present to provide electrons, a base will not accept any.

Therefore, an acid-base reaction:

• acids react with bases to form the conjugate base of the reactant acid
• bases react with acids to form the conjugate acid of the reactant base

The reactants and conjugates have the following labels:

• reactant acid = acid 1
• conjugate base = base 1
• reactant base = base 2
• conjugate acid = acid 2

Therefore, a general acid-base reaction can be represented as:

acid 1 + base 2 ? base 1 + acid 2

For example:

HCl + H₂O ? H₃O⁺ + Cl^–

acid 1 base 2 acid 1 base 1

Substance which are amphoteric are able to undergo acid-base reactions with themselves. For example:

H₂O + H₂O ? H₃O + OH^–

acid 1 base 2 base 1 acid 2

An acid-base reaction which takes place in an aqueous solution will show which of the two acids is the strongest. For example;

HCl(aq) + H₂O(l) ? H₃O⁺(aq) + Cl^–(aq) goes 95% therefore HCl is stronger than H₃O⁺.