Acids neutralise bases. They have a pH < 7 and liberate CO2 from carbonates. A characteristic of an acid is that it has the ability to transfer H+ ions to something else. A substance which accepts H+ is called a base. The theory of H+ transfer is known as the Bronsted-Lowry theory of acids and bases.
Since a hydrogen atom contains only one proton and one electron, a H+ ion corresponds to one proton. Hence we sometimes refer to acids as proton donors and bases as proton acceptors.
An alkali is a special kind of base, because it dissolves in water to produce hydroxide ions, OH-.
When hydrogen chloride is added to water a reaction takes place and ions are produced:
The HCl is donating H+ to the water, hence it is acting as an acid. The water is accepting H+, so it is acting as a base. H3O+ is called an oxonium ion. It is present in every solution of an acid in water. The familiar properties of an acid are due to the presence of the oxonium ion.
Once an acid has donated an H+ ion, there is always the chance that it will take it back again. When ethanoic acid is added to water, ethanoate ions and oxonium ions are formed. Ethanoate ions can accept an H+ ion and go back to ethanoic acid. The reaction is reversible. In this reaction, the ethanoate ion is acting as a base. It is called the conjugate base of ethanoic acid.
Every acid has a conjugate base, and every base has a conjugate acid. They are called a conjugate acid-base pair.
If we represent a general acid as HA, then we have:
|conjugate acid||conjugate base|
The conjugate base of HCl is Cl-; the conjugate base of H2SO4 is HSO4-.
Not all acids have the same strength. Some are powerful H+ donors and are called strong acids. Those which are weaker donors are called weak acids. A strong acid has a weak conjugate base; a weak acid has a strong conjugate base. Examples of strong acids are hydrochloric acid, sulphuric acid and nitric acid. Examples of weak acids are ethanoic acid, citric acid and oxalic acid. Sodium hydroxide and potassium hydroxide are strong bases because they readily form OH- ions in solution. On the contratry, ammonia is a weak base.
Many acid-base indicators are weak acids. The conjugate acid and base forms have different colours. We can represent an inicator as HIn which ionises in solution:
|HIn (aq)||H+ (aq)||+||In- (aq)|
|Colour A||Colour B|
When acid, H3O+, is added, the reaction is shifted to the left, producing colour A.
When alkali, OH-, is added, this neutralises the H+(aq) and the reaction shifts to the right, producing colour B.
Litmus paper is an example of this, shewing red in its base state, when an acid is present, and it shews blue in its acidic state - when a stronger base is present.
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