Acids and bases I

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1) Bronsted-Lowry Theory of Acids and Bases

According to the Bronsted Lowry theory, an acid is defined as a substance that can donate proton to another substance, i.e. a proton donor. An example of an acid is hydrochloric acid. Students also need to know what a conjugate base is. An acid whhich has donated its proton becomes a conjugate base. Using hydrochloric acid as an example, hydrochloric acid is the acid and the chloride ion is the conjugate base. Using ethanol as an example, if ethanol behaves as a Bronsted acid, it loses its proton and becomes ethanoate ion. Therefore the conjugate base is the ethanoate ion.


A base is a substance which accepts a proton from another substance, i.e. a proton acceptor. An example is ammonia. Ammonia is a Bronsted base, it accepts a proton and becomes the ammonium ion. In this case ammonia is the base and the ammonium ion is the conjugate acid. Note that a bronsted base is related to its conjugate acid and a bronsted acid is related to its conjugate base.

Note that certain acids and bases do not conform to the bronsted acid and base definition. Take for example sodium hydroxide, it is well know that sodium hydroxide is a base. However NaOH is not a proton acceptor hence it is not a Bronsted base.

There is another definition of acids and bases proposed by Arrhenius. According to the Arrhenius definition, an acid dissociates in water to form hydrgen ions and a base dissociates in water to produce hydroxide ions. NaOH will fit the Arrhenius definiton of a base. There are other definitions such as the Lewis acid definition. For more information students can refer to the wikipedia entry.

2) Differences between a weak and strong acid.

A strong acid such as hydrochloric acid dissociates fully in water to form hydrogen ions and chloride ions. A weak acid dissociates partially in water. An example is ethanoic acid. When ethanoic acid is dissolved in water, some of the ethanoic acid molecules will dissociate to form the ethanoate ion and hydrogen ions. However not all of the ethanoic acid molecules dissociate so there will be ethanoic acid molecules in the wtaer in addition to ethanoate ions and protons.

Students must be careful not to mix up concentration and strength of acid. A concentrated acid might not be a strong acid. A 5 M ethanoic acid is a concentrated acid solution but ethanoic acid is not a strong acid.

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