ionic equilibrium I - definition of acid and base

In this chapter besides being able to calculate equilibrium constants and equilibrium concentrations, we will need to know what is the definition of an acid (and base) as well as the strength of the acid (and base).

Acid and Base defintion

There are various definition of acid and base. Currently, we are apply the Bronsted-Lowry acid base theory, where the acid is a proton donor and the base is a proton acceptor.

Bronsted-Lowry acid:

HX + H₂O -> H₃O⁺ + X^-

In the above equation, you can see that HX has donated a proton (H⁺ to H₂O. Thus, it is an acid. At least, in the A level syllabus, we generally identify acids which has a hydrogen atom that is attached to an electronegative atom (except for N).

Hence, H₂SO₄, H₃PO₄ and CH₃COOH are acid based on the above pattern as all these compounds have an O-H bond. In addition, HF, HCl, HBr and HI are also acids.

If an acid has only one hydrogen atom attached to an electronegative atom, the acid is a monbasic acid (e.g HCl). While, H₂SO₄ is an example of a dibasic acid as it has two H-O bonds. Whereas, H₃PO4 can be seen as a tribasic acid as it has three H-O bonds.

Bronsted-Lowry base:

B + H₂O -> BH⁺ + OH^-

In the above equation, you can see that B has accepted a proton from H₂O. Hence, B is a base based on the above definition. As the proton do not contain an electron, hence in order for B to react with the H⁺, B must form a dative covalent bond with H. Hence, the presence of a donatable lone pair of electrons is required. Hence, the following molecules such as NH₃, CH₃NH₂ (= methylamine) and CH₃CH₂NH₂ (= ethylamine) are bases.

Hence, with the above paragraph we have introduced the Lewis base and it states that a base is someone who has a lone pair to donate.

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Article written by Kwok YL 2007 (edited in Apr 2009).

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