CHEMISTRYCAFE

8.1 Arrhenius Theory

• An acid ionises in solution to form H3O+(hydronium) ions.

  • HA(aq) + H2O(l) ⇌ A- (aq) + H3O+(aq)

• A base ionises in solution to form OH- ion.

  • B(aq) + H2O(l)  ⇌ HB+(aq) + OH-(aq)

8.2 Bronsted-Lowry Theory **

• There are many definitions of acids and bases in existence, but the most useful one is the Bronsted-Lowry definition

• An acid is a substance which can behave as a proton donor. A proton can be represented as a hydrogen ion, H+.

• HCl == H+ + Cl-

• H2SO4 == H+ + HSO4-

• CH3COOH == H+ + CH3COO-

• A base is a substance which can behave as a proton acceptor. Any species which has a lone pair of electrons can thus behave as a base

• NH3 + H+ == NH4+

• OH- + H+ == H2O

• CO32- + H+ == HCO3-

8.2.1 Conjugate Acid-Base Pair

• Since all the above reactions are reversible, the species formed when an acid gives up a proton can accept a proton and thus behave as a base. The species formed when a base accepts a proton can give up a proton and behave as an acid.

• Cl- + H+ == HCl

• HSO4- + H+ == H2SO4

• CH3COO- + H+ == CH3COOH

• Acids and bases thus come in pairs; every acid can lose a proton to become a base and every base can accept a proton to become an acid. Species related in this way are known as conjugate acid-base pairs.

• Acid ⇌ Conjugate Base + H+

• Base ⇌ Conjugate Acid + OH-

• E.g. Cl- is the conjugate base of HCl; HCl is the conjugate acid of Cl-

• E.g. NH3 is the conjugate base of NH4+; NH4+ is the conjugate acid of NH3

• Stronger acids form weaker conjugate bases, weaker acids form stronger conjugate bases.

• Stronger bases form weaker conjugate acids, weaker bases form stronger conjugate acids.

8.2.2 Acid-Base Reactions

• Protons are not given up by acids unless there is a base which can accept them, and bases can only accept protons if there is an acid present to provide them. Reactions between acids and bases thus involve proton transfer from acid to base.

• In acid-base reactions, acids react with bases to give the conjugate base of the reactant acid and the conjugate acid of the reactant base. The reactant acid is given the label "acid 1" and its conjugate base is given the label "base 1". The reactant base is given the label "base 2" and its conjugate acid is given the label "acid 2".

• A general acid-base reaction can thus be represented as follows:

  • Acid 1 + Base 2 == Base 1 + Acid 2

  • HCl +    H2O ==    H3 O+ +   Cl-

    Acid 1   Base 2      Acid 2    Base 1