C3aL7 Acids and Base Theories (Higher Level Only)

Key Words

Acid - a compound that produces hydrogen ions in aqueous solution

Aqueous systems - substances dissolved in water.

Base - a substance that is a hydroxide, carbonate, or oxide.

Dissociate - split apart into ions.

Hydroxide ion - OH-.

Lewis Acid - any molecule that accepts a single pair of electrons (or just one electron).

Lewis Base - any molecule that gives a single pair of electrons.

Test Yourself

Homework
Physics GCSE
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Grade C

Until about 150 years ago, acids and bases were not well understood.  We have the following chemists to thank for what we know now:

  • Svante Arrhenius (1859 - 1927);
  • Johannes Nicolaus Brønsted (1879 - 1947);
  • Thomas Martin Lowry (1874 - 1936);
  • Gilbert Newton Lewis (1875 - 1946).

Arrhenius, a Swedish chemist, stated that acids produced hydrogen ions and hydroxides produce hydroxide ions.  However he applied his theory only to aqueous systems, and it did not answer questions like why HCl dissolved in water, but methane, which contains hydrogen, did not.  The theory only considered hydroxide ions, not carbonates which can act like bases.

 

Grade B

 

The Danish chemist, Brønsted, and the British chemist, Lowry, worked independently, but arrived at the same theory, so it's called the Lowry-Brønsted Theory.

They defined an acid as a substance that could donate a proton.  A base was a substance that could accept a proton.

Water accepts a proton from an acid to become the hydronium ion:

H+(aq) + H2O(l) ® H3O+(aq)

Strong acids dissociate almost completely into ions. 

HCl(aq) + H2O(l) « H3O+(aq) + Cl-(aq)

The hydrogen chloride has donated the proton to the water to make the hydronium ion.  When this reaction reaches equilibrium, 0.004 % of the HCl exists as molecules; all the rest, 99.996 % exists as ions.

Ethanoic acid is a poor proton donor.  If we look at the way it dissolves in water, we get this reaction:

CH3CO2H(aq) + H2O(l) « H3O+(aq) + CH3CO2-(aq)

In this reaction, 98.7 % of the ethanoic acid remains as molecules; only 1.33 % is ionised.

The Lowry-Brønsted Theory applies to solutions in solvents other than water, and in gas and solid phases.  It allows all sorts of compounds to be considered as acids or bases.

Grade A/A*

The American physical chemist, G N Lewis gave us Lewis diagrams.  He also introduced the idea of a Lewis acid, any molecule that can accept a single pair of electrons from another molecule to form a compound.

A + :B ® A-B

Note that no other substance is formed.  The molecule A has accepted the electrons from B.   B is the electron donor, so it's a Lewis base.  For example:

H+ + :OH- ® H2O