Acids and Bases

Arrhenius Definitions

Svante Arrhenius was the Swedish scientist who first defined acids and bases.

Hydrogen-containing Substance that dissociates in solution to give H+ ions in water
Hydroxide Group-containing substance that dissociates to produce OH- ions in water

Bronsted-Lowry Definitions

Substance that donates protons
Substance that accepts a proton

In this theory:

  • Bases don’t have to contain OH (so NH3 is a base)
  • Acid-base reaction doesn’t have to occur in water

Compounds that tend to be Acidic

  • Ones with hydrogen attached to group 16-17 elements
  • Protons to be measurably acidic must be bound to another atom with an ‘appreciable acidic bond’

Compounds that tend to be Basic

  • Requires the presence of one or more lone pairs (electron pairs in an outer shell that aren't shared with another atom)
  • Usually contain deprotonated group 15-16 atoms

Strong v Weak

Strength of an acid/base refers to its ionisation (dissociation) ability; in acids its their ability to donate a proton, and in bases its their ability to accept a proton (and hence form OH- ions).

Strength is hence correlated to electrolytic rating, as ionisation correlates with losing/gaining electrons. And the dissolved species of these acids/bases exist as ions in solution.

Strong Acids

The chemicals we wil encounter in first year will be largely weak. Hence we learn the strong ones. The more oxygen molecules around the centre increases the likelihood of strength. Examples:

  • Hydrohalogic acid halogens (not HF, but HCl, HI etc)
  • HNO3
  • H2SO4
  • halogen oxyanion acids - halogic acid and halogous acid and per and hypo

Strong Bases

A strong base is able to deprotonate very weak acids in solution. The cations of these strong bases appear in the first and second groups of the periodic table


  • Potassium hydroxide KOH
  • Barium hydroxide Ba(OH)2
  • Lithium hydroxide LiOH

Multiprotic Acids

Bronsted acids can release more than one proton (H+ ion). These release one proton per hydrogen atom in the acid.

  • Monoprotic: HCl
  • Diprotic: H2SO4
  • Triprotic: H3PO4


  • acid + base → salt+water
  • acid + carbonate → salt + water + carbon dioxide
  • acid + metal → salt + hydrogen
    • needs a metal that is more electropositive than hydrogen