Acids, Bases and pH

• Acids and alkalis release H+ and OH- ions in water - this is the Arrhenius model of acids and bases:

  acids dissociate and release H+ ions in aqueous solution

  alkalis dissociate and release OH- ions in aqueous solution

  H+ ions are neutralised by OH- ions to form water
• Arrhenius model of acids and bases:

  H+ (aq) + OH- (aq) = H2O (l)
• an alkali is a soluble base
• The Bronsted-Lowry model for acids and bases extends this model to emphasise the role of proton transfer between species
• A Bronsted-Lowry acid is a proton donor
• A Bronsted-Lowry base is a proton acceptor
• A conjugate base pair contains two species that can be interconverted by transfer of a proton
• For example: in the forward direction: HCl releases a proton to form its conjugate base Cl-

  in the reverse direction Cl- accepts a proton to form its conjugate acid, HCl
• acid base

  nitric acid: HNO3 (aq) = H+ (aq) + NO3- (aq)

  sulfuric acid: H2SO4 (aq) = H+ (aq) + HSO4- (aq)

  ethanoic acid: CH3COOH (aq) = H+ (aq) + CH3COO- (aq)
• Neutralisation can be shown by a simple equation: H+ + OH- = H2O

  In HCl, H+ ions have been supplied by dissociation: HCl = H+ + OH-
• Combining the two equations gives the acid base equilibrium. There are now two conjugate acid-base pairs which are labelled: acid-1, base-1 and acid-2, base-2
• HCl (aq) + OH- (aq) = H2O (l) + Cl- (aq)

  acid 1 base 2 acid 2 base 2
• In the forward reaction:

  1. HCl is an acid because it donates H+
  2. OH- is a base because it accepts H+
• In the reverse reaction/direction:

  1. H2O is an acid as it donates H+
  2. Cl- is a base as it accepts H+