Acid Base Equilibria

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Bronstead-Lowry acids and bases

  • A Bronstead-Lowry acid is a proton donor.
  • A Bronstead-Lowry base is a proton acceptor.

Calculating pH

  • pH= -log[H+]

Strong acids

  • A strong acid means that it is completely dissociated.
  • The concentration of hydrogen ions in a monoprotic strong acid will be the same as the concentration of the acid.
  • [H+]= 1 x 10^-pH

Ionic product of water

  • The equation for the dissociation of water is H20 -> H+ + OH-
  • Kw= [H+] [OH-]
  • At 25 degrees the value of Kw for all aqueous solutions is 1 x 10^-14 mol2 dm-6

Weak acids

  • Weak acids are only slightly dissociated.
  • Ka= [H+][A-]         [HA]
  • pKa= -logKa
  • Ka= 10^-pKa

Half Equivalence

  • When a weak acid has been reacted with exactly half the neutralisation volume of alkali we can assume that [H+]= Ka.
  • pH= pKa

Buffer solutions

  • A buffer solution is one where the pH does not change significantly if small amounts of acid or alkali are added to it.
  • An acidic buffer is made from a weak acid and a salt of that weak acid e.g. ethanoic acid and sodium ethanoate.
  • A basic buffer solution is made from a weak base and a salt of that weak base, which is made from reacting the weak base with a strong acid e.g. ammonia and ammonium chloride.
  • Buffers are used in shampoos as human hair becomes rough when exposed to alkaline conditions.
  • They are also used in biological washing powders…

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