Acid/Base equilibria -4

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  • Created by: Shannon
  • Created on: 02-04-15 15:46
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  • Acid/base equilibria
    • Defintions
      • Lewis:
        • Acid is an electron acceptor
        • Base is an electron donor
      • Bronsted-Lowry:
        • Acid is a proton donor
        • Base is a proton acceptor
    • The extent of dissociation
      • Strong acids dissociate completely in aqueous solution
      • Weak acids only partially dissociate in aqueous solution. The majority of molecules remain undissociated
        • Acid dissociation constant, Ka
          • Ka = [H3O+] [A-] / [HA]
            • HA = intial concentration of weak acid
          • Assumptions
            • 1) Concentration of acid at equilibrium is same as initial concentraion
            • 2) All the H3O+ comes from the acid
    • Conjugate pairs
      • In a reaction if an acid loses a H+, the resulting species is referred to as a conjugate base
      • The species resulting from a base gaining a H+ is called a conjugate acid
      • An amphoteric species can act as an acid and a base e.g H2O
    • pH
      • pH = -log[H3O+]
        • You can calculate [H30+] if you have been given pH by using the following - 10^-pH
      • In alkaline solution pH is calculated by - pOH = -log[OH-]
        • pH = 14 - pOH
        • Calculating pH of strong bases
      • pKa = pH @ half equivalence point
        • Known as buffer region
      • pH = pKa + log[A-]/[HA]
        • Henderson-Hasselbach equation
    • The dissociation of water - Kw
      • Kw = [H3O+] [OH-]
      • Pure liquids do not appear in the equilibrium expression
    • Titration curves
      • Strong acid v strong base
        • pH @ equivalence point = 7
      • Strong acid v weak base
        • pH @ equivalence point = 5 - (due to H3O+)
      • Weak acid v weak base
        • pH @ equivalence point = 7
      • Weak acid v strong base
        • pH @ equivalence point = 9 (due to OH-)
      • The end point is where the indicator changes colour
    • Buffer solutions
      • A buffer solution resists changes in pH upon small additions of acid or base
      • A buffer is made up of a weak acid and its conjugate base or a weak base and its conjugate acid

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