Buffers

AQA Chemistry A2 unit CHM4. Buffer Solutions.

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  • Created by: Phoebe
  • Created on: 14-12-12 08:55
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  • Buffers
    • Definition
      • A buffer solution minimises change in pH during the addition of an acid or an alkali.
      • It is a mixture of a weak acid (HA), and its conjugate base (A-).
        • The concentration of H+ is very small compared with HA or A-.
    • Addition of an acid (H+)
      • H+ is increased.
      • The pH change is opposed and the equilibrium moves to the left, removing H+ and forming HA.
      • The conjugate base A- removes most of any added H+.
    • Addition of an alkali (OH-)
      • The added OH- reacts with the small concentration of H+.
      • The pH change is opposed - the equilibrium moves to the right, restoring H+ as HA dissociates.
      • The weak acid HA restores most of any H+ that has been removed.
    • Common Buffer Solutions
      • Acidic
        • Aq. solution containing a mixture of ethanoic acid and the ethanoate ion.
        • Ethanoic acid acts as the weak acid (CH3COOH).
        • Sodium ethanoate (CH3COO-Na+) acts as a source of the conjugate base (CH3COO-).
      • Alkaline
        • Aq. solution containing a mixture of the ammonium ion and ammonia.
        • Ammonium  chloride (NH4+Cl-) acts as a source of the weak acid (NH4+)
        • Ammonia acts as the conjugate base (NH3).
    • Calculations
      • The pH of the buffer depends upon ....
        • The acid dissociation constant (Ka) of the buffer system.
        • The ratio of the weak acid (HA) and its conjugate base (A-).
      • [H+] = Ka x  [HA]/[A-]

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