Buffers and Neutralisation (Chapter 21)

Buffers and Neutralisation (Chapter 21)

1. BUFFER SOLUTIONS (21.1)

   • a buffer solution is a system that minimises pH changes
     • it does this by containing two components - a weak acid and its conjugate base
   • when acids or alkalis are added to a buffer, the two components in the buffer will react and eventually be used up
     • the pH does change but only by a small amount - it doesn't stay completely constant
   • acidic buffers
     • acidic buffers can be prepared in two ways:
       • mix a weak acid with the salt of its conjugate base
         • i.e. ethanoic acid and sodium ethanoate
           • salt fully dissociates into CH₃COONa → CH₃COO⁻ + Na⁺
             • acid only slightly dissociates into CH₃COOH ⇌ H⁺ + CH₃COO⁻
               • This sets up the equilibrium CH₃COOH (from undissociated weak acid) ⇌ H⁺ + CH₃COO⁻ (formed from salt or base)
       • Mix an excess of weak acid with a strong alkali
         • i.e. ethanoic acid and sodium hydroxide
           • All the base reacts with the acid CH₃COOH +OH⁻ → CH₃COO⁻ + H₂O
             • acid only slightly dissociates into CH₃COOH ⇌ H⁺ + CH₃COO⁻
   • Action of the buffer solution
     • when an acid is added, [H+] increases, H+ ions react with conjugate base (A-), the equilibrium position shifts to the left, removing most of the H+ ions
     • When an alkali is added, [OH-] increases, H+ ions react with OH- ions to form water which means equilibrium shifts to the right to restore H+ ions

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