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Module 5 Chapter 21 (21.1 Buffer Solutions (pH range (Effective buffer=…

- - - - From weak acid and its salt
      - 1) HA(aq)⇌ H+(aq) + A-(aq)
      - Component 1= HA= need large reservoir
      - 2) NaA (aq) -> Na+ (aq) + A- (aq)
      - Step 1 only produces a small amount of A-
      - Component 2= A- = need large reservoir
      - Step 2 is complete dissociation so provides A-
    - - 1) Add aqueous solution of alkali to an excess of weak acid
      - 2) Weak acid partially neutralised forming conjugate base
      - 3) Some acid remains unreacted
  - - - Equilibrium shifts to left to reduce conc. of H+ ions
      - A- ions react with excess H+ forming HA
      - Fall in pH is minmised
    - - Concentration of H+ ions falls as they react with OH- forming water (doesn't change pH)
      - Equilibrium shifts to right to produce H+ ions (HA dissociates to generate H+)
  - - - 1) Ka - large effect- coarse tuning
      - 2) [acid] : [salt] ratio- small effect- fine tuning
- - - - Increased H+ conc- equilibrium to left
      - Reservoir of HCO3- reacts with H+ forming H2CO3
    - - Increase OH- conc. - Reacts with H+ forming H2O
      - Equilibrium to right to restore H+ ion concentration - H2CO3 dissociates to achieve this
- - - - weak acid (HA) - red
      - Conjugate base (A-)- yellow
      - End point (neutral) - colour in between as [HA] = [A-]
    - - Addition of OH-
        
        OH- react with H+ forming water
        
        HA (weak acid) dissociates- equilibrium shifts to right
        
        Colour changes to orange [H+]= [OH-]
        
        Colour changes to yellow when more OH- than H+
      - Addition of H+
        
        H+ reacts with A- and equilibrium shift to left
        
        Colour changes to orange and then red when equilibrium has shifted