Buffers
AQA Chemistry A2 unit CHM4. Buffer Solutions.
- 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).
- Acidic
- 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-]
- The pH of the buffer depends upon ....
- Definition
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