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Summary Chemistry Alevel Buffer Action Notes
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- AQA
Chemistry Alevel Buffer Action Notes
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Buffer Action and CalculationsA buffer is a solution which resists changes in pH when small amounts of acid or alkali are
added.
It does not stop the changes from occurring completely but it makes them slighter. They only
work for small amounts of acid/base.
They consist of a weak acid/conjugate base or a weak base/conjugate acid.
Acid Buffers
pH of less than 7 - containing a weak mixture of weak acid and one of its salts. They can resist
changes when either an acid or base is added to the solution.
HA <=> H+ + A-
Resisting an acid
CH3COO-Na is a buffer. The large number of CH3COO- ions make sure that the buffer can
cope with the addition of an acid.
If a small amount of acid is added the concentration of H+ ions increases. The extra H+ ions
combine with CH3COO- ions forming CH3COOH shifting equilibrium to the left reducing the H+
value.
CH3COOH <=> H+ + CH3COO-
<--- addition of acid
H+ are added.
Equilibrium shifts to the left as the H+ reacts with CH3COO- decreasing the concentration
forming CH3COOH until equilibrium is established.
There is a large supply of CH3COOH so the concentration is not affected after the
reaction.
pH remains constant.
Resisting a base
If a small amount of base (NaOH) is added the OH concentration increases.Most of the extra
ions react with H+ ions to form water removing H+ ions. This causes more CH3COOH to
disassociate forming H+ ions shifting equilibrium to the right.
CH3COOH <=> H+ + CH3COO-
----> addition of OH
OH- is added.
Equilibrium shifts to the right as the OH- reacts with H+ to form water decreasing the OH-
concentration.
The large supply of CH3COOH means that the concentration is not affected after the
reaction.
pH remains constant.
Base Buffers
pH greater than 7 - they contain a mixture of a weak base and one of its salts.
, B + H2O <=> BH+ + OH-
NH4Cl is a weak base and can act as a buffer.
NH4Cl ---> NH4+ + Cl-
NH3 + H2O <=> NH4+ + OH-
Resisting an acid:
H+ concentration increases.
Equilibrium shifts to the right as the H+ react with OH- forming water - equilibrium
decreases the concentration of H2O and increases the concentration of OH-.
OH- increases.
pH remains constant.
Resisting a base:
OH- concentration increases.
Excess concentration of OH- reacts with NH4+ causing equilibrium to shift to the left.
This causes OH- to decrease.
pH remains constant.
Dilution
Acid and base buffers can resist changes in pH when diluted with water. If a small amount of
water is added to the buffer the water slightly dissociates so the H+ and OH- ions push
equilibrium the same amount in both directions leaving it unchanged.
Applications of Buffers
Shampoos
Biological Washing Powders
In humans HCO3- acts as a buffer to keep the blood pH at optimum level. CO2 is produced
during respiration and reacts with H+ ions in the blood to produce carbonic acid. This means
that equilibrium between CO2 and HCO3- is very important.
If blood pH drops this causes acidosis a condition where there is too much acid in body fluids -
blood.
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