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, Buffer solutions
A buffer solution is a system that minimises pH changes when small
amounts of an acid or a base are added.
Buffer solutions contain 2 components:
The weak acid, HA, removes added alkali
The conjugate base, A-, removes added acid.
These 2 components act as 2 reservoirs that can act independently
to remove acid & alkali by shifting the equilibrium position.
When alkalis & acids are added to the buffer, the 2 components in
the buffer solution react & will eventually be used up. As soon as
one component has completely reacted, the solution loses its
buffering ability towards added acid or alkali.
On addition of an acid: On addition of an alkali:
1. [H+(aq)] increases 1. [OH-(aq)] increases
2. H+(aq) ions react with the 2. The small concentration of H+(aq) ions
conjugate base A-(aq) react with the OH-(aq) ions producing
3. The equilibrium position H₂O: H+(aq) + OH-(aq) H₂O(l)
shifts to the left, 3. HA dissociates, shifting the equilibrium
removing most of the position to the right to restore most of
H+(aq) ions. the H+(aq) ions
HA(aq) H+(aq) + A-(aq) HA(aq) H+(aq) + A-(aq)
added acid added alkali
,Preparing a weak acid buffer solution- ethanoic acid
In the ethanoic acid equilibrium, the equilibrium position lies well to the
left. When CH₃COO - ions are added to CH₃COOH the equilibrium shifts
even further left, reducing the already small [H +(aq)] & leaving a solution
containing mainly the 2 components, CH₃COOH & CH₃COO -
CH₃COOH(aq) H+(aq) + CH₃COO - (aq)
Preparation from a weak acid and its salt
A buffer solution can be prepared by mixing a solution of ethanoic acid,
CH₃COOH, with a solution of one of its salts e.g. sodium ethanoate
CH₃COONa
When ethanoic acid is added to water, the acid partially dissociates & the
amount of ethanoate ions in solution is very small. Ethanoic acid is the
source of the weak acid component of the buffer solution.
CH₃COOH(aq) H+(aq) + CH₃COO-(aq)
Salts of weak acids are ionic compounds & provide a convenient source of
the conjugate base. When added to water, the salt is the source of the
conjugate base component of the buffer solution
CH₃COONa(s) + aq CH₃COO-(aq) + Na+(aq)
Preparation by partial neutralisation of the weak acid
A buffer solution can also be prepared by adding an aqueous solution of an
alkali to an excess of the weak acid. The weak acid is partially neutralised
by the alkali, forming the conjugate base. Some of the weak acid is left over
unreacted. The resulting solution contains a mixture of the salt of the weak
acid & any unreacted weak acid.
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