Chapter 21: Buffers and Neutralisation (21.1-21.3) Flashcards
what are buffer solutions used for?
to minimise changes in pH to a solution on addition of small amounts of an acid or base
how does an acid buffer work
Acid buffer solutions contain two components, a weak acid HA and its conjugate base A-:
HA (aq) < – > H+ (aq) + A- (aq)
When an acid (H+) is added, it reacts with the conjugate base (A-) to form HA and the equilibrium shifts to the left. When alkali (OH-) is added, it reacts with the H+ to form H2O and the equilibrium shifts towards the right.p
preparing a buffer solution from a weak acid and a salt
a buffer solution can be prepared by mixing together a solution of a weak acid (eg CH3COOH) and a solution of a salt of that weak acid (eg CH3COONa).
Work out concentrations of acid and salt, assume the concentration of the salt is the same as the concentration of the conjugate base ( [A-] )
preparing a buffer solution by partial neutralisation of a weak acid
a buffer solution can be prepared by mixing together an excess of a weak acid (eg CH3COOH) and a solution of an alkali (NaOH)
Ka value for buffer solution
Ka = [H+][A-] / [HA]
buffer system used in the blood
carbonic acid-hydrogencarbonate
The H2CO3/HCO3- buffer is present in blood plasma maintaining a pH between 7.35 and 7.45
explain the shape of a pH titration curve
pH on y-axis, volume of base/acid added on x-axis
Initial shallow curve: pH changes slowly as there is still an excess of the original acid/base.
Vertical section: pH increases rapidly on addition of a very small amount of base/acid, acid and base concentrations are similar.
Equivalence point: the centre of the vertical section aka the point of neutralisation
Final shallow curve: pH changes slowly as it is now the base/acid that was added that is now in excess
what is an indicator
An acid-base indicator is a weak acid, HA, that has a different colour from its conjugate base, A-.
The colour changes as a response to the shift in equillibrium
at the end point of a titration
- There are equal concentrations of HA and A-
- The colour of the indicator is between the two extreme colours
how to chose an indicator
Use an indicator where the vertical section goes completely through the range which the indicator would change colour.
No indicator is suitable for a weak acid/weak base titration
What is the pH of a buffer where HA and A- have the same concentrations?
[H+] = Ka x [HA]/[A-] therefore [HA] and [A-] cancel so pH is equivalent to the pKa value and [H+] is equivalent to Ka
How do you work out H+ in a buffer?
Ka x [HA]/[A-]
