Chapter 21 - Buffers and Neutralisation Flashcards
What is a buffer solution?
A system that minimises pH changes when small amounts of an acid or base are added.
What 2 components does a buffer solution contain?
A weak acid and its conjugate base.
How can a weak acid buffer solution be prepared from a weak acid and its salt?
Mix a solution of a weak acid with a solution of its salt.
When the weak acid is added to water, it partially dissociates a small number of ions.
The salt completely dissolves when added to water and provides source of conjugate base.
How can a weak acid buffer solution be prepared by partial neutralisation?
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 and some of the weak acid is still leftover.
In a buffer solution, what occurs to increase pH when acid is added?
[H+(aq)] increases
H+ ions react with conjugate base A-
Equilibrium shifts to the left, removing most H+ ions
In a buffer solution, what occurs to decrease pH when alkali is added?
[OH-(aq)] increases
H+ ions react with OH-
HA dissociates, shifting equilibrium to the right to restore H+ ions.
When is a buffer solution most effective?
When there are equal concentrations of the weak acid, HA, and its conjugate base, A-
Why is a buffer solution most effective with equal concentrations of weak acid and its conjugate base ? (When [HA] = [A-])
Because the pH of the buffer solution is the same as the pKa value of HA.
Because the pKa value is centred at the midpoint of the operating pH range of the buffer solution.
Where does blood pH operate between?
Between 7.35 and 7.45
What happens if the blood pH falls out of its range?
Below - acidosis causing fatigue, shortness of breath and in extreme cases, death.
Above - alkalosis causing muscle spasms and nausea.
What is the most important buffer system in blood?
The carbonic acid - hydrogencarbonate buffer system
What is the equilibrium for the buffer system in blood?
H2CO3(aq) —> H+(aq) + HCO3-(aq)
What happens when acid is added to blood?
{H+(aq)] increases
H+ ions react with HCO3-
Equilibrium position shifts to the left removing most of the H+ ions
What happens when alkali is added to blood?
[OH-(aq)] increases
H+ ions react with OH- ions
Equilibrium position shifts to the right and H2CO3 dissociates to restore the H+ ions.
What are the significant points on a pH titration curve?
First stretch = excess of acid. pH increases slowly.
Vertical section = pH increases rapidly on addition of very small volume of base
Equivalence point = centre of vertical section
Last stretch = excess of base, pH increases slowly
