Chapter 21 - Buffers and Neutralisation

Question 1

What is a buffer solution?

Answer 1

A system that minimises pH changes when small amounts of an acid or a base are added.

Question 2

What two components does a buffer solution contain?

Answer 2

A weak acid and its conjugate base.

Question 3

How can a weak acid buffer solution be prepared from a weak acid and its salt?

Answer 3

When the weak acid is added to water, it partially dissociates and the number of ions is very small.
When added to water, the salt completely dissolves into ions.

Question 4

How can a weak acid buffer solution be prepared by partial neutralisation?

Answer 4

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.

Question 5

In a buffer solution, what occurs to increase pH when acid is added?

Answer 5

[H+] increases
H+ ions react with conjugate base A-
Equilibrium shifts to the left removing most H+ ions

Question 6

In a buffer solution, what occurs to decrease pH when alkali is added?

Answer 6

[OH-] increases
H+ ions react with OH-
HA dissociates, shifting equilibirum to the right to restore H+ ions

Question 7

When is a buffer solution most effective?

Answer 7

Equal concentrations of the weak acid and its conjugate base.

Question 8

A buffer solution is most effective with equal concentrations of weak acid and its conjugate base
Why is this the case?

Answer 8

The pH of the buffer solution is the same as the pKa value of HA.
Operating pH is typically over two pH units, centred at pH of pKa value.

Question 9

How can the pH of a buffer solution be calculated?

Answer 9

When Ka, [HA] and [A-] are known:

Ka=[H+][A-]/[HA]

Question 10

How is pH calculated if [A-]=[HA]?

Answer 10

Ka=[H+]

pKa=pH

Question 11

What is the Henderson-Hasselbalch equation for calculating pH?

Answer 11

pH=pKa + log([A-]/[HA])

Question 12

Where does blood pH operate between?

Answer 12

7.35 and 7.45

Question 13

What happens if the blood pH falls out of this range?

Answer 13

Below - acidosis causing fatigue, shortness of breath and in extreme, death.
Above - alkalosis causing musle spasms and nausea.

Question 14

What is the most important buffer system in blood?

Answer 14

Carbonic acid-hydrogencarbonate

H2CO3/HCO3-

Question 15

What is the equilibrium for the buffer system in blood?

Answer 15

H2CO3(aq) H+(aq) + HCO3-(aq)

Question 16

When acid is added to blood, what happens?

Answer 16

[H+] increases
H+ ions react with HCO3-
Equilibrium position shifts to the left removing most of the H+ ions

Question 17

When alkali is added to blood, what happens?

Answer 17

[OH-] increases
H+ ions react with OH- ions
H2CO3 dissociates to restore H+
Equilibrium shifts to the right

Question 18

What are the significant points on a pH titration curve?

Answer 18

At first, excess of acid, pH increases slowly.
Vertical section, pH increases rapidly on addition of very small volume of base.
Equivalence point, centre of the vertical section.
At the end, excess of base, pH increases slowly.

Question 19

What is the equivalence point?

Answer 19

The volume of one solution that exactly reacts with the volume of the other solution.

Question 20

What is an acid-base indicator?

Answer 20

A weak acid, HA, that has a distinctively different colour from its conjugate base, A-.

Question 21

When is an indicator suitable for a titration?

Answer 21

The pH indicator range is passed through by the vertical section of the pH titration curve.