Chapter 21 - Buffers and neutralisation

Question 1

What is a buffer solution?

Answer 1

A solution that minimises pH change when small amounts of acid or base are added

Question 2

What are the two components of a buffer solution?

Answer 2

A weak acid and it’s conjugate base

Question 3

What is the role of the weak acid in a buffer solution?

Answer 3

Removes any alkali

Question 4

What is the role of the base in a buffer solution?

Answer 4

Removes any acid

Question 5

What are the two ways in which a buffer solution can be made?

Answer 5

Partial neutralisation of an acid and preparing the weak acid from its salt

Question 6

How do you make a buffer solution using a weak acid and its salt?

Answer 6

Mix the two together
The acid partially dissolves in the solution and the salt fully dissociates. This means that the ions produced by the salt act as the conjugate base

Question 7

How do you make a buffer solution by partially neutralising a weak acid?

Answer 7

An add excess of weak acid to a strong alkali. Once the two have reacted, you are still left with some weak acid because it was in excess

Question 8

What is the function of the two reservoirs?

Answer 8

If you add acid, the H+ ions combine with the conjugate base ions to form more acid and shift the equilibrium to the left and reduce the H+ concentration so the pH stays fairly constant
If you add alkali, the OH- ions combine with the H+ ions in the acid to form water. This removes H+ ions from the mixture, so more weak acid has to dissociate to form more, shifting the equilibrium to the right

Question 9

When is a buffer solution most effective?

Answer 9

When there are equal concentrations of weak acid and conjugate base

Question 10

What can be observed when the concentrations of acid and base are equal?

Answer 10

The pH of the buffer solution is roughly the same as the pKa of the acid
The operating pH range is typically 2 units

Question 11

How do you calculate the pH of a buffer solution?

Answer 11

[H+] = Ka x [HA]

[A-]

Question 12

What assumptions have to be made to calculate the pH of a buffer solution?

Answer 12

The salt of the conjugate base is fully dissociated

HA is only slightly dissociated

Question 13

What organic molecules are particularly sensitive to pH?

Answer 13

Enzymes

Question 14

What happens when blood pH falls below 7.35?

Answer 14

A condition called acidosis can develop, which can lead to fatigue and shortness of breath

Question 15

What happens when blood pH rises above 7.45?

Answer 15

A condition called alkalosis can develop, causing nausea and muscle spasms

Question 16

What buffer system primarily controls blood pH?

Answer 16

The carbonic acid-hydrogencarbonate buffer system

Question 17

What happens when H+ is added to the carbonic acid-hydrogencarbonate buffer system?

Answer 17

[H+] increases, so H+ ions react with the conjugate base (HCO3-)
This causes the equilibrium to shift to the left to remove the H+ ions

Question 18

What happens when OH- is added to the carbonic acid-hydrogencarbonate buffer system?

Answer 18

[OH-] increases, so OH- ions react with H+ ions to produce water
H2CO3 dissociates to restore the H+ ions and the equilibrium shifts to the right

Question 19

Describe the graph of a typical pH titration curve

Answer 19

1) When the base is first added, the increase in pH is very slight as the acid is still in excess
2) The equivalence point - the vertical section of the graph where the addition of a small amount of base results in a large increase in pH
3) Towards the end of the graph, the base is in excess, so the gradient decreases

Question 20

What is the equivalence point?

Answer 20

The volume of one solution that reacts exactly with the volume of the other solution. It is the central point of the vertical section

Question 21

What colour will the end point of a titration be when using methyl orange?

Answer 21

Orange

Question 22

What happens when OH- ions are added to an acidic solution?

Answer 22

OH- ions react with H+ ions in the indicator
The weak acid dissociates, shifting the equilibrium to the right
The colour changes to yellow as the equilibrium shift

Question 23

What happens when H+ ions are added to a basic solution?

Answer 23

H+ ions recap with the conjugate base
The equilibrium shifts to the left
The colour changes to red as the equilibrium shifts

Question 24

How do you select an indicator to use for a titration?

Answer 24

It must have a colour change that coincides with the vertical section of the graph