Chapter 21 - Buffers and Neutralisation

Question 1

What is meant by a buffer solution?

Answer 1

Is a system that minimised pH changes when small amounts of an acid or alkali are added

Question 2

What does a buffer solution contain in equilibrium?

Answer 2

A weak acid HA
Its conjugate base A⁻

Question 3

What happens when an acid is added/ H⁺ ions are added to the buffer system?

Answer 3

Initially [H⁺] increases
The large reservoir of A⁻ ions reacts with excess H⁺ ions
The equilibrium position moves to the left
[H⁺] and therefore pH are restored

Question 3

What must be present for the bugger to work?

Answer 3

A large reservoir of both HA and A⁻

Question 4

What happens when an alkali is added/OH⁻ ions are added to the buffer system?

Answer 4

OH⁻ ions react with H⁺ ions
[H⁺] decreases
The reservoir of HA molecules dissociate
The equilibrium position moves to the right
[H⁺] and therefore pH are restored

Question 5

What are the two approaches in preparing a buffer solution?

Answer 5

1. Mix a solution containing the weak acid with a solution that contains a salt of the weak acid
2. Or add an aqueous solution of an alkali to an excess of a weak acid
   The weak acid will therefore be partially neutralised and the resulting solution will contain both unreacted HA as well as the A⁻ salt produced in the neutralisation reaction

Question 6

When is the buffer solution most effective?

Answer 6

When [HA] = [A⁻]

Question 6

How do we calculate the pH of a buffer solution?

Answer 6

Rearrange the weak acid equation

Question 7

What does the [acid]:[salt] ratio provide in this case?

Answer 7

A ‘fine tuning’ of the buffer’s pH

Question 8

Why shouldn’t the ratio of [acid]:[salt] not be too large or small in this case?

Answer 8

As there will be insufficient HA or A⁻ for the buffer to be effective

Question 9

What does the value of Kₐ provide?

Answer 9

A ‘coarse tuning’ of the buffer’s pH

Question 10

What do we assume for the acid:conjugate base ratio?

Answer 10

That its going to be 1:1
Kₐ will be as close to the desired [H⁺] as possible

Question 11

What pH does blood plasma need to be maintained at?

Answer 11

Between 7.35 and 7.45

Question 12

How is the pH controlled in the blood?

Answer 12

By the carbonic-acid hydrogencarbonate buffer system

Question 13

What does a drop in pH in the blood ultimately suggest?

Answer 13

Means [H⁺] increases
H⁺ ions react with the conjugate base, HCO₃⁻ (aq)
The equilibrium position moves to the left
[H⁺] and therefore pH are restored

Question 14

What does as increase in pH in the blood ultimately suggest?

Answer 14

Means [OH⁻] increases
OH⁻ ions react with the H⁺ ions
[H⁺] decreases
H₂CO₃ dissociates
The equilibrium position moves to the right
[H⁺] and therefore pH are restored

Question 15

How can the pH changes during an acid-base titration be monitored?

Answer 15

Using a pH meter

Question 16

What can the data collected from the pH meter used to plot?

Answer 16

A pH titration curve
Which is plotted against the volume of acid or base

Question 17

What does a pH meter consist of?

Answer 17

An electrode
Which is dipped into the solution
Connected to a meter that displays the pH reading

Question 18

What is the purpose of carrying out a titration?

Answer 18

To find the equivalence point

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

Describe the practical procedure in preparing a titration

Answer 20

Pipette 25cm³ HCl into a conical flask
Then add the NaOH from the burette in 1cm³ portions
Swirl after each addition and record the new pH and the total vol. added
As the pH starts to change more rapidly add the solution dropwise for each reading, until the pH changes less rapidly
Go back to adding 1cm³ at a time until there is little change after several additions

Question 21

Label and describe the pH titration curve

Answer 21

Question 22

Strong acid - strong base titration curve:

Answer 22

Question 23

Weak acid - strong base titration curve:

Answer 23

Question 24

Strong acid - weak base titration curve:

Answer 24

Question 25

weak acid - weak base titration curve:

Answer 25

Question 26

What is an acid-base indicator and what does it do?

Answer 26

Is a weak acid, HA, that has distinctly different colour from its conjugate base, A⁻ Enables us to find the equivalence point in a titration without plotting a titration curve

Question 27

How does an acid-base indicator help us find the equivalence point?

Answer 27

Should have a colour change (or end point) which coincides with the vertical section of the titration curve

Question 28

Examples of acid-base indicators:

Answer 28

Question 29

In a titration in which a strong base is added to a strong acid what indicator is used and describe the whole process in which the solution changes colour

Answer 29

Methyl orange is used Initially red as the high concentration of H⁺ ions forces the equilibrium position well to the left When the alkali is added the OH⁻ react with the H⁺ ions More weak acid, HA, dissociates, shifting the equilibrium position further and further to the right The END POINT of the titration is when concentrations of HA and A⁻ are equal (the colour will be in-between i.e. orange) As more alkali is added, the equilibrium shifts even further to the right Now an excess of A⁻ in this case and the indicator is yellow

Question 30

What occurs if methyl orange is added initially to a basic solution and acid added?

Answer 30

Methyl orange is initially yellow as the low concentration of H⁺ ions forces the equilibrium position well to the right When the acid is added the H⁺ ions react with the A⁻ ions The equilibrium position shifts to the left The END point of the titration is when concentrations of HA and A⁻ are equal (the colour will be in-between i.e. orange) As more acid is added the equilibrium shifts even further to the left Now an excess of HA and the indicator is red

Question 31

At the end point [HA] = ?

Answer 31

[A⁻]

Question 32

What is the end point?

Answer 32

Point where the indicator changes color

Question 33

How do we choose a suitable indicator?

Answer 33

Use one which has a colour change which coincides with the vertical section of the titration curve