Chapter 21 - Buffers and Neutralisation

Question 1

Define a buffer solution

Answer 1

It minimises the pH changes when small amounts of an acid or a base are added.

Question 2

What are the 2 components in a buffer solution

Answer 2

They contain a weak acid and its conjugate base.

Question 3

What does the weak acid do

Answer 3

It removes the added alkali

Question 4

What does the conjugate bas do

Answer 4

It remove the added acid

Question 5

How can you prepare the weak acid buffer solutions?

Answer 5

1. Preparation from a weak acid and its conjugate base

2. Preparation by partial neutralisation of weak acids

Question 6

Preparation from a weak acid and its conjugate base

Answer 6

This needs a weak acid and its conjugate base.
Use an acid and one of it’s salts. When the acid is added to water, it will partially dissociate, and the acid itself is the weak acid.
Any weak acid salt is an ionic compound, so it will fully dissociate. So the salt is the conjugate base component. (As in CH3COO- is the component)

Question 7

Preparation by partial neutralisation of weak acids

Answer 7

Adding an aqueous solution of an alkali to an excess f the weak acid.
The weak acid is partially neutralised by the alkali forming the conjugate base.
Some of the weak acid is left unreacted.

Question 8

Action of the buffer solutions.
Adding more H+ to:
HA H+ +A-

Answer 8

1. Add more H+
2. H+ reacts with the conjugate base
3. EQP moves to the left.
4. Added acid is removed.

Question 9

Action of the buffer solutions.
Adding more OH - to:
HA H+ +A-

Answer 9

1. Adding more alkali
2. A small concentration of the H+ reacts with the OH-.
3. HA dissociates shifting right to restore the H+ ions.

Question 10

How can you choose the components for a buffer solution

Answer 10

A buffer is most effective when there are equal concentrations of the weak acid and its conjugate base. So when:

• pH of buffer is same as pKa value of the HA.
• The operating pH is typically over about 2 pH units, centred at the pH of the pKa value.

Question 11

Equation to calculate the pH of a buffer solution

Answer 11

{H+(aq)] = Ka x [HA]/[A-]

Question 12

If the pKa is given and the [HA] =[A]

Answer 12

Then the pH of the buffer = pKa value.

Question 13

What pH should the blood plasma be at?

Answer 13

Between 7.35 and 7.45.

Question 14

How id the blood plasma pH controlled?

Answer 14

With the carbonic acid-hydrogencarbonate buffer system, which is the most important.

Question 15

What happens if the pH falls below 7.35?

Answer 15

Develop acidosis.

Which causes fatigue, shortness of breath and in extreme cases, shock or death.

Question 16

What happens if the pH falls rises above 7.45

Answer 16

Develop alkalinosis.

Which causes muscle fatigue, light-headedness and nausea.

Question 17

What happens when you add acid to the carbonic acid-hydrogencarbonate buffer system

Answer 17

1. [H+] increases
2. The H+ ions react with the conjugate base, HCO3-.
3. The equilibrium position shifts to the left, removing most of the H+ ions.

Question 18

What happens when you add alkali to the carbonic acid-hydrogencarbonate buffer system

Answer 18

1. [OH-] increases
2. The small concentration of H+ ions react with the OH- ions.
3. H2CO3 dissociates, shifting the equilibrium position to the right to restore most of the H+ ions.

Question 19

How can you use a pH meter

Answer 19

1. Using a pipette, add a measured volume of acid to a conical flask.
2. Place the electrode to the pH meter in the flask.
3. Add the aqueous base to the burette and add the acid in the conical flask 1cm3 at a time.
4. After each addition, swirl the contents, record the pH and the total volume of the aqueous base added.
5. Repeat steps 3 and 4 until the pH starts to change more rapidly. Then, add the aqueous base dropwise for each reading until the pH changes less rapidly.
6. Now, add the aqueous base 1cm3 at a time again until the excess has been added and the pH has been basic with little change for several additions.
7. A graph of pH against total volume is plotted then.

Question 20

What is an alternative method in using a pH meter?

Answer 20

Attach the pH meter to a datalogger and use a magnetic stirrer in the flask. Connect the logger to a computer and a graph could be plotted automatically.

Question 21

What does the pH against volume of base added graph look like

Answer 21

An S, but the centre is a vertical straight line.

The centre of the vertical section is the equivalence point.

Question 22

Define the equivalence point

Answer 22

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

Question 23

The end point of the acid- base indicators

Answer 23

At the end point, the indicator contains equal concentration of HA and A-. And the end colour will be a mix of the 2 colours.

Question 24

What is an acid-base indicator

Answer 24

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

Question 25

What happens to an acid- base indicators when you add a basic solution

Answer 25

You tend to increase OH- ions.
Those extra OH- ions react with H+ ions to make H20
So, the weak acid dissociates, shifting the EP right, to produce more of the H+ ions.

Question 26

What happens to an acid- base indicators when you add an acid solution

Answer 26

You increase the H+ ions, so the EP moves left to produce more of the HA, and reduce the extra H+ concentration.

Question 27

How sensitive is the end point

Answer 27

Since indicators are just weak acids, they have a different Ka value and they change colours at different pH ranges. Essentially, the pH = pKa.

Question 28

Choosing an indicator

Answer 28

• Use one that has a colour change that coincides with the vertical section of the pH titration curve.
• Ideally, the end point + equivalence point should coincide.