Chapter 21 Buffers And Neutralisation

Question 1

State what is meant by a buffer

Answer 1

A system which minimises pH changes on addition of small amounts of an acid or base

Question 2

When can the pH of a buffer system change?

Answer 2

Adding large amounts of acid or base will change the pH of the buffer system

Question 3

Describe how a buffer is formed

Answer 3

-An acidic buffer can be formed from a weak acid (eg, ethanoic acid) and the salt of a weak acid (eg sodium ethanoate)
-The reaction between excess weak acid (eg, ethanoic acid) and a strong alkali (eg, NaOH)
Because the acid is in excess it will be able to react with the salt formed from the neutralisation reaction between the weak acid and strong base to form the buffer

Question 4

What is an acidic buffer?

Answer 4

A system that buffers the pH below 7

Question 5

Give the equation for the buffer system of ethanoic acid.

Answer 5

CH3COOH <—> CH3COO^- + H+

Question 6

Describe how the buffer system works when more H+ is added

Answer 6

The negative ions (eg, ethanoate ions CH3COO^-) will combine with the extra H+ ions shifting equilibrium to the left forming more of the weak acid to restore pH.

Question 7

Describe how the buffer system works when more base is added

Answer 7

The base will remove the H+ ions
This means that the weak acid has to dissociate more to replace the lost H+ ions causing equilibrium to shift to the right restoring pH.

Question 8

What is the equation used to find [H+] of buffers?

Answer 8

Ka x [ACID]
[H+] = —————-
[SALT]
PH = -log [H+]

Question 9

What is the equation used to find Ka of buffers?

Answer 9

[H+] [SALT]
Ka= —————-
[ACID]

Question 10

What is the formula used to find Acid: Salt ratio?

Answer 10

Ka [SALT] [SALT]
——- = ———- = ———-
[H+] [ACID] 1

Question 11

Describe how the blood pH is controlled by a buffer system

Answer 11

The pH of blood is maintained at 7.40 by a buffer system
Contain carbonic acid H2CO3 and hydrogen carbonate ions HCO3-
H2CO3 <—> HCO3- + H+

Question 12

Describe how the buffer system of the blood counteracts an increase of H+ ions

Answer 12

The H+ will combine with the Hydrogen carbonate ions ( HCO3-) and shift equilibrium back to the left to restore pH forming more carbonic acid.
The carbonic acid will then dissociate into H2O and CO2 to prevent it from building up in the blood.

Question 13

State the name that describes an increase of H+ ions in the blood

Answer 13

Acidosis where the pH lowers <7.35

Question 14

Describe how the buffer system of the blood counteracts a decrease of H+ ions

Answer 14

Carbonic acid (H2CO3) will dissociate more shifting equilibrium to the right to produce more H+ ions.

Question 15

State the name that describes a decrease of H+ ions in the blood

Answer 15

Alkalosis where pH rises >7.45

Question 16

What type of buffer calculations may be asked?

Answer 16

• Direct combination of the weak acid and the salt forming the buffer
• From the combination of excess acid and a base where the excess acid will react with the salt formed (same number of moles as the limiting base) to form a buffer

Question 17

What is the half equivalence point?

Answer 17

In the middle of the buffer region for weak acid and strong base pH titration curve
This is where there is equal concentration of weak acid and salt meaning it will have the maximum buffer capacity to deal with changes in OH- and H+

Question 18

Name two indicators which can be used for acid- base titrations

Answer 18

Methyl orange
Phenolphthalein

Question 19

How is the correct indicator selected for an acid base titration?

Answer 19

The indicator will need to change colour at the equivalence point of titration.
Therefore, the pH range of the indicator needs to lie within the pH range of the vertical section of the curve.

Question 20

What is the indicator colour of methyl orange at low pH?

Answer 20

Red

Question 21

What is the indicator colour of phenolphthalein at low pH?

Answer 21

Colourless

Question 22

What is the approximate pH range for the colour change of methyl orange?

Answer 22

3.1 - 4.4

Question 23

What is the approximate pH range for the colour change of phenolphthalein?

Answer 23

8.3 - 10

Question 24

What is the colour change of methyl orange at high pH?

Answer 24

Yellow

Question 25

What is the colour change of phenolphthalein at high pH?

Answer 25

Pink

Question 26

What indicators can be used for strong acid/ strong base titrations?

Answer 26

Methyl orange
Phenolphthalein

Question 27

What indicators can be used for weak acid/ strong base titrations?

Answer 27

Phenolphthalein only

Question 28

What indicators can be used for strong acid/ weak base titration?

Answer 28

Methyl orange only

Question 29

What indicators can be used for weak acid/ weak base titrations?

Answer 29

Neither indicators
PH meter needs to be used instead of

Question 30

How do indicators work?

Answer 30

Indicators are weak acids and work as they have different coloured conjugate pairs.
As the pH of the solution changes during a titration, the equilibrium concentrations of the conjugate pairs also change. The colour will change depending on whether the indicator is protonated or deprotonated.

Question 31

State what the vertical section of a pH titration curve shows

Answer 31

The volume of the substance in the burette needed to neutralise the substance in the conical flask.