Chapter 21 - Buffers and Neutralisation

Question 1

What is a buffer solution ?

Answer 1

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

Question 2

What are the two components of a buffer solution ?

Answer 2

A weak acid and its conjugate base

Question 3

What is the role of the weak acid ?

Answer 3

It removes the added alkali

Question 4

What is the role of the conjugate base ?

Answer 4

It removes the added acid

Question 5

Does the pH change when a buffer works ?

Answer 5

• The pH does change but only by a small amount
• Do not assume it stays constant

Question 6

What are the two methods of preparing a weak acid buffer solution ?

Answer 6

• From a weak acid and its salt
• From the partial neutralisation of a weak acid

Question 7

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

Answer 7

• Mixing a solution of a weak acid with one of its salt
• The acid is the source of the weak acid component
• The salt of the weak acid dissociates and provides the conjugate base

Question 8

How can a buffer be prepared from the partial neutralisation of a weak acid ?

Answer 8

• Adding an aqueous solution of an alkali to an excess of the weak acid
• Weak acid is partially neutralised by the alkali forming the conjugate base
• Some of the weak acid is left over unreacted providing the weak acid component

Question 9

What does the conjugate acid base pair in an acid buffer solution control ?

Answer 9

It controls the pH

Question 10

How does the conjugate base remove added acid ?

Answer 10

• H+ conc increases
• H+ ions react with the conjugate base
• The eqm position shifts to the left, removing most of the H+ ions

Question 11

How does the weak acid remove alkali ?

Answer 11

• OH- conc increases
• The small conc of H+ ions reacts with the OH- ions
• HA dissociates, shifting the eqm position to the right to restore most of the H+ ions

Question 12

When is a buffer most effective at removing the added acid or alkali ?

Answer 12

When there are equal concentrations of the weak acid and its conjugate base

Question 13

What are the two assumptions when [HA] = [A-] ?

Answer 13

• Ka = [H+]
• pKa = pH

Question 14

What is the pH of the blood plasma like ?

Answer 14

Needs to be maintained at a pH between 7.35 and 7.45

Question 15

What is the pH of the blood plasma controlled by ?

Answer 15

• A mixture of buffers
• With the carbonic acid - hydrogencarbonate buffer system

Question 16

What is the pH of normal healthy blood ?

Answer 16

7.4

Question 17

What happens if the pH falls below 7.35 ?

Answer 17

People can develop a condition called acidosis

Question 18

What is acidosis ?

Answer 18

Can cause fatigue, shortness of breath and in extreme conditions, shock or death

Question 19

What happens if the pH rises above 7.45 ?

Answer 19

People can develop a condition called alkalosis

Question 20

What is alkalosis ?

Answer 20

Can cause muscle spasms, light-headedness and nausea

Question 21

What happens on the addition of an acid in the blood plasma buffer system ?

Answer 21

• H+ concentration increases
• H+ ions react with the conjugate base, HCO3-
• The equilibrium position shifts to the left, removing most of the H+ ions

Question 22

What happens on the addition of an alkali in the blood plasma buffer system ?

Answer 22

• OH - concentration increases
• The small concentration of H+ ions react with OH- ions to create H2O
• H2CO3 dissociates, shifting the equilibrium position to the right to restore most of the H+ ions

Question 23

How does the body prevent the build up of H2CO3 ?

Answer 23

It is converted into CO2 gas, which is then exhaled by the lungs

Question 24

What are the significant points on a pH titration curve ?

Answer 24

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

Question 25

What is the equivalence point ?

Answer 25

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

Question 26

How can you find the equivalence point from a graph ?

Answer 26

The equivalence point is the centre of the vertical section of the pH titration curve

Question 27

What is an acid-base indicator ?

Answer 27

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

Question 28

What is the end point of the titration ?

Answer 28

• Where the indicator contains equal concentrations of HA and A-
• The colour will be in between the colour of the acid and the base

Question 29

What is equal to what at the end point ?

Answer 29

• [HA] = [A-] and therefore Ka = [H+]
• Ka = [H+] and pKa = pH

Question 30

How must you choose an indicator, in a titration ?

Answer 30

You must use an indicator that has a colour change that coincides with the vertical section of the pH titration curve

Question 31

Is there a suitable indicator for a weak acid and a weak base ?

Answer 31

No indicator is suitable

Question 32

Why is no indicator suitable for a weak acid and a weak base ?

Answer 32

There is no vertical section

Question 33

What indicator is suitable for a strong acid and a strong base ?

Answer 33

Phenolphthalein and methyl orange are both suitable

Question 34

What indicator is suitable for a strong acid and a weak base ?

Answer 34

• Methyl orange is suitable
• Phenolphthalein is not suitable

Question 35

What indicator is suitable for a weak acid and a strong base ?

Answer 35

• Phenolphthalein is suitable
• Methyl orange is not suitable