Chapter 21 - Buffers and Neutralisation

Question 1

What is a buffer solution?

Answer 1

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

Question 2

What are buffers made out of?

Answer 2

A weak acid and its conjugate base
The weak acid HA removes added alkali
The conjugate base A- removes added acid

Question 3

What are the two ways a weak acid buffer solution can be prepared?

Answer 3

• mixing a weak acid with one of it’s salts
• adding an aqueous solution of an alkali to an excess of the weak acid

Question 4

How is a buffer prepared from a weak acid and its salt?

Answer 4

Mixing solution of weak acid (e.g. CH3COOH) with a solution of one of its salts (e.g. CH3COONa)
When ethanoic acid is added to water, the acid partially dissociates. Salts of weak acids are ionic compounds and provide conjugate base.
Acid = acid, salt = base

Question 5

How is a buffer prepared by the partial neutralisation of a weak acid?

Answer 5

Adding aqueous solution of an alkali to excess of weak acid, which becomes partially neutralised to form conjugate base. Some of weak acid is left unreacted which = acid. Mixture of salt of weak acid and unreacted weak acid.

Question 6

How does the conjugate base in a buffer solution remove added acid?

Answer 6

On adding acid:
- [H+] increases
- H+ ions react with conjugate base, A-
- equilibrium shifts to the left, removing most of the H+ ions

Question 7

When is a buffer most effective?

Answer 7

When there are equal concentrations of the weak acid and its conjugate base
or when [HA] = [A-]

Question 8

What is the pH of a buffer solution typically the same as, and what is the range of operation?

Answer 8

pH = same as pKa value of HA
The operating pH is typically over about two pH units, centred at the pH of the pKa value

Question 9

How can you calculate the pH of a buffer solution?

Answer 9

[H+] = Ka x [HA]/[A-]

Question 10

What is the pH of normal healthy blood?

Answer 10

7.40
Blood plasma needs to be maintained between 7.35 and 7.45

Question 11

Why must blood be buffered?

Answer 11

The well being of the human body relies on precise pH control - enzymes are particularly sensitive and each have an optimum pH

Question 12

What happens to the body if pH slips outside the ideal range?

Answer 12

Below 7.35 = acidosis (fatigue, shortness of breath, death)
Above 7.45 = alkalosis (muscle spasms, light headedness, nausea)

Question 13

What is the equilibrium for the blood buffer carbonic acid-hydrogencarbonate?

Answer 13

H2CO3 ⇌ HCO3- + H+

Question 14

How does the carbonic acid-hydrogencarbonate buffer system work?

Answer 14

Add acid:
[H+] increases, reacts with conjugate base HCO3- and eqm shifts to left, removing most of H+ ions
Add alkali:
[OH-] increases, small conc of H+ ions reacts with OH- ions, H2CO3 dissociates shifting eqm right to restore H+ ions

Question 15

How can you calculate the concentration ratio of HCo3-/H2CO3?

Answer 15

Rearrange Ka equation
[HCO3-]/[H2CO3] = Ka/[H+]

Question 16

What are the key features of a typical pH curve (adding base to acid)?

Answer 16

When base is first added, acid in great excess so pH increases very slightly. At the vertical section, the pH increases rapidly during addition of a very small volume of base. After this section, the pH will rise very slightly as the base is now in great excess

Question 17

What is the equivalence point of a titration?

Answer 17

The volume of one solution that exactly reacts with the volume of the other solution. Centre of the vertical section of the pH curve
Moles of base = moles of acid

Question 18

What is an acid base indicator made up of?

Answer 18

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

Question 19

What happens at the end point of a titration for an indicator?

Answer 19

The indicator contains equal concentrations of HA and A-, so the colour will be in between the 2 extreme colours

Question 20

How does an indicator (such as methyl orange) change colour with the addition of an acid or a base?

Answer 20

ADDING ACID:
- H+ ions react with conjugate base A-, shifting eqm to left so colour will change to that of HA (red for methyl orange)
ADDING BASE:
- OH- ions react with H+, causing HA to dissociate and shift eqm to right, colour changes to that of A- (yellow for methyl orange)

Question 21

What is the pH of the end point the same as?

Answer 21

The pKa value of HA

Question 22

How must you choose an indicator for a titration?

Answer 22

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