CL 21- buffers and neutralisation

Question 1

what is a buffer solution?

definition

Answer 1

• a mixture that minimises pH changes on addition of small amounts of an acid or a base

Question 2

what does a buffer solution contain?

Answer 2

• a weak acid
• a salt of the weak acid (a conjugate base)

Question 3

dissociation of ethanoic acid

equation

Answer 3

CH₃COOH ₍ₐ₎ ⇌ CH₃COO⁻ ₍ₐ₎ + H⁺ ₍ₐ₎

Question 4

dissociation of solid sodium ethanoate

equation

Answer 4

CH₃COONa ₍ₐ₎ ⇌ CH₃COO⁻ ₍ₐ₎ + Na⁺ ₍ₐ₎

Question 5

how can you use ethanoic acid to form a buffer solution?

Answer 5

• ethanoic acid is the source of the weak acid component of the buffer solution
• when dissociated, a very small amount of CH₃COO⁻ is made
• CH₃COONa provides a convenient source of the conjugate base
• when CH₃COONa is added, it will produce more CH₃COO⁻ as seen in it’s dissociation
• this shifts the equilibrium of ethanoic acid to the left

Question 6

HA ⇌ H⁺ + A⁻

how does a buffer solution control pH?
(a) when acid is added

use equation to illustrate

Answer 6

if some H⁺ is added
* H⁺ + A⁻ ⇌ HA
* added acid reacts with conjugate base, A, to produce more of the weak acid HA
* this shifts the equilbrium of the buffer solution to the left (as [H⁺] decreased)

Question 7

HA ⇌ H⁺ + A⁻

how does a buffer solution control pH?
(b) when alkali is added

use equation to illustrate

Answer 7

if NaOH (eg) is added
* H⁺ + OH⁻ ⇌ H₂O
* added alkali reacts with H⁺ to make water
* therefore [H⁺] decreases, so equilbrium of buffer solution shifts to the right to replace H⁺ ions and oppose the change

Question 8

Ka for buffer solution

calculation

Answer 8

Ka = [H⁺][A⁻]
……..[HA]

Question 9

assumptions made for Ka of buffers

two assumptions

Answer 9

1. [A⁻] comes from salt
2. [HA] = initial conc of HA

Question 10

explain how blood pH is controlled by the carbonic acid-hydrogencarbonate buffer system

Answer 10

H₂CO₃ ⇌ H⁺ + HCO₃⁻

on addition of an acid,
* [H⁺] increases
* H⁺ ions react with the conjugate base, HCO₃⁻
* equilbrium shifts to the left

on addition of an alkali,
* [OH⁻] increases
* small conc of H⁺ ions reacts with the OH⁻ ions
* H₂CO₃ dissociates and equilbrium shifts to the right

Question 11

what is the equivalence point?

Answer 11

• the point in a titration at which the volume of one solution has reacted completely

Question 12

general shape of a pH curve

Answer 12

• s-shaped
• vertical section length depends on strength

Question 13

strong acid-strong base curve

features

Answer 13

• longest vertical section
• starts at 1-2 pH
• ends at 13-14 pH
• equivalence point = 7

Question 14

weak acid-strong base curve

features

Answer 14

• starts at 4-5 pH
• ends at 13-14 pH
• equivalence point > 7

Question 15

strong acid-weak base curve

Answer 15

• starts at 1-2 pH
• ends at 9-10 pH
• equivalence point < 7

Question 16

weak base-weak acid curve

Answer 16

• no vertical section
• equivalence point = 7
• cant use an indicator

Question 17

features of a good indicator

Answer 17

• change colour over a narrow pH range
• two definitive identifiable colour changes
• one of the colours should be intense

Question 18

how to choose a suitable indicator

Answer 18

• choose an indicator which changes colour as close as possible to equivalence point
• pH range of indicator must lie within vertical portion of graoh
  e.g strong acid-strong base reactions are easier to find indicators for, as they have such a large vertical section
  but weak acid-weak base reactions can’t use indicators due to no vertical section