[21.1] buffer solutions

Question 1

what is a buffer?

Answer 1

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

Question 2

what 2 components of buffers mean pH changes are minimised?

Answer 2

• weak acid, HA, removes added alkali
• conjugate base, A-, removes added acid

Question 3

what 2 ways can buffers be prepared?

Answer 3

• mixture of weak acid and its salt
• mixture of an excess of WA and strong base (partial neutralisation of WA)

Question 4

how can buffers be prepared from a weak acid and its salt? (based on ethanoic acid)

Answer 4

• when ethanoic acid is added to water, the acid partially dissociates and the amount of ethanoate ions in solution is very small
• when salts of weak acids are added to water, it completely dissolves
• ethanoic acid = source of weak acid component
• salts of weak acids = source of conjugate base

CH₃COOH (aq) ⇌ H⁺(aq) + CH₃COO⁻ (aq)
CH₃COONa (s) + aq -> CH₃COO⁻ (aq) + Na⁺ (aq)

Question 5

how can control of pH be explained in terms of shifts in the equilibrium positions using le chatelier’s principle when alkali is added?

Answer 5

HA (aq) ⇌ H+ (aq) + A- (aq)
1. [OH-] increases
2. small conc of H+ ions react with OH- ions: H⁺ + OH⁻ -> H₂O
3. HA dissociates
4. POE shifts to right to restore most of H+ ions

Question 5

how can control of pH be explained in terms of shifts in the equilibrium positions using le chatelier’s principle when acid is added?

Answer 5

HA (aq) ⇌ H+ (aq) + A- (aq)
1. [H+] increases
2. H+ ions react with conjugate base, A-
3. equilibrium position shifts to the left, removing most of the H+ ions

Question 5

when is a buffer most effective at removing either added acid or alkali?

Answer 5

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

Question 5

how can buffers be prepared by partial neutralisation of the weak acid (ethanoic acid)?

Answer 5

• aqueous solution of an alkali + excess of weak acid
• weak acid is partially neutralised by alkali, forming the conjugate base
• resulting solution contains a mixture of salt of WA and any unreacted WA
• CH₃COOH and CH₃COO⁻ act as two resevoirs that are able to remove added acid and alkali by shifting the buffer’s equilibrium

CH₃COOH (aq) ⇌ H⁺ (aq) + CH₃COO⁻ (aq)

Question 6

why is a buffer most effective when [HA] = [A-]?

Answer 6

• pH of buffer solution is same as pKa value of HaA
• operating pH is around 2 pH units, centred at pH of pKa value

Question 7

how do you calculate the pH of a buffer solution?

Answer 7

[H+] = Ka × ([HA] / [A-])

Question 8

how do you calculate the pH of a buffer solution when [HA] = [A-]?

Answer 8

• Ka = [H+]
• pKa = pH