Chapter 21 - Buffers and Neutralisation

Question 1

What is a buffer solution?

Answer 1

System that minimises pH change when a small amount of acid or base is added

Question 2

What are the two components of a buffer?

Answer 2

1) weak acid (HA) which removes added alkali
2) conjugate base (A^-) which removes added acid

Question 3

How is a buffer solution prepared from a weak acid and it’s salt?

Answer 3

Mix weak acid with it’s salt
- Weak acid added to water: partially dissociates so source of weak acid ONLY
- Salt added to water: fully dissociates so source of conjugate base ions

Question 4

How is a buffer solution prepared from partial neutralisation of weak acid?

Answer 4

• Add aqueous solution of alkali to excess of weak acid (excess will allow some weak acid to remain)
• Reaction will form a salt as weak acid is partially dissociated to it’s conjugate base

Question 5

Explain buffer action when acid is added

Answer 5

• [H⁺] increases
• H⁺ ions react with conjugate base A^-
• Equilibrium shifts to left to remove H⁺ ions

Question 6

Explain buffer action when alkali is added

Answer 6

• [OH^-] increases
• Small concentration of H⁺ ions from acid react with OH^- ions to form water so H⁺ used up
• HA dissociates
• Equilibrium shifts to right to restore H⁺ ions

Question 7

When is a buffer solution most effective?

Answer 7

When there is equal concentrations of weak acid and conjugate base [HA] = [A^-]

Question 8

What is the pH of a buffer solution when [HA] = [A^-]?

Answer 8

pKa of HA because Ka = [H⁺]

Question 9

What assumption for weak acids is no longer valid for buffer solutions?

Answer 9

[H⁺] ≠ [A^-]

Question 10

What is the buffer system in blood?

Answer 10

H₂CO₃ <-> HCO₃^- + H⁺

Question 11

What is the ratio of HCO₃^- : H₂CO₃?

Answer 11

[HCO₃^-] = Ka
[H₂CO₃] [H⁺]

Question 12

What is the equivalence point?

Answer 12

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

Question 13

What is the end point?

Answer 13

Indicator contains equal concentrations of HA and A^- and the colour is in between the two extremes

Question 14

What does the pH equal at the end point?

Answer 14

pKa value of HA

Question 15

Why does the pH titration curve plateau slightly at the beginning of a weak acid - strong/weak base reaction?

Answer 15

Acts as a buffer to minimise pH change

Question 16

When is an indicator suitable?

Answer 16

When the pH range of the indicator passes through the vertical line of the pH titration curve