Buffers

Question 1

what is a buffer solution?

Answer 1

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

Question 2

what are the two methods of preparing a buffer solution?

Answer 2

• a weak acid and a salt of the weak acid (e.g.CH3COOH/CH3COONa)
• excess of a weak acid and a strong alkali (preparation by partial neutralisation of a weak acid) (e.g.excess CH3COOH/NaOH)

Question 3

how do you prepare a buffer solutions from a weak acid and its salt?

Answer 3

• CH3COOH would partially dissociate: CH3COOH(aq) <–> H+(aq) + CH3COO-(aq)
• the salt completely dissolves: CH3COONa(s) + aq –> CH3COO-(aq) + Na+(aq)

Question 4

how do you prepare a buffer solution from excess weak acid and a strong alkali?

Answer 4

• weak acid is partially neutralised by the alkali to form its conjugate base (salt)
• some of the weak acid is left over unreacted

Question 5

what are the 2 components of a buffer solution?

Answer 5

• a weak acid
• its conjugate base

Question 6

what does the weak acid do?

Answer 6

HA
- removes added alkali

Question 7

what does the conjugate base do?

Answer 7

A-
- removes added acid

Question 8

how do conjugate base in an acid buffer minimise changes in pH?

Answer 8

conjugate base removes added acid
- on addition to acid [H+], it increases
- H+ ions react with the conjugate base (A-)
- the equilibrium position shifts left, removing most of the H+ ions

Question 9

how do weak acid in an acid buffer minimise changes in pH?

Answer 9

weak acid removes added alkali
- on addition to the alkali, OH-
- [OH-] increases
- H+ ions reacts with the OH- ions: H+ +OH- –> H2O
- HA dissociates shifting equilibrium position to shift to the right to restore most of the H+ ions

Question 10

what is an acidic buffer?

Answer 10

weak acid + its conjugate base

Question 11

what is the general equations for buffers?

Answer 11

HA <–> H+ + A-