buffers

Question 1

what is a buffer

Answer 1

a buffer is a solution that minimises changes in pH when small amounts of acid or base are added

Question 2

acid buffer

Answer 2

acid buffers has a pH of less than 7

Question 3

how to make an acidic buffer (1)

Answer 3

1) mix a weak acid with the salt of its conjugate base
- salt fully dissociated into its ions when it dissolves
- weak acid only slightly dissociates

Question 4

how to make an acidic buffer (2)

Answer 4

2) mix an excess of weak acid with a strong alkali
- all the base reacts with the acid
- weak acid was in excess so there’s still some left in solution once all the base has reacted, this acid slightly dissociates

Question 5

acid buffer equilibrium

Answer 5

CH3COOH <-> H+ + CH3COO-
- lots of undissociated weak acid (HA)
- lots of the acids conjugate base (A-)
- enough H+ ions to make the solution acidic

Question 6

what happens if you add a small amount of acid

Answer 6

• H+ concentration increases
• most of extra H+ ions combine with CH3COO- ions to form CH3COOH
• this shifts equilibrium to the left reducing the H+ concentration close to original value
• pH doesn’t change much

Question 7

what happens if you add a small amount of alkali

Answer 7

• the OH- concentration increases
• most of the extra OH- ions react with H+ ions to form water to remove H+ ions from the solution
• this causes more CH3COOH to dissociate to form H+ ions which shifts equilibrium to the right
• H+ concentration increases until close to original value so pH doesn’t change much

Question 8

buffer solutions in blood

Answer 8

• blood needs to be kept between 7.35 and 7.45
• pH controlled by using a carbonic acid-hydrogen carbonate buffer system
• levels of H2CO3 are controlled by respiration, CO2 is breathed out and HCO3 is reduced so equilibrium shifts right
• HCO3- levels are controlled by the kidneys with excess being excreted in urine

Question 9

equilibrium reactions in blood

Answer 9

H2CO3 <-> H+ + HCO3-
H2CO3 <-> H2O + CO2

Question 10

how to calculate pH of a buffer solution

Answer 10

• write expression for Ka of the weak acid
• rearrange expression and put data in to calculate [H+]
• convert [H+] to pH
  pH= -log10[H+]

Question 11

to calculate pH of a buffer, 2 assumptions

Answer 11

• salt of the conjugate base is fully dissociated, equilibrium concentration of A- is the same as the initial concentration of the salt
• HA is only slightly dissociates so its equilibrium concentration is the same as its initial concentration