13. Buffers

Question 1

a buffer solution is a

Answer 1

mixture that minimises PH changes on addition of small amounts of acid or base

Question 2

blood PH below 7.35

Answer 2

acidosis

Question 3

blood PH above 7.45

Answer 3

alkalosis

Question 4

acidic buffers contain

Answer 4

weak acid and its conjugate base

Question 5

acidic buffers have a PH of less than

Answer 5

7

Question 6

how are acidic buffers made

Answer 6

by setting up an equilibrium between  weak acid and its conjugate base 
e.g. CH3COOH(aq) -> H+(aq) + CH3COO-(aq)
contains lots of undissociated acid HA 
lots of acids conjugate base A-
enough H+ ions to make it acidic

Question 7

2 ways in which acidic buffer can be made

Answer 7

1. weak acid + salt of its conjugate base

2. excess of weak acid + strong alkali

Question 8

making an acidic buffer
1. weak acid + salt of its conjugate base
how is the equilibrium set up?

Answer 8

e.g. ethanoic acid and sodium ethanoate

salt fully dissociates into its ions
CH3COO-Na+(aq) -> CH3COO-(aq) + Na+(aq)

weak acid slightly dissociates
CH3COOH(aq) -> H+(aq) + CH3COO-(aq)

= equilibrium set up CH3COOH(aq) -> H+ + CH3COO-

Question 9

making an acidic buffer
2. excess of weak acid + strong alkali
how is the equilibrium set up?

Answer 9

e.g. ethanoic acid and sodium hydroxide

all base reacts with the acid
CH3COOH(aq) + OH-(aq) -> CH3COO-(aq) + H2O

some weak acid left (excess). slightly dissociates
CH3COOH(aq) -> CH3COO-(aq) + H+(aq)

= equilibrium set up CH3COOH(aq) -> H+ + CH3COO-

Question 10

how does the equilibrium set up minimise changes in PH

Answer 10

conjugate base mops up an excess of H+

conjugate acid releases H+ if theres too much base

Question 11

acidic buffers

if you add a small amount of acid H+ concentration increases

Answer 11

CH3COOH(aq) -> H+(aq) + CH3COO-(aq)

H+ ions combine with CH3COO- ions to form CH3COOH
this shifts equilibrium to the left reducing H+ concentration to its original value so PH doesn’t change much

Question 12

acidic buffers

if you add a small amount of alkali e.g. NaOH the OH- concentration increases

Answer 12

CH3COOH(aq) -> H+(aq) + CH3COO-(aq)

most of the extra OH- reacts with H+ ions to form water
removing H+ ions from solution
causes CH3COOH to dissociate to form H+ ions shifting equilibrium to the right
H+ concentration increases until its close to original value so PH doesn’t change much

Question 13

in our bodies blood needs to be kept between PH

Answer 13

7.35 and 7.45

Question 14

PH in the blood is controlled by

Answer 14

a carbonic acid-hydrogen carbonate buffer system

H2CO3(aq) -> H+(aq) + HCO3-(aq)

H2CO3(aq) -> H2O(l) + CO2(aq)

Question 15

buffer solution in the blood

how are the levels of H2CO3 controlled

Answer 15

by respiration
breathing out CO2
level of H2CO3 is reduced as it moves equilibrium to right
H2CO3(aq) -> H2O(l) + CO2(aq)

Question 16

buffer solution in the blood

how are the levels of HCO3- controlled

Answer 16

by the kidneys
excess is secreted in the urine
H2CO3(aq) -> H+(aq) + HCO3-(aq)

Question 17

what do you need to know to calculate the PH of a buffer solution

Answer 17

Ka of the weak acid

concentrations of the weak acid and its salt

Question 18

assumptions when calculating PH of a buffer solution

Answer 18

salt of the conjugate base is fully dissociated. assume equilibrium conc A- is the same as the initial conc of the salt
HA is only slightly dissociated. assume equilibrium conc is the same as the initial conc