chapter 21

Question 1

what are buffer solutions?

Answer 1

a system that minimises pH changes when small amounts of an acid or alkali are added
contains weak acid and conjugate base

Question 2

what is needed to make a buffer solution?

Answer 2

a larger reservoir of both HA and A-

Question 3

what happens when you add an acid to a buffer system?

Answer 3

[H+] increases initially
large reservoir of A- ions reacts with excess H+
equilibrium moves to the left
[H+] and pH are restored

Question 4

what happens when you add alkali to a buffer system?

Answer 4

OH- ions react with H+ ions
[H+] decreases
large reservoir of HA molecules dissociate
equilibrium position moves to the right
[H+] and pH are restored

Question 5

how do you prepare a buffer solution?

Answer 5

1. mix solution containing weak acid with a solution that contains a salt of the weak acid
   or
2. add aqueous solution of an alkali to an excess of a weak acid, then partially neutralise weak acid, solution then contains unreacted HA and A- salt

Question 6

how do you calculate the pH of a buffer?

Answer 6

[H+] = Ka x [HA]/[A-]

Question 7

what do you change for coarse tuning of pH?

Answer 7

Ka

Question 8

what do you change for fine tuning of pH?

Answer 8

[HA]/[A-]

Question 9

what does the blood plasma pH need to be maintained at?

Answer 9

between 7.35 and 7.45

Question 10

what is the pH in the body controlled by?

Answer 10

the carbonic acid - hydrogencarbonate buffer system

Question 11

what happens when their is a drop in pH of blood plasma?

Answer 11

[H+] increases
H+ ions react with conjugate base HCO3-
equilibrium moves to the left
[H+] and pH are restored

Question 12

what happens when their is an increase in pH of blood plasma?

Answer 12

[OH-] increases
OH- ions react with H+ ions
[H+] decreases
H2CO3 dissociates
equilibrium moves to the right
[H+] and pH are restored

Question 13

what is the purpose of titrations?

Answer 13

to find equivalence point for a reaction volume of one solution that exactly reacts with the volume of the other solution

Question 14

what does the titration curve look like for strong acid and strong base?

Answer 14

vertical section centred at pH 7

Question 15

what does the titration curve look like for strong acid weak base?

Answer 15

vertical section centred at pH<7

Question 16

what does the titration curve look like for weak acid strong base?

Answer 16

vertical section centred at pH> 7?

Question 17

what does the titration curve look like for weak acid weak base?

Answer 17

no vertical section

Question 18

what do acid-base indicators do?

Answer 18

enable us to find equivalence point in a titration without curve
causes colour change which coincides with vertical section of curve

Question 19

what are examples of acid-base indicators?

Answer 19

methyl orange
phenolphthalein

Question 20

what are the colour changes for methyl orange?

Answer 20

red —> yellow
HA —-> A-

Question 21

what are acid-base indicators?

Answer 21

weak acid
distinctly different colour from conjugate bases

Question 22

what are the colour changes for phenolpthalein?

Answer 22

colourless —> pink
HA ——> A-

Question 23

what happens when you add a strong base to a strong acid with methyl orange?

Answer 23

initially red due to high [H+]
base added so OH- reacts with H+
weak acid dissociates
equilibrium shifts right
end point of titration when HA=A- and colour is in between two extremes (orange)

Question 24

what happens when you add acid to a basic solution with methyl orange?

Answer 24

methyl orange initially yellow due to low [H+]
acid is added so H+ ions react with A- ions
equilibrium shifts left
end point when HA=A- (orange colour)

Question 25

what is true when [HA]=[H+]?

Answer 25

[H+]=Ka
pH=pKa

Question 26

what is the equivalence point?

Answer 26

when the volume of one solution has reacted exactly with the volume of the second solution

Question 27

what is the end point of a titration?

Answer 27

when indicator colour changes
[HA]=[H+]

Question 28

what should you consider when choosing an indicator for a titration?

Answer 28

choose one that has a colour change that coincides with the vertical section of the titration curve