Chapter 21 Buffers

Question 1

what are buffer solutions ?

Answer 1

• Buffer solutions are systems that minimise pH changes with the addition of small ammounts of acid or base
• they are made of weak acids and their conjugate bases
• HA removes alkali, A- removes acid

Question 2

what do bufffers contain?

Answer 2

• HA and A- act as 2 idependent reservoirs that are capable of reacting with added acid and alkali
• Buffer is no longer effective when 1 reservoirs is used up

Question 3

how to prepare a buffer from a weak acid and salt ?

Answer 3

• mixing ethanoic acid (weak acid) with solution of one of its salts e.g sodium ethanoate in solution
• salt of weak acid is conjugate base and since ionic they completely dissociate to provide [A-]
• dissociation of weak acid is small so [HA] is the same

Question 4

how to prepare a buffer by partial neutralisation of weak acid ?

Answer 4

• aqueous solution of alkali is added to excess weak acid
• weak acid is partially neutralised forming conjugate base
• some weak acid remains so solution contains conjugate base and weak acid

Question 5

how do buffers work ?

Answer 5

• as buffers have 2 independent reservoirs a reaction with one will cause a shift in equilibrium to negate the change

Question 6

what happens when an acid is added to a buffer?

Answer 6

• adding acid increases the ammout of H+ ions
• the extra H+ ions react with A- to form HA
• the equilibrium position shifts to the left to reduce [H+]
• this limits decreases in pH

Question 7

what happens when a base is added to a buffer?

Answer 7

• adding base increase the ammount of OH- ions
• the extra OH- reacts with H+ to form water
• [H+] decreases shifting equilibrium to the right
• more HA discociates to replace H+ reducing change in pH

Question 8

when are buffers most effective

Answer 8

• most effective when [HA] = [A-]
• different weak acids form buffer solutions that work over different ranges
• the pH of a buffer = pKa of HA
• Operating range often +- 1 pH about pKa

Question 9

Why does [H+] not equal [A-] in buffers?

Answer 9

• A- has been added as one of the components of the buffer

Question 10

an example of calculating pH of a buffer

weak acid and its salt

Answer 10

Question 11

an example of calculating pH of a buffer

partial neutralisation of weak acid

Answer 11

Question 12

typical buffer 7 marker

Answer 12

Question 13

what buffer is found in blood plasma?

Answer 13

• Carbonic acid [HA] and hydrogencarbonate [A-]
• Blood plasma has a pH of 7.35-7.45

Question 14

what happens when the pH in blood plasma is below 7.35 ?

Answer 14

• pH below 7.35, can lead to acidosis
• acidosis - fatigue, shortness of breath and potential death

Question 15

what happens when the pH in blood plasma is above 7.45 ?

Answer 15

• pH above 7.45 can lead to alkalosis
• alkalosis - spasms, light-headed, and nausea

Question 16

how to conduct a strong acid-base titration with a pH meter ?

Answer 16

Question 17

what affects the shape of a titration curve?

Answer 17

• substance being titrated, e.g acid or base and which one starts in the buerette
• the type of acid or base, weak or strong

Question 18

what are the 4 key features to pH titration curves ?

Answer 18

• excess of base
• vertical section
• equivilance point
• excess of acid

Question 19

what is the equivalence point ?

Answer 19

• the equivalence point is the volme required to have completely reacted the acid and base together
• the pH at the equivalence point depends on the type of acid and base
• strong acid + strong base = pH 7
• Strong acid + weak base = pH < 7
• weak acid + strong base = pH > 7

Question 20

what are acid-base indicators and how do they work ?

Answer 20

• acid base indicators are weak acids that have different coloured conjugate bases
• in acidic conditions the indicator equilibrim is shifted towards the weak acid (HIn)
• as the system becomes more basic the equilibrium position shift towards the conjugate base (In-), altering the color

Question 21

what is the end point ?

Answer 21

• end point is when [HA] = [A-]
• the colour will lie between both extremes
• each indicator will have a different pH value for the end point as they have different K_a values

Question 22

how to determine pH of endpoint ?

Answer 22

• the pH can be found from pK_a value of HA at end point
• [HA] = [A-], This means K_a = [H+]

Question 23

how to choose a suitable indicator for titrations?

Answer 23

• need to choose an indicator that has a colour change that coincides with the vertical section of the pH curve
• due to small vertical section for weak acid + weak base there is NO SUITABLE INDICATOR