21 - Buffers and neutralisation

Question 1

what is a buffer solution?

Answer 1

a system that minimises pH changes when small amounts of an acid or base are added

Question 2

what does a buffer solution contain?

Answer 2

two components that remove added acid or alkali
- a weak acid
- its conjugate base

Question 3

what is the role of a weak acid in a buffer solution

Answer 3

it removes added alkali

Question 4

what is the role of a conjugate base in a buffer solution

Answer 4

removes added acid

Question 5

when does the buffer stop working?

Answer 5

when one of the components is used up, the solution starts becoming acidic or alkali

Question 6

state two ways to prepare a weak acid buffer solution?

Answer 6

• Preparation from a weak acid and its salt
• preparation by partial neutralisation of the weak acid

Question 7

how does a buffer that is prepared from a weak acid and its salt work?

Answer 7

• the weak acid in the buffer only partially dissociates so [A-] is very small. here is the source of the weak acid
• the salt of the weak acid completely dissolves and provides the source of the conjugate base

Question 8

a buffer solution contains CH3COOH and CH3COONa. explain using equations how this buffer solution works?

Answer 8

• this buffer solution uses a weak acid and its salt to minimise the pH change when acid or base is added
• ethanoic acid partially dissociates in water, however the amount of ethanoate ions produced is small- here is the source of the weak acid

CH3COOH <-> H+ + CH3COO-
^component 1

sodium ethanoate is the salt of ethanoic acid, when dissolved in water it forms the conjugate base CH3COO-

CH3C))Na -> Ch3COO- + Na=

Question 9

how does a buffer that is prepared by partial neutralisation of the weak acid?

Answer 9

add an aqueous solution of an alkali to an excess of weak acid.

The weak acid is partially netralised b te alkali forming the conjugate base, and some of the weak acid is left over.

Question 10

What is the action of the conjugate base in a buffer solution?

• use HA <—> H+ + A-

Answer 10

• it minimises pH change when acid is added to the colution
• [H+] increases
• H+ ions react with the conjugate base [A-]
• the equilibrium shifts to the left, removing the H= ions

Question 11

What is the action of the weak acid in a buffer solution?

• use HA <—> H+ + A-

Answer 11

• it minimises the pH change when alkali is added
• [OH-] increases
• H+ reacts with the OH- ions to form water
• HA dissociate, as [H+] has decreased, shifting the position to the right to restore H+

Question 12

Complete the equilibrium and label the conjugate acid-base pairs as A1, B1 and A2, B2.

HCOOH + CH3COOH ⇌ ________ + ____________

Ka(HCOOH) =1.82 × 10−4
Ka(CH3COOH) = 1.78 × 10−5

Answer 12

HCOOH + CH3COOH ⇌ HCOO– + CH3COOH2+
A1 B2 B1 A2

Question 13

The student calculates the pH of 0.0800 mol dm−3 HIO3(aq). The student assumes that the
equilibrium concentration of HIO3(aq) is the same as the initial concentration of HIO3(aq).
The student measures the pH, and finds that the measured pH value is different from the
calculated pH value.

Explain why the measured pH is different from the calculated pH

Answer 13

HIO3 dissociation is not negligible / dissociates to a significant extent

OR

Large Ka and HIO3 is ‘stronger’ (weak) acid

OR

[HIO3 ]eqm is significantly lower than [HIO3] initial/undissociated ✓

because Ka was relatively high, the
resultant acid strength would mean that the
degree of dissociating was significant
enough so initial [HIO3] was significantly
less than [HIO3] at equilibrium. In other
words, the assumption [HIO3]initial =
[HIO3]equilibrium is not valid in this case.

Question 14

Write an ionic equation to show how carbonate ions form an alkaline solution in water

Answer 14

CO32– + H2O → OH– + HCO3–

OR

CO32– + H2O → 2OH– + CO2

Question 15

when is a buffer most effeective?

Answer 15

when there are equal concentrations of weak acid and conjugate base

Question 16

what happens when
[HA] = [A-]
in a buffer solution

Answer 16

• the pH of the buffer solution is the same as the pKa value of HA
• the operating pH is over two pH units, centred at the pH of the pKa value

Question 17

what is true for the calculations of a weak acid, but isnt true for a buffer solution?
why?

Answer 17

[H+] = [A-] for weak acids
- not true for buffers as A- has been added as a component of the buffer

Question 18

what does the pH of a buffer solution depend on?

Answer 18

• the pKa of the weakk acid
  = the ratio of the concentration of the HA and A-

Question 19

how is the pH of the blood maintaind?
what is its range?

Answer 19

pH: 7.35-7.45
- a mixture of buffers including carbonic acid-hydrogencarbonate (H2C3 / HCO3 -)

Question 20

What is it called when the pH of blood falls lower than 7.35?
what will people experience?

Answer 20

• acidosis
• fatigue, shortness of breath, shock and death

Question 21

What is it called when the pH of blood goes higher than 7.45?
what will people experience?

Answer 21

• alkalosis
• muscle spasm, light-headedness and nausea

Question 22

What happen when acid is added to the blood?

Answer 22

[H+] increases
-H+ reacts with the conjugate base HCO3-
- equilibrium shifts o the left removing most of the H+ ions

H2CO3 <—–> H+ + HCO3-

Question 23

weak acid and conjugate base of the blood?

Answer 23

weak acid - H2CO3
conjugate base - HCO3-

Question 24

What happen when an alkali is added to the blood?

Answer 24

• [OH-] increases
• small concentration of H+ reacts with OH-
  H+ + OH- —–> H2O
• H2CO3 dissociates, shifting the equilibrium to the right, restoring H+

H2CO3 <—–> H+ + HCO3-

Question 25

the henderson-hasselbalch equation

Answer 25

pH = pKa + log [A-]/ [HA]

Question 26

How to monitor the pH as base (aq) is added to an acid solution?

Answer 26

• using a pipet, add a measured volutme of the acid to a conical flask
• olace electrode of the pH in the flask
• add aqueous base to the burette and add to the acid 1cm3 at a time
• swirl the contents and recrd the pH and volume of base added
• repeat and stop when pH changes rapidly, than add base dropwise until reading changes less dramatically
• ## now add base until excess is added

Question 27

describe the pH curve of a strong base added to a weak acid

Answer 27

• when base is added there is excess of acid and the pH is very low, it increases slightly
• as more base is added the acoid is used u more quickly and forms a virtical section
• after the vertical section, as more base is added, the pH rises gradually

Question 28

what is the equivalence point of a titration curve?

Answer 28

is the volume of one solution that exactly reacts ith the volume of the other solution.
- it is the centre of the vertical section

Question 29

what is an acid-base indicator?
give an example
what is its end point

Answer 29

• it is a weak acid that had a distinctive colour from its conjugate base
• methyl orange
  -> weak acid is red
  -> conjugate base is yellow
• orange

Question 30

what is the end point of a titration?

Answer 30

the indicater contains equal concentrations of HA and A- and the colour of the solutions is between the two colours

Question 31

what happens when methyl orange is added initially to an acidic solution and an alkali is added?

Answer 31

• OH- reacts with the H+ of the indicator
  H+ + OH- —–> H2O
• the weak acid dissociates, shifting the equilibrium to the right

red-> orange -> yellow

Question 32

what is an indicator

Answer 32

a weak acid

Question 33

what happens when methyl orange is added initially to a basic solution and acid is added?

Answer 33

• H+ reacts with the conjugate base
• equilibriumm shifts to the left
• yellow –> orange –> red

Question 34

in terms of Ka, what happens at the end point

Answer 34

[HA] = [A-]
Ka= [H+]
pKa = pH

Question 35

what are ther most effective indicators?

Answer 35

have a colour change over a range of 2 pH unuts