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Chemistry > C21 - Buffers and Neutralisation > Flashcards

C21 - Buffers and Neutralisation Flashcards

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1
Q

What is a buffer solution?

A

A system that minimises pH Chang’s when small amounts of an acid or base are added

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2
Q

What are the two components of a buffer?

A

A weak acid (component 1) and its conjugate base (component 2)

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3
Q

What is the role of the weak acid in the buffer?

A

Weak acid (HA) removes added alkali

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4
Q

what is the role of the conjugate base in the buffer?

A

Conjugate base (A-) removes added acid

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5
Q

What happens when alkalis and avoids are added to a buffer?

A

The two components in the buffer solution react and will eventually be used up

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6
Q

What happens as soon as one component has all reacted?

A

The solution loses its buffering ability towards added acids or alkalis

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7
Q

Does pH change as the buffer work?

A

Yes, but only but a small amount - should not assume that pH stays completely constant

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8
Q

What components does a buffer based on a weak acid need?

A

Weak acid and its conjugate base

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9
Q

What is an example of a salt solution that can be mixed with ethanoic acid to prepare a buffer solution?

A

CH3COONa

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10
Q

What happens when ethanoic acid is added to water?

A

Partially dissociates and amount of ethanoate ions in solution is very small
Ethanoic acid is the source of the weak acid component of the buffer solution

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11
Q

What kind of substances are salts of weak acid and what is the advantage of this?

A

Ionic compounds

Provide a convenient source of conjugate base

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12
Q

What happens when salts of weak acids are added to water?

A

Salt completely dissolves
Dissociation of ions is complete
Salt so spruce of the conjugate base component of the buffer solution

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13
Q

What is the first method of preparing a weak acid buffer solution?

A

Preparation from a weak acid and its salt

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14
Q

What is the second method of preparing a weak acid buffer solution?

A

Preparation by partial neutralisation of the weak acid

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15
Q

What is an example of an alkali that can be added to a weak acid to prepare a buffer solution?

A

NaOH (aq)

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16
Q

What happens when an alkali is added to an excess of weak acid?

A

Weak acid is partially neutralised by the alkali, forming the conjugate base
Some of the weak acid is left over unreacted
Resulting solution contains a mixture of the salt of the weak avoid and any unreacted acid

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17
Q

where does the equilibrium position lie in the ethanoic acid equilibrium?

A

Well towards ethanoic acid (left side)

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18
Q

What happens when CH3COO- ions are added to CH3COOH?

A

Equilibrium position shifts even further to the left, reducing the already small concentration of H+ ions
Leaves a solution containing mainly the two components, CH3COOH and CH3COO-

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19
Q

How do CH3COOH and CH3COO- act as two reservoirs?

A

Able to act independently to remove added acid and alkali

Achieved by shifting the buffer’s equilibrium system either to the right or left

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20
Q

What controls the pH in an acid buffer solution?

A

Conjugate acid-base pair, HA (aq)/A-(aq)

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21
Q

What is the equilibrium for an acid buffer solution?

A

HA(aq) —> H+(aq) + A-(aq)

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22
Q

What happens to the acid buffer solution when an acid (H+) is added?

A
  • [H+(aq)] increases
  • H+ (aq) ions react with the conjugate base A-(aq)
  • Equilibrium position shifts to the left removing most of the H+ ions
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23
Q

What happens to the acid buffer solution when an alkali (OH-) is added?

A
  • [OH-(aq)] increases
  • small concentration of H+ ions react with the OH- ions
  • HA dissociates, shifting the equilibrium position to the right, restoring most of the H+ ions
24
Q

When is a buffer most effective at removing either added acid or alkali?

A

When there are equal concentrations of the weak acid and it’s conjugate base

25
Q

What applies when there are equal concentrations of the weak acid and its conjugate base
([HA(aq)] = [A-(aq)] ?

A
  • the pH of the buffer solution is the same as the pKa value of HA
  • the operating pH is typically over about two pH units, centred at the pH of the pKa value
    (eg. CH3COOH, pKa = 4.76, operating pH range: 3.76-5.76)
26
Q

How can a buffer solution be fine tuned?

A

Ratio of the contractions of the weak acid and its conjugate base can be adjusted

27
Q

What is the Ka expression for a weak acid?

A

Ka = [H+(aq)][A-(aq)]
———————
[HA(aq)]

28
Q

What is the expression for calculating to pH of a buffer solution?

A
  • [H+(aq)] = Ka x [HA(aq)]
    —————
    [A-(aq)]
  • pH = -log[H+(aq)]
29
Q

What is the Ka expression for a buffer solution when the conc. of HA and A- are the same?

A
  • Ka = [H+(aq)]

- pH = pKa

30
Q

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

A

7.35-7.45

31
Q

How is the blood plasma pH controlled?

A
  • By a mixture of buffers

- Carbonic acid-hydrogen carbonate (H2CO3/HCO3-) buffer system being the most important

32
Q

What is the pH of normal healthy blood?

A

7.40

33
Q

What could happen if the blood plasma pH falls below 7.35?

A
  • Could develop a condition called acidosis

- acidosis can cause fatigue, shortness of breath, shock, death

34
Q

What could happen if the blood plasma pH rises above 7.45?

A
  • Could develop condition called alkalosis

- Alkalosis can cause muscle spasms, light-headedness, nausea

35
Q

What is the equilibrium of the carbonic acid-hydrogen carbonate buffer system?

A

H2CO3 (aq) —> H+ (aq) = HCO3- (aq)

36
Q

What happens when an acid (H+) is added to the carbonic acid-hydrogen carbonate buffer system?

A
  • [H+(aq)] increases
  • H+ ions react with the conjugate base, HCO3-
  • Equilibrium position shifts to the left removing most of the H+ ions
37
Q

What happens when an alkali (OH-) is added to the carbonic acid-hydrogen carbonate buffer system?

A
  • [OH-(aq)] increases
  • Small concentration of H+ ions reacts with OH- ions forming water
  • H2CO3 dissociates, shifting ther equilibrium position to the right to restore most of the H+ ions
38
Q

How does H2CP3 build up in the body?

A

The body produces far more acidic materials than alkaline,w high the conjugate base HCO3- converts to H2CO3

39
Q

How does the body prevent build of H2CO3?

A

By converting it to carbon dioxide gas which is then exhaled by the lungs

40
Q

What does a pH meter consist of?

A
  • An electrode that is dipped into a solution and connected to a meter that displays the pH readings
41
Q

How are values typically recorded by a pH meter?

A

To 2 decimal places

42
Q

What is the advantage of using a pH meter instead of indicator paper?

A
  • Indicator paper is usually matched from colour charts to the nearest whole number
  • pH meter is able to give more accurate measurements of pH during a titration
43
Q

What is the procedure that monitors the pH as an aqueous base is added to an acid solution?

A
  1. Using a pipettes, add a measured volume of acid to a. Conical flask
  2. Place the electrode pot the pH meter in the flask
  3. Add the aqueous base to the beretta and add the acid in the conical flask, 1cm3 at a time
  4. After each addiction, swirl the contents. record the pH and total volume of the aqueous base added
  5. Reaper steps 3& 4 until pH starts to change more rapidly
  6. Add the aqueous base dropwise for each reading until pH changes less rapidly
    Add the aqueous base 1cm3 at a time again until and excess has been added and the pH has been basic, with little change, for several additions
44
Q

How is the data from the experiment using a pH meter recorded?

A

Ploy a graph of pH against total volume of aqueous base added (s-shaped curve)

45
Q

What is an alternative method of recording the pH as an aqueous base is added to an acid solution?

A
  • Attach the pH meter to a data logger and use magnetic stirrer in the flask
  • Aqueous base would then be added from the burette to the flask slowly and pH titration curve could be plotted automatically using data logger or appropriate software Ona. Computer
46
Q

What happens when the base is first added to the acid solution?

A

Acid is in great excess and the pH increases very slowly

47
Q

What happens as the vertical section is approached?

A

The pH starts to increase more quickly as the acid is used up more quickly

48
Q

What happens during the vertical section?

A
  • PH increases rapidly during of a very small volume of base
  • Acid and vase concentrations similar
  • Only drops of solution will be needed for the whole vertical section
49
Q

What happens after the vertical section?

A

PH increases very slowly as the base is now in great excess

50
Q

What is the equivalence point?

A
  • The volume of one solution that reacts exactly reacts with the volume of the other solution
  • Centre of the vertical section of the curve
  • pH = 7
51
Q

Why might the titration curve vary in shape?

A

Different combinations of acid and base strengths

52
Q

Why might the pH curve be plotted from high pH to low pH ?

A

If the acid is added from the burette to the base in the flask.
The shape is essentially the same just flipped

53
Q

What is an acid base indicator?

A

A weak acid, HA, that has a distinctively different colour from is conjugate base, A-

54
Q

What is an example of a common acid-base indicator and what colours correspond to its weak acid and conjugate base?

A

Methyl orange
Weak acid, HA = orange
Conjugate base, A- = yellow

55
Q

What happens at the end point of a titration?

A

Indicator contains equal concentrations of HA and A- and the colour will be in between the two extreme colours

56
Q

What is the end point colour of methyl orange?

A

Orange

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