Z A Level Chemistry > Buffers and Neutralisation > Flashcards

Buffers and Neutralisation Flashcards

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1
Q

What is a buffer solution?

A

A buffer solution is a system that minimises pH changes when small amounts of acid or base are added

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2
Q

What do buffer solutions contain?

A
  • Buffer solutions contain two components to remove added acid or alkali
  • A weak acid (as component 2)
  • And its conjugate base (as component 2)
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3
Q

What does the weak acid,HA do?

A

Removes added alkali

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4
Q

What does the conjugate base, A- do?

A

Removes added acid

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5
Q

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

A
  • When alkalis and acids are added to a buffer, the two components in the buffer solution react and will eventually be used up
  • As soon as one component has all reacted, the solution loses its buffering ability towards added acid or alkalis
  • As the buffer works, the pH does change but only by a small amount (you should not assume that the pH stays completely constant
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6
Q

What are the ways of preparing weak acid buffer solutions?

A
  • A buffer solution based on a weak acid needs a weak acid and its conjugate base
    1. Preparation from a weak acid and its conjugate base
    2. Preparation by partial neutralisation of the weak acid
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7
Q

How do you prepare a buffer from a weak acid and its salt?

A

-A buffer solution can be prepared by mixing a solution of ethanoic acid, CH3OOH, with a solution of one of its salts e.g. sodium ethanoate CH3COONa

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8
Q

What happens when ethanoic acid is added to water?

A

-The acid partially dissociates
-The amount of ethanoate ions in solution is very small
-Ethanoic acid is the source of the weak acid component of the buffer solution
CH3COOH (equilibrium arrow) H+ (aq) + CH3COO- (aq)

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9
Q

What are salts of weak acids?

A

-Salts of weak acids are ionic compounds and provide a convenient source of the conjugate base
-When added to water the salt completely dissolves
-Dissociation into ions is complete and so the salt is the source of the conjugate base component of the buffer solution
CH3COONa(s) + aq –> CH3COO- (aq) + Na+(aq)

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10
Q

How do you prepare a buffer by partial neutralisation of the weak acid?

A

-A buffer solution can also be prepared by adding an aqueous solution of an alkali, such as NaOH (aq) to an excess of the weak acid

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11
Q

What happens to the weak acid in preparing a buffer by partial neutralisation of the weak acid?

A
  1. The weak acid is partially neutralised by the alkali forming the conjugate base
  2. Some of the weak acid is left over unreacted
    - The resulting solution contains a mixture of the salt of the weak acid and any unreacted weak acid
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12
Q

What are the two reservoirs to remove added acid or alkali?

A
  1. In the ethanoic equilibrium, the equilibrium position lies well towards the ethanoic acid
  2. When CH3COO- ions are added to CH3COOH, the equilibrium position moves even further to the left, reducing the already small concentration of H+ (aq) ions, and leaving a solution contains mainly the two components CH3COOH and CH3COO-
    CH3COOH (aq) (equilibrium arrow) H+ (aq) + CH3COO-(aq)
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13
Q

What do CH3COOH and CH3COO- acts as?

A
  • Two reservoirs that are able to act independently to remove added acid or alkali
  • This is achieved by shifting the buffer’s equilibrium system either to the right or the left
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14
Q

What controls the pH?

A

The conjugate acid-base pair HA(aq)/A-(aq)

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15
Q

How can the control of pH be explained?

A

The control of the pH can be explained in terms of shifts in the equilibrium position using le Chatlier’s principle

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16
Q

How does the conjugate base remove added acid?

A
  • On addition of an acid H+(aq)
    1. [H+(aq)] increases
    2. H+ (aq) ions react with the conjugate base, A- (aq)
    3. The equilibrium position shifts to the left, removing most of the H+ (aq) ions
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17
Q

How does the weak acid remove added alkali?

A
  • On addition of alkali, OH- (aq)
    1. [OH-(aq)] increases
    2. The small concentration of H+ (aq) ions reacts with the OH- (aq) ions H+(aq) + OH-(aq) –> H2O (l)
    3. HA dissociates, shifting the equilibrium position tot he right to restore most of H+(aq) ions
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18
Q

How do you know which acid to use?

A
  • Choosing the components for a buffer solution:
    1. Different weak acids, result in buffer solutions that operate over different pH ranges
    2. A buffer solution is most effective at either removing either added acid or alkali where there are equal concentrations of the weak acid and its conjugate base
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19
Q

What happens when the [HA (aq)]=[A-(aq)]?

A
  1. The pH of the buffer solution is the same as the pKa value of HA
  2. The operating pH is typically over about two pH units centred at the pH of the pKa value
  3. The ratio of the concentrations of the weak acid and its conjugate base can then be adjusted to fine tune the pH of the buffer solution
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20
Q

How do you calculate the pH of a buffer solution?

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A

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

21
Q

What approximation is made in the pH of a weak acid that is not valid for a buffer solution?

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A

-When you calculate the pH of a weak acid, you make an approximation that [H+(aq)] = [A-(aq)] but for a buffer solution this is no longer true as A-(aq) has been added as one of the components of the buffer

22
Q

What do you need to calculate the pH of a buffer solution?

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A
  1. The value of Ka
  2. The ratio of weak acid and conjugate base used
    - Provided that Ka and the concentrations of HA and A- are known [H+(aq)] and the pH can be calculated
23
Q

What happens when the concentrations of HA and A- are the same?

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A

Ka = [H+(aq)] and pKa=pH

24
Q

Describe buffer solutions in the body

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A
  • Buffer solutions are widespread in living systems
  • The well-being of the human body relies on the precise pH control with different parts of the body requiring specific pH value for effective functioning
  • Enzymes are particularly sensitive and each has an optimum pH
  • The role of pH control in the body falls to buffer solutions e.g. in the plasma of the blood
25
Describe blood plasma control
1. Blood plasma needs to be maintained at a pH between 7.35 and 7.45 2. The pH is controlled by a mixture of buffers with the carbonic acid-hydrogen carbonate (H2CO3/HCO3-) buffer system being the most important 3. Normal healthy blood should have a pH of 7.40
26
What happens if the pH is out of this range?
1. If the pH falls below 7.35, people can develop acidosis which can cause fatigue, shortness of breath and in extreme cases shock or death 2. If the pH rises above 7.45, the condition is called alkalosis which can cause muscle spasms light headedness and nausea
27
What happens in the carbonic acid-hydrogencarbonate buffer system on addition of acid, H+(aq)?
1. [H+(aq)] increases 2. H+(aq) ions react with the conjugate base, HCO3- (aq) 3. The equilibrium position shifts to the left removing most of the H+(aq) ions H2CO3 (aq) (equilibrium arrow) H+ (aq) + HCO3- (aq)
28
What happens in the carbonic acid-hydrogencarbonate buffer system on addition of alkali, OH-(aq)?
1. [OH-(aq)] increases 2. The small concentration of H+(aq) ions react with the OH-(aq) ions H+(aq) + OH-(aq) --> H2O (l) 3. H2CO3 dissociates, shifting the equilibrium position to the right to restore most of H+(aq) ions H2CO3 (aq) (equilibrium arrow) H+ (aq) + HCO3- (aq)
29
How does the body prevent H2CO3 building up?
- The body produces far more acidic materials than alkaline which the conjugate base HCO3- converts to H2CO3 - The body prevents H2CO3 building up by converting it into carbon dioxide gas which is then exhaled by the lungs
30
What do acid-base titrations use?
- Acid-base titrations used indicators to monitor neutralisation reactions accurately - The results can then be analysed to find out some unknown information about the acid or base
31
How do you monitor the pH changes that take place during a titration?
Using a pH meter
32
Describe a pH titration curve at the start
1. When the base is first added, the acid is in great excess and the pH increases very slightly 2. As the vertical section approaches the pH starts to increase more quickly as the acid is used up more quickly - Excess of acid: pH increase slowly as basic solution is added
33
What happens in the middle of a pH titration curve?
3. Eventually the pH increases rapidly during addition of a very small volume of base, producing the vertical section, only drops of solution will be needed for the whole vertical section - Vertical section: pH increases rapidly on addition of a very small volume of bases. Acid and base concentrations similar - Equivalence point: The centre of the vertical section of the pH titration curve
34
What happens at the end of a pH titration curve?
4. After the vertical section, the pH will rise very slightly as the base is now in great excess - Excess of baseL pH increases slowly as basic solution is added
35
What is the equivalence point of the titration?
- The volume of one solution that exactly reacts with the volume of the other solution - The solutions have then exactly reacted with one another and the amounts used matching the stoichometry of the reaction
36
What is the equivalence point of the titration graphically?
The centre of the vertical section of the pH titration curve
37
Can the titration curve be different?
The titration curve may be a different shape for combinations of acid and base with different strengths
38
What is the pH titration curve for adding an acid to a base?
- A pH titration curve can also be plotted with the acid added from the burette to the base in the flask - The shape is essentially the same, just the other way around going from high pH to low pH
39
What is an acid-base indicator?
- An acid base indicator is a weak acid HA, that has distinctively different colour from its ocnjuabet base A-, for example methyl orange 1. The weak acid HA is red 2. The conjugate base A- is yellow
40
What happens at the end point of a titration?
At the end point of a titration, the indicator contains equal concentrations of HA and A- and the colour will be in between the two extreme colours e.g. for methyl orange the colour at its end point is orange
41
Why does an indicator change colour?
- An indicator is a weak acid - The equilibrium position is shifted towards the weak cid in acidic conditions or towards the conjugate base in basic conditions, changing the colour as it does so - In a titration in which a strong base is added to a strong acid, methyl orange is initially red as the presence of H+ ions force the equilibrium position well to the left
42
Describe and explain the colour change on addition of a basic solution containing OH-(aq) ions
1. OH-(aq) ions react with the H+(aq) in the indicator H+(aq) + OH-(aq) --> H2O (l) 2. The weak acid HA dissociates shifting the equilibrium position to the right 3. The colour changes first to a orange at the end point and finally to yellow as the equilibrium position is shifted to the right HA(aq) (equilibrium arrow) A- (aq) + H+(aq)
43
Describe and explain the colour change if methyl orange is aded initially to a basic solution and acid added
1. H+(aq) ions react with the conjugate base A-(aq) 2. The equilibrium position shifts to the left 3. The colour changes first to orange at the end point and finally to red when the equilibrium position has shifted to the left HA(aq) (equilibrium arrow) A- (aq) + H+(aq)
44
What is indicator change similar to?
Indicator change is very similar to the mode of action of a buffer and all acid-base indicators work in a similar way
45
How is the sensitivity of end points different?
- Different indicators have different Ka values and change colour over different pH ranges - The sensitivity of the indicator depends upon the indicator itself and eyesight and most indicators change colour over a range of about two pH units
46
What happens at the end point?
[HA(aq)]=[A-(aq)] and Ka=[H+(aq)] and pKa=pH | -The pH of the end point is the same as the pKa value of HA
47
How do you choose the indicator?
- In a titration, you must use an indicator that has a colour change which coincides with the vertical section of the pH titration curve - Ideally the end point and equivalence point would coincide - However this may not be possible and the end point may give a volume that is slightly different from the equivalence point - but any difference will be very small in the order of one or two drops
48
Why is no indicator suitable for a weak acid-weak base titration?
As there is no vertices section, and even at its steepest, the pH requires several cm^3 to pass through a typical pH indicator range of 2 pH units
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