21 Buffers and neutralisation Flashcards
What is a buffer solution? (2 marks)
Buffers are solutions that can resist changes in pH, despite the addition of small quantities of acids or alkalis.
What is an acidic buffer? (1 mark)
A mixture of a weak acid and it’s conjugate base.
What happens if the conjugate base component of the buffer solution gets used up? (1 mark)
The buffer solution would not work for an acid.
Describe two ways in which buffer solutions can be prepared, and where the conjugate base comes from in each. (4 marks)
- Weak acid + one of its salts –> the salt is the source of the conjugate base.
- Weak acid (excess) + alkali –> The weak acid becomes partially neutralised by the alkali, forming the conjugate base.
Describe how a buffer solution counteracts a small amount of acid being added. (3 marks)
If an acid is added:
1. [H+] increases
2. H+ reacts with A-
3. Equilibrium position shifts to the left, removing most of the H+ ions
Describe how a buffer solution counteracts a small amount of alkali being added. (3 marks)
If an alkali is added:
1. [OH] increases
2. H+ reacts with OH-
3. HA dissociates. the equilibrium shifts to the right, restoring most of the H+
When is a buffer most effective? (1 mark)
When [HA(aq)] = [A-(aq)]
When [HA(aq)] = [A-(aq)], what is the value of pH? (1 mark)
pH = pKa of HA
How do you calculate the pH of a buffer solution prepared by mixing a weak acid and its salt? (3 marks)
- Rearrange the Ka expression:
[H+]= Ka x [HA]/[A-] - Use pH= -log[H+(aq)]
How do you calculate the pH of a buffer solution prepared by partial neutralisation? (3 marks)
- Find the excess moles of HA
- Use Ka expression (with [HA]= excess HA/volume)
- Use pH= -log[H+(aq)]
What is the really quick equation you can learn to calculate pH? (2 marks)
pH= pKa + log([A-]/[HA])
State the equilibrium equation for a buffer that contains equal concentrations of butanoic acid and sodium butanoate. (2 marks)
CH3(CH2)2COOH (aq) <—> H+ (aq) + CH3(CH2)2COO- (aq)
What pH does blood plasma need to be maintained at? (1 mark)
Between 7.35 and 7.45
How is pH of the blood maintained? (1 mark)
By a mixture of buffers.
What is the most important buffer system used to control the pH of the blood? (2 marks)
Carbonic acid and hydrogen carbonate ions.
H2CO3/HCO3-
What could happen if the pH of the blood rises above 7.45? (1 mark)
Alkalosis
State two symptoms of alkalosis. (2 marks)
Nausea
Muscle spasms
Light-headedness
What could happen if the pH pf the blood falls below 7.35 (1 mark)
Acidosis
State two symptoms of acidosis. (2 marks)
Fatigue
Shortness of breath
Shock or death
How does the body prevent a build up of carbonic acid? (2 marks)
It converts the carbonic acid to water and carbon dioxide.
The carbon dioxide is then exhaled.
How does the body ensure the concentration of hydrogen carbonate ions is not too high? (1 mark)
Kidney excretes less hydrogen carbonate ions
How many decimal places does a pH meter usually read? (1 mark)
2 decimal places
Describe how you would monitor the pH as an aqueous bas is added to an acid solution. (5 marks)
- Measure volume of acid into a conical flask, using a pipette.
- Put electrode of pH meter in flask.
- Add base gradually, using a burette, swirling and recording the pH and volume of base added at regular intervals.
- Repeat until the pH changes more rapidly, then add dropwise until rate slows.
- Add faster again, until an excess has been added and the pH has been basic (with little change) for several additions.
- Plot a graph of pH/total volume added.
How could you automate the monitoring of the pH of a titration? (2 marks)
Attach the pH meter to a data logger
Use a magnetic stirrer in the flask
Graph plotted automatically using appropriate software
What is the equivalence point? (1 mark)
The volume of solution that exactly reacts with the volume of the other solution.
What is an acid-base indicator? (1 mark)
A weak acid that has a different colour from its conjugate base.
What colour is methyl orange in a weak acid? (1 mark)
Red
What colour is methyl orange in a conjugate base? (1 mark)
Yellow
What colour is the end point of a titration indicated by when using methyl orange? (1 mark)
Orange
What happens when a basic solution is added to methyl orange? (4 marks)
- Solution is initially red.
- OH- ions react with the H+ ions in the indicator, forming water.
- The weak acid in the indicator dissociates more, shifting the position of equilibrium to the right.
- The colour changes, first to orange then to yellow.
What happens when an acid solution is added to methyl orange? (4 marks)
- Solution is initially yellow
- H+ ions react with the conjugate base in the indicator (A-)
- The equilibrium shifts toward the left
- The colour changes, first to orange then to red.
What colour is bromothymol blue in a weak acid? (1 mark)
Yellow
What colour is bromothymol in a conjugate base? (1 mark)
Blue
What colour is phenolphthalein in a weak acid? (1 mark)
Clear
What colour is phenolphthalein in a conjugate base? (1 mark)
Pink
What is the pH at the equivalence point for a strong acid-strong base/ weak acid-weak base titration? (1 mark)
pH= 7
What is the pH at the equivalence point for a strong acid-weak base titration? (1 mark)
pH < 7
What is the pH at the equivalence point for a weak acid-strong base titration? (! mark)
pH > 7
