Chapter 21 Flashcards
What is a buffer solution?
- A system that minimises pH changes on addition of small amounts of an acid or a base
What do buffer solutions consist of?
- Some undissociated acid (which is why WEAK acids must be used)
- Its conjugate base
How can buffer solutions be formed?
- By mixing a weak acid and a salt of the weak acid (e.g. ethanoic acid and sodium ethanoate)
- By reacting excess weak acid with an alkali (e.g. ethanoic acid and sodium hydroxide)
How do the 2 methods of forming buffer solutions provide weak acid and conjugate base?
- Method 1: the salt of the weak acid supplies the conjugate base (as it would break up in solution)
- Method 2:
- the acid and the alkali react to form the conjugate base (the salt formed would break up in solution)
- an EXCESS of acid is used to supply the weak acid (otherwise it would all be neutralised)
How do buffer solutions control pH when acid is added?
- The weak acid dissociates, creating an equilibrium
- If H+ is added, it reacts with the conjugate base to form the acid
- Equilibrium shifts to the left, which removes H+ ions
How do buffer solutions control pH when alkali is added?
- If alkali is added, OH- is added
- The OH- ions react with H+ ions to form water
- This decreases [H+], which shifts equilibrium to the right, and restores [H+]
Why do buffers only work when a small amount of acid or alkali is added?
- If too much alkali or acid is added, buffers cannot remove or replenish enough H+ ions
How do you calculate the pH of a buffer solution? What shortcut is there to this method?
- [H+]= Ka × [HA] / [A-]
- You can use the moles of HA and A- if it’s faster, since they would have the same volume
What special quality do certain buffer solutions have, and why?
- For some, [HA] = [A-], so they would cancel out in the Ka expression, and [H+] = Ka, and pH = pKa
- This is because they contain equal concentrations of a weak acid and its conjugate base (in the reaction method of making a buffer, you would add half as much as alkali as weak acid)
What approximation is used for weak acids, but not for buffers?
- [H+] = [A-]
- (A- was added in the buffer, so it’s no longer true)
What range of values should the pH of blood plasma be between?
- 7.35-7.45
How is the pH of blood maintained? Give the equation to demonstrate this.
- Using the carbonic acid-hydrogencarbonate buffer system
- H2CO3 <-> H+ + HCO3-
How do pH titration curves work? (Use the example of acid in the conical flask, and you’re adding base.)
- You plot volume of base added against pH
- Twice as much base as needed is used
- This causes the graph to be flat at first, have a steep rise, and then go flat again (s-shaped)
List 4 main features of pH titration curves (going from acidic to alkaline).
- At first, there is an excess of acid, so the pH slowly increases
- Then there is vertical line called the vertical section, where pH increases rapidly on addition of a very small amount of base
- The equivalence point is at the centre of the vertical section
- At the end, there is an excess of base, so the pH slowly increases
What is the equivalence point of a reaction?
- The point in a titration where the volume of one solution has reacted exactly with the volume of the other solution
What does the pH titration curve of a reaction between a strong acid and a strong base look like?
- The graph starts at roughly pH 1
- It ends at pH 13
- The vertical section is from about pH 4 to 10
- The pH steadily increases at first
What does the pH titration curve of a reaction between a strong acid and a weak base look like?
- The graph starts at roughly pH 1
- It ends at pH 11
- The vertical section is from about pH 4 to 7
- The pH steadily increases at first
What does the pH titration curve of a reaction between a weak acid and a strong base look like?
- The graph starts at roughly pH 3
- It ends at pH 13
- The vertical section is from about pH 7 to 10
- The pH rapidly increases at first
What does the pH titration curve of a reaction between a weak acid and a weak base look like?
- The graph starts at roughly pH 3
- It ends at pH 11
- There is no vertical section
- The pH rapidly increases at first
How do pH indicators work?
- They’re weak acids where the acid and conjugate base have different colours
- When hydroxide ions are added, they react with H+ ions, which shifts equilibrium to the right, making the solution the colour of the conjugate base
- If H+ ions are added, equilibrium shifts to the left, so the solution becomes the colour of the acid
How are appropriate indicators chosen for titrations?
- Each indicator has a pH range that it works for
- Its range has to be within the vertical section for it to be able to show the end point of a titration
What is the end point of a titration?
- When the indicator contains equal concentrations of acid and conjugate base; the point where the indicator changes colour
What are the pH ranges for methyl orange and phenolphthalein? Therefore, which types of titrations can they be used for?
- Methyl orange: 3 to 4
- Any with a strong acid
- Phenolphthalein: 9 to 10
- Any with a strong base
What colours does phenolphthalein change between, and in what conditions?
- Colourless in acids
- Pink in alkalis
What colours does methyl orange change between, and in what conditions?
- Red in acids
- Yellow in alkalis
What colour is methyl orange at the end point?
- Orange
How is the end point found for weak acid-weak base titrations? Why?
- No indicator can be used for weak acid-weak base reactions, as there is no vertical section
- A pH probe and meter are used instead
How are pH probes and meters used?
- You start by measuring your acid/ alkali into a conical flask with a pipette
- You put the electrode of the pH probe into the solution to measure its pH and get a reading from the pH meter
- You then add the alkali/ acid from the burette, swirling after each addition, and recording the volume added and the pH after every each addition
- Continue doing this until the pH starts to change more rapidly
- After the substance has been neutralised, keep adding alkali/ acid so you can draw the entire pH titration curve
