Module 5: Chapter 21 - Buffers and Neutralisation Flashcards
What are pH indicators?
pH indicators are weak acids where HA and A⁻ are different colours. At low pH, HA is the main species present. At high pH A⁻ is the main species present. The pH at which the colour changes varies from one indicator to another.
HA ⇌ H⁺ + A⁻
What is universal indicator?
Universal indicator is a mixture of indicators and so shows many colours at different pH’s
What is the colour of HA in methyl orange?
red
What is the colour of A⁻ in methyl orange?
Yellow
What is the colour of HA in phenolphthalein?
colourless
What is the colour of A⁻ in phenolphthalein?
pink
What is the pH range of colour change for methyl orange?
3.2 - 4.4
What is the pH range of colour change for phenolphthalein?
8.2 - 10.0
What is the significance of the pH range of colour change for an indicator?
In a titration, the pH changes rapidly at the end point as the last drop of acid/alkali is added. For an indicator to change colour at this moment where the moles of acid = moles of base, the indicator must change colour within the range of the rapid pH change at the end point (vertical section on a pH titration curve). Ideally the end point and equivalence point would coincide.
What are pH curves?
graphs showing how the pH changes as an alkali is added to an acid (or vice versa)
What is the equivalence point?
The equivalence point of a titration is the volume of one solution that exactly reacts with the volume of another solution - the moles of alkali added equals the moles of acid present
When does the pH curve usually show a rapid change in pH for and acid-alkali reaction?
around the equivalence point
What is the end point of a titration?
The point in a titration where the indicator changes colour; the end point indicates when the reaction is just complete. It is when the indicator contains equal concentrations of HA and A⁻
Why is choosing a suitable indicator important for titrations?
So that the end point coincides with the equivalence point
What are the 4 distinct parts to this question?
You must find the pH:
1. Before adding the acid
2. When the NaOH is still in excess
3. At the equivalence point
4. When the acid is in excess
Calculate the pH before adding the acid:
pH = 13
Calculate the pH when the NaOH is still in excess (When 10cm³ and 24cm³ have been added)
After 10cm³, pH = 12.63
After 24cm³, pH = 11.31
Calculate the pH at the equivalence point
pH = 7
How can you identify the equivalence point on a titration curve?
It is in the middle of the near vertical portion of the curve
Describe each point on this pH curve:
- No base added, so pH is the pH of the original acid
- Start of vertical portion of curve (close to equivalence point)
- Equivalence point (this is the middle of the near vertical portion of the curve)
- End of the vertical portion of the curve
- Base is in excess, so final pH ≈ pH of base
What is the significance of the pKa of a pH indicator?
An indicator would be suitable if its pKa value falls within the vertical section of the titration curve
Why may the pH of solution at the equivalence point not be 7?
When neutralised, weak acids and bases can form solution with pH ≠7. For example, titrating NH₃ with HCl forms NH₄Cl, which has acidic properties (pKa = 9.3) and therefore forms a weakly acidic solution at the equivalence point
What is a buffer solution?
A solution that resists cahnges in pH when small amounts of acid or alkali are added (the pH does slightly change but it is miniscule)
What is the pH of an acidic buffer?
less than 7
What is the pH of a basic buffer?
greater than 7
How are acidic buffers produced?
- From a mixture of a weak acid and one of its salts, HA and A⁻ (e.g. ethanoic acid & sodium ethanoate)
- From a mixture of an excess of a weak acid with a strong alkali (as it results in a mixture of HA and A⁻)
What is important with an acidic buffer?
[acid] and [salt] is much greater than [H⁺]
How is a basic buffer produced?
- From a mixture of a weak alkali and one of its salts (e.g. ammonia & ammonium chloride)
- From a mixture of an excess of a weak alkali with a strong acid
What is important with a basic buffer?
[base] and [salt] is much greater than [OH⁻]
What are the components of a basic buffer?
Weak base + one of its salts
[base] & [salt]»_space; [OH⁻]
What are the components of an acidic buffer?
Weak acid + one of its salts
[acid] & [salt]»_space; [H⁺]
What happens when you add a little H⁺ to an acidic buffer?
HA ⇌ H⁺ + A⁻
The added H⁺ is removed by the reaction with A⁻ to form HA
The [A⁻] falls slightly and the [HA] rises slightly, but as [HA] & [A⁻]»[H⁺], the ratio of [HA]/[A⁻] remains roughly constant and therefore the pH stays constant
What happens when you add a little OH⁻ to an acidic buffer?
HA ⇌ H⁺ + A⁻
The added OH⁻ reacts with H⁺, and so some of the HA breaks down to replace the H⁺.
The [A⁻] rises slightly and the [HA] falls slightly, but as [HA] & [A⁻]»_space; [H⁺], the ratio of [HA]/[A⁻] remains roughly constant and therefore the pH stays constant
What happens when you add water to an acidic buffer?
The ratio of [HA]/[A⁻] remains constant and so the pH remains constant
How do you find the pH of an acidic buffer?
The pH of an acidic buffer is found using the Ka expression, therefore the pH of an acidic buffer depends on the ratio of [HA] to [A⁻]
What are the 4 types of buffer calculation questions?
- Type A: Calculate the pH of a buffer solution produced by mixing a weak acid with their salt
- Type B: Calculate the pH of a buffer solution produced by mixing an excess of weak acid with a strong base
- Type C: Calculate the new pH of this buffer after a strong base has been added to it
- Type D: Calculate the new pH of this buffer after a strong acid has been added to it
Each type can also be reversed, i.e starting with the pH and determining the amount of substance added
How would you solve a Type A buffer calculation (Calculate the pH of a buffer solution produced by mixing a weak acid with their salt)?
- Set up Ka expression
- Calculate number of moles of salt added
- Calculate number of moles of A⁻
- Calculate concentration of A⁻
- Substitute in data to Ka expression and solve for [H⁺]
- Calculate pH
How would you solve a Type B buffer calculation (Calculate the pH of a buffer solution produced by mixing an excess of weak acid with a strong base)?
- Set up Ka expression
- Calculate intial moles of HA and OH⁻
- Set up ICE for the equation “HA + OH⁻ -> A⁻ + H₂O” (A⁻ and H₂O are intially at 0)
- Calculate the concentration of each at equilibrium where [OH⁻] falls to 0
- Substitute in data to Ka expression and solve for [H⁺]
- Calculate pH
How would you solve a Type C buffer calculation (Calculate the new pH of this buffer after a strong base has been added to it)?
- Set up Ka expression
- Calculate moles of base added
- Calculate moles of OH⁻ added
- Set up ICE for the equation “HA + OH⁻ -> A⁻ + H₂O” (A⁻ and H₂O are initially not at 0)
- Calculate the concentration of each at equilibrium where [OH⁻] falls to 0
- Substitute in data to Ka expression and solve for [H⁺]
- Calculate pH
How would you solve a Type D buffer calculation (Calculate the new pH of this buffer after a strong acid has been added to it)?
- Set up Ka expression
- Calculate moles of acid added
- Calculate moles of H⁺ added
- Set up ICE for the equation “A⁻ + H⁺ -> HA” (HA is initially not at 0)
- Calculate the concentration of each at equilibrium where [H⁺] falls to 0
- Substitude in data to Ka expression and solve for [H⁺]
- Calculate pH
Calculate the pH of the buffer solution formed
IMPORTANT
4.72
Calculate the pH of the buffer solution formed
IMPORTANT
Solve:
IMPORTANT
Solve:
IMPORTANT
Solve:
IMPORTANT
Solve:
IMPORTANT
Final pH = 4.60
pH change = 0.02
What is a healthy pH of blood plasma?
7.35 - 7.45
What is acidosis?
When the blood plasma pH falls below 7.35
pH < 7.35
What is alkalosis?
When the blood plasma pH exceeds 7.45
pH > 7.45
What is carbonic acid?
H₂CO₃
What is the carbonic acid-hydrogencarbonate buffer system?
H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)
How does the blood maintain a constant pH?
The carbonic acid buffer system (H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)) is present within the blood plasma and it maintains the pH of the blood plasma between 7.35-7.45
19.95
What is the Henderson-Hasselbalch equation?
When is pH = pKa?
When the acid/conjugate base concentrations are the same
What is the optimum pH for a buffer to work in?
Optimum pH = pKa of buffer
This occurs when [HA] = [A⁻]
Is the hydrogencarbonate buffer an acidic or basic buffer?
It is an acidic buffer as the optimum pH is 6.36 (its pKa) and this is less than 7
What component of the buffer solution removes added acid?
The conjugate base, A⁻
What component of the buffer solution removes added alkali?
The weak acid, HA
When does a buffer solution stop working?
When lots of acid or alkali is added. If all of the weak acid or conjugate base is used up, then the solution loses its buffering ability towards acids of alkali
How is an acidic buffer produced from a weak acid and its salt?
- When the weak acid is added to water, it partially dissociates and the amount of conjugate base ions in the solution is very small. It is the source of the weak acid component
- Salts of the weak acid are ionic compounds and provide a convenient source of the conjugate base. When added to water, the salt completely dissociates putting lots of conjugate base ions into the solution. It is the source of the conjugate base component
How is an acidic buffer produced from partial neutralisation of the weak acid?
A buffer solution can also be prepared by adding an aqueous solution of an alkali, such as NaOH, to an excess of the weak acid. The weak acid is partially neutralised by the akali, forming the conjugate base. 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
What is the operating pH of a buffer?
It is over 2 pH units, centred at the pH of the pKa value
What is the pH of normal healthy blood?
7.40
What does acidosis cause?
- Fatigue
- Shortness of breath
- Shock
- Death
What does alkalosis cause?
- Muscle spasms
- Light-headedness
- Nausea
Why must the blood pH be maintained across such a short range?
A small change in pH causes a large change in the concentration of H⁺
The body produces far more acidic materials than alkaline, causing the conjugate base (HCO₃⁻) in the blood to convert it into H₂CO₃. How does your body prevent a build up of H₂CO₃ in the blood?
By converting it to carbon dioxide gas, which is then exhaled by the lungs
Explain what happens to the indicator when the concentration of OH⁻ increases:
On addition of a basic solution containing OH⁻ ions:
* OH⁻ reacts with H⁺ in the indicator to form water
* The weak acid, HA, dissociates, shifting the equilibrium position to the right
* The colour changes as the concentration of A⁻ increases and HA decreases
HA ⇌ H⁺ + A⁻
Explain what happens to the indicator when the concentration of H⁺ increases:
On addition of an acidic solution containing H⁺ ions:
* H⁺ ions react with the conjugate base, A⁻
* The equilibrium position shifts to the left
* The colour changes as the concentration of HA increases and A⁻ decreases
Why is pH = pKa at the equivalence point?
Why is no indicator suitable for a weak acid/weak base titration?
There is no vertical section, and even at its steepest, the pH requires several cm³ to pass through a typical pH indicator range of 2 pH units
What does the pH titration curve look like for a strong acid/strong base titration?
When adding base
What does the pH titration curve look like for a strong acid/weak base titration?
When adding base
What does the pH titration curve look like for a weak acid/weak base titration?
When adding base
What does the pH titration curve look like for a weak acid/strong base titration?
When adding base
What is the rough equivalence point of strong acid/strong base titration?
Under standard temperature and pressure
~7
What is the rough equivalence point of a strong acid/weak base titration?
Under standard temperature and pressure
~5
What is the rough equivalence point of a weak acid/strong base titration?
Under standard temperature and pressure
~9
What is the rough equivalence point of a weak acid/weak base titration?
Under standard temperature and pressure
No distinguishable value due to shape of titration curve
How can you determine the ratio of [A⁻(aq)] to [HA(aq)] in a buffer system?
If you are given the pH and the pKa?
The Henderson-Hasselbalch equation
Derive the Henderson-Hasselbalch equation
How can you determine the pH of a buffer solution?
- Working with the Ka expression
- The Henderson-Hasselbalch equation
