A7 Acids, Bases + Buffers Flashcards
Define a Brønstead-Lowry acid
A species that donates a proton
Define a Brønstead-Lowry base
A species that accepts a proton
How to tell conjugate acid-base pairs
- Differ by only an H+
- Opposite sides of the equation
What is a monobasic acid
Acid that donates one proton
What is a dibasic acid
Acid that donates two protons
What is a tribasic acid
Acid that donates three protons
Draw a mechanism for the reaction between methanoic acid and water
Equation for pH of strong acids
Formula to work out pH from [H+]
Formula to determine pH from [H+]
Procedure to determine the pH of weak acids
What assumption is made when determining the pH of weak acids to allow [H] = [A-]
[H] = [A-], as dissociation of water is negligable
What is the limitation with the assumptions made when calculating the pH of weak acids
With ‘stronger’ weak acids, [HA] in equilibrium being equal to [HA] undissociated may no longer be valid
Formula to convert pKa to Ka
Formula to convert pKa to Ka
Steps to calculate the pH of strong bases
Why do acids still contain some OH-
- Water dissociates
Define a buffer system
A system that minimises pH changes on addition of small amounts of an acid or a base
Steps to calculating the pH of a buffer system of a weak acid and salt of a weak acid
- Find new concentrations of acid and salt
Steps to calculate the pH of a buffer solution with a weak acid and a strong alkali
- Find the moles left of acid remaining, and the moles of salt now present
- Find their concentrations
With reference to equalibrium shifts, explain what happens to a buffer solution on addition of alkali
- Alkali reacts with HA
- Equilibrium shifts RHS
- Towards H+ and A-
With reference to equalibrium shifts, explain what happens to a buffer solution on addition of acid
- H+ reacts with A-
- Equilibrium shifts towards LHS
- Towards HA
What is the equation for the buffer system in our blood
What does the buffer system of our blood ensure
pH range is between 7.35 and 7.45
How would the pH of a buffer solution change if the volume changes
- pH remains the same
- As the ratio of HA / A- remains the same
Where will the lower horizontal curve be on a pH curve with a weak acid
3-6
Where will the lower horizontal curve be on a pH curve with a strong acid
1-3
Where will the upper horizontal curve be on a pH curve with a weak base
8-11
Where will the upper horizontal curve be on a pH curve with a strong base
11-13
Where will the vertical line be on a pH curve with a weak acid and strong base
6-11
Where will the vertical line be on a pH curve with a weak acid and weak base
6-8
Where will the vertical line be on a pH curve with a strong acid and weak base
3-8
Where will the vertical line be on a pH curve with a strong acid and a strong base
3-11
What is the equivalence point on a pH graph
The volume of alkali added at the vertical line
What is true at the equivalence point on a pH graph
mol Acid = mol Base
How to explain that an indicator is suitable
The vertical section matches the pH range where the colour change occurs
How to explain the colour of an indicator at a low pH
- H+ reacts with A- so equibilbrium shifts towards LHS/RHS
- Towards HA
- Gives … colour
How to explain the colour of an indicator at a high pH
- Alkali reacts with HA so equilibrium shifts towards LHS/RHS
- Towards A-
- Gives … colour
How to explain the colour of an indicator at end point
- Moles of HA = Moles of A-
- So final colour is a mix of the other colours
How can pH be measured experimentally
Using a pH probs and a data logger
What assumption is made when determining the pH of weak acids to allow [HA] to remain unchanged
- [HA] in equilibrium is equal to [HA] undissociated
- As there is negligible dissociation of HA
- [HA] in equilibrium = [HA] undissociated]
