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
conj acid
species after a base accepts proton - gains proton from original
Conj base
species formed when acid denotes proton- loses proton from original
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 assumptions are made when determining the pH of weak acids
[HA] in equilibrium is equal to [HA] undissociated (inital)
[H+] in equilibrium is equal to [A-], as there is negligible dissociation of water
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
KW value
1x10-14
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 of alkali
find moles of acid
find new moles of acid –> old - alkali
FInd new conc
With reference to equilibrium 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 equilibrium shifts, explain what happens to a buffer solution on addition of acid
H+ reacts with A-
Equilibrium shifts towards LHS
Towards HA
more HA formed
HA =
acid
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 equilibrium shifts towards LHS
Towards HA
Gives … colour
How to explain the colour of an indicator at a high pH
Alkali reacts with HA so equilibrium shifts towards 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 probe and a data logger
strong acids
fully dissociate
weak acids
partially dissociate
alkali
OH- donor
How can an aqueous solution of an acid contain hydroxide ions
water dissocites
- equation
HNO2
weak acid
body temp means
neutral
neutral defintion
OH = H+
how do you label conj acid base pairs
acid 1 base 1
acid 2 base 2
numbers = pairing numbers
or acid 1 base 1
conj acid conj base
Ph curve
if Ka is bigger is it a weak or stronger acid
stronger acid
