Acids & Bases Flashcards
What is an acid and a base
Acid - Proton donor
Base - proton acceptor
What’s the formula of Kw
[H+] [OH-]
What’s the value of Kw at 25C
10^-14
How do you find [H+] from a pH
10^-pH
How does concentration of H+ ions [H+] effect pH?
Increasing [H+] = decreased pH
Decreasing [H+] = increased pH
How do you find pH of pure water
when neutral Kw = [H+]^2
so [H+] = square root Kw
How does increasing the temperature of water effect pH
The dissociation of water is endothermic
- increasing the temp would push the equilibrium to the right
- so [H+] increases
- so pH decreases
how to get Ka from pKa
Ka = 10^-pKa
What’s the equation for weak acids dissociation
HA —> H+ + A-
HA is the weak acid
A- is the salt
How do you find pH of a weak acid on its own
Ka = [H+]^2 / [HA]
How do you work out pH of a DILUTED strong acid
- [H+] old x old volume/new volume
- pH -log
How do you work out pH DILUTED base
- [OH-] old x old volume/new volume
- [H+] = Kw/[OH-]
- pH -log
Why is H20 not shown in the Kw expression
H20 is almost constant
Why does Kw increase as temperature increases
Equilibrium is endothermic in the forward reaction
The equilibrium shifts right to oppose the increase in temperature
Why is the pH probe washed in between readings
So the different solutions don’t contaminate eachother
How does a buffer resist changes in pH when a small amount of acid is added
1 - write down reversible reaction for the buffer eg CH3COOH -> H+ + CH3COO-
2 - when acid is added [H+] increases
3- so the equilibrium shifts to the left to reduce the conc of H+ ions to what it was originally
4- overall [H+] remains roughly constant
How does a buffer resist changes in pH when a small amount of base is added
1 - write down reversible reaction for the buffer eg CH3COOH -> H+ + CH3COO-
2 - when OH- is added it reacts with H+ ions so [H+] decreases
3- so the equilibrium shifts to the right to increase the conc of H+ ions to what it was originally
4- overall [H+] remains roughly constant
What is the equation that represents when a base is added to a buffer solution
HA + OH- —> A- + H20
What is the equation that represents when a acid is added to a buffer solution
H+ + A- —> HA
Explain why diluting with water does not change the pH of a buffer solution
[HA] / [A-] is constant upon addition of water therefore [H+] is constant
Describe a method to construct a pH curve 7m
- Transfer 25cm3 of acid to a conical flask using a pipette
- Measure initial pH of the acid with a pH meter
- Add alkali in small amounts 2cm3 noting the volume added
- Stir the mixture to equalise the pH
- Measure and record the pH to 1.d.p
- Repeat 3-5 but when approaching endpoint add smaller volumes of alkali
- Add until the alkali is in excess
Why do you need to calibrate the pH meter
Because they can lose accuracy when they’re stored for periods of time
How can you improve the accuracy of pH meter
By maintaining constant temperature
What are examples of acids strong and weak
Strong - HCl, H2SO4, HNO3
Weak - CH3COOH
What are examples of bases strong and weak
Strong - NaOH, Ca(OH)2
Weak - NH3
What is the pH at equivalence point of a strong acid + strong base
pH 7
What is the pH at equivalence point of a strong acid + weak base
Less than pH 7
What is the pH at equivalence point of a weak acid + strong base
More than pH 7
What is the pH at equivalence point of a weak acid + weak base
Normally 7 but hard to determine
In a buffer when you add ACID how do you work out moles of HA and A-
Mol of HA = initial moles + moles of acid
Mol of A- = initial moles - moles of acid
In a buffer when you add a BASE how do you work out moles of HA and A-
Mol of HA = initial moles - moles of base
Mol of A- = initial moles + moles of base
What’s the equation to calculate [H+] in a buffer solution
Ka x [HA] / [A-]
How do you work out Ka from pH
10^-pH
How does a COOH group in a acid effect it’s strength
- it contains two very electronegative oxygen atoms
- therefore has a negative inductive effect
- making the acid stronger
How does a CH3 group in a acid effect it’s strength
- has a positive inductive effect
- making the acid weaker
How does polarity of OH effect acid strength
- the O-H bond is more polarised
- so there is more dissociation info H+ ions
- making the acid stronger
How do you do a buffer calc when it’s a neutralisation
- Work out initial moles of acid and base
- Take excess away from the other
- Do excess/total vol to get [HA]
- Do initial moles of thing not in excess/total vol to get [A-]
- Find [H_] by rearranging Ka expression
How does the Ka value effect acid strength
The larger the Ka value the stronger the acid
