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
