Topic 12: Acid-base Equilibria 1️⃣ Flashcards
Define a bronsted lowry acid
Proton donor
Define a bronsted lowry base
Proton acceptor
Define a bronsted lowry conjugate acid
Species formed when a base accepts a proton
Define a bronsted lowry conjugate base
The species formed when a proton is removed from a base
Define strong acids
An acid that almost completely dissociates into ions when in aqueous solution
Define a weak acid
An acid that only partially dissociates when in aqueous solution
Give the equation to calculate pH
pH = -log[H+]
Give the equation to calculate conc of H+ ions using pH
[H+] = 10^-pH
Give the equation to calculate the [H+] in weak acids
[H+] [A-]
Ka = ————-
[HA]
Give the equation to calculate pKa
pKa = -logKa
How do you calculate Ka from pKa
Ka = 10^-pKa
What does a low pKa value indicate
Strong acid
What is the effect of diluting strong acids
If you dilute a strong acid 10 times its pH will increase by one unit
Diluting it 100 times would therefore increase the pH by two units
How do weak acids behave upon dilution
Weak acids are not fully dissociated in solution so diluting them causes the equilibrium to shift to oppose the change.
This means a 10 times dilution of a weak acid would increase the pH by less than one unit
What is the equilibrium constant for Kw
Kw = [H+][OH-]
How does temp affect the value of Kw
The forward reaction in the equilibrium of water is endothermic and is therefore favoured when the temp of water is increased.
As a result as temp increases, more H+ ions are produced making water more acidic
Give the equation for pKw
pKw = -log Kw
Give the equation for Kw from pKw
Kw = 10^-pKw
What is the equivalence point
When the acid and base have reacted together in the same proportions as dictated by the stoichiometric equation
What does the end point refer to
When the colour of the indicator just changes colour
What does the pH at the equivalence point depend on ?
The combination of acid and base used
Eg strong acid strong base, weak acid strong base etc.
Describe the pH titration curve for a strong acid strong base titration
- starts at approx pH 14 finishes at approx pH 0.5 (uses full scale)
- pH only falls by a very small amount until large steep section quite near the equivalence point
What is plotted on the x and y axis of a pH titration curve ?
x axis = volume of acid added
y axis = pH
Describe the pH titration curve for a weak acid strong base titration
- starts at approx pH 14 finishes at approx pH 3 (uses 3/4 of scale)
- curve like that of strong acid strong base pH titration up to equivalence point
- past the equivalence point the mixture acts as a buffer resisting any fluctuations in pH upon further addition of acid
Describe the pH titration curve for a strong acid weak base titration
- curve starts at approx pH 11 finishes at approx pH 0.5 (uses 3/4 of scale)
- when acid is first added curve starts to fall but then levels out because buffer solution has been formed
- steep section to meet equivalence point
Describe the pH titration curve for a weak acid weak base titration
- starts at approx pH 11 finishes at approx pH 3
- no steep section to curve , instead ‘point of inflexion’ - why it is difficult to do titration of weak acid weak base with indicator
An acid-base indicator is either…
A weak acid or weak base
Most are weak acids (HIn)
How is the dissociation of an indicator that is a weak acid in aq solution represented
HIn (aq) ⇌ H+ (aq) + In- (aq)
HIn and In- have different colours in aqueous solution
What does the equilibrium constant KIn give us
There will be a stage where [HIn] = [In-] indicator in the case of methyl orange will appear orange
The exact pH when this is reached can be determined using KIn
How can we determine a suitable indicator for an acid-base titration using the pH titration curve
Each indicator has an exact pH range
So long as the exact pH range of the indicator falls within the steep section of the curve it is a suitable indicator
Give the Henderson-Hasselbalch equation used to calculate the pH of a buffer solution
pH = pKa + log [salt] / [acid]
What is a buffer solution
A solution that minimises the change in pH when a small amount of either acid or alkali is added
