3.9 Acid Base Equilibrium Flashcards
What is a weak acid buffer made of? Give an example.
Weak acid and salt of acid with a strong base.
HCOOH and HCOONa
What happens when you add an acid [H+] to a weak acid buffer solution?
[H+] increases, and system decreases it by promoting the formation of the weak acid. The dissociation of the salt provides the reservoir of anions.
E.g. HCOONa -> HCOO- + Na+
Added H+ + HCOO- HCOOH
What happens when you add an alkali [OH-] to a weak acid buffer?
The OH- reacts with the free H+ in solution, creating H20.
This reduces the amount of H+, so the system counteracts this by promoting the dissociation of the acid into H+ and its anions.
What is a weak base buffer made of? Give an example.
A weak base and the salt of the weak base with a strong acid.
E.g., NH3 and NH4Cl
(Ammonia and ammonium chloride)
What happens when you add an acid (H+) to a weak base buffer?
[H+] increases, and the system reverses it by using it to react with the base. E.g.,
(H+) + (NH3) NH4+
As NH3 is a weak base, there is plenty of it unionised that is available for this reaction.
What happens when you add an alkali (OH-) to a weak base buffer?
The OH- reacts with free H+ to form H20. This reduces the [H+] p, so the system reverses it by promoting the reaction that creates H+
E.g., the reverse reaction of NH3 + H+ NH4+.
The dissociation of the salt provides the reservoir of NH4+.
What type of buffer would you need to maintain a low pH of 1-3?
Strong acid and its strong salt.
Which equation do you use to calculate the pH of a strong acid?
pH = -log[H+]
Which two equations do you use to calculate pH of a weak acid?
Ka = [H+].[A-] / [HA]
Then pH = -log[H+]
Which two equations do you use to calculate the pH of a strong base?
Kw = [H+].[OH-] = 1x10^-14
Therefore, [H+] = Kw / [OH-]
The concentration of OH- is same as concentration of strong base
What is the value given for the ionic product of water? What is the original equation to show this ionisation?
1x10^-14
H2O H+ and OH-
Ka = [H+].[OH-] / [H2O = 1x10^-14 mol dm-3
What is the Henderson - Hasselbalch Equation?
Combines all steps for calculation of pH of buffer.
pH of buffer = pKa + log(SALT/ACID)
Describe a strong acid, strong base titration curve.
The pH starts low at around 1, and it slowly and gradually increases in pH as the 20cm3 of alkali is added. There is a sudden increase at about pH2 to pH12 as the strong alkali is now in excess after 25ml has been added. The pH slowly and gradually increases as the remaining 25cm3 of alkali is added.
What does the vertical region of a titration curve tell us? What is this called?
The point at which the amount of moles of alkali added is equal to the amount of moles in the original acid solution. It is called the equivalence point.
What causes differences in titration curves?
The strength of the acid or base used, resulting in molecules not being fully dissociated.
