Acids and Bases Flashcards
Define an acid and base respectively.
Acid : Proton donor/substance that produces H+ ions when dissociated.
Base : Proton acceptor/substance that extracts a proton, H+ and generates OH-
Relationship of Ka and acidity.
Higher Ka means a stronger acid and weaker conjugate base
Relationship of Kb and alkalinity.
Higher Kb means a stronger base and weaker conjugate acid.
Ka and pKa
Higher Ka = strong acid
Higher pKa = weak acid
*The same is true for Kb and pKb
Why was a pH scale developed?
It was developed for practicality.
Explain weak acids
The acid dissociation constant, Ka, is the equilibrium constant for the reaction in which a weak acid is in equilibrium with its conjugate base and the hydroxonium ion in an aqueous solution.
Explain weak bases
The dissociation of a weak base, which results in the production of hydroxide ions, is represented by the base dissociation constant Kb
An example and calculation of a weak acid reacting with water
Ethanoic acid as an example: CH3COOH (aq) + H2O (l) ⇌ CH3COO- (aq) + H3O+ (aq)
Ka = [CH3COO-(aq)][H3O+(aq)]/ [CH3COOH(aq)] = 1.8 x 10-5 M at 25°C
pKa = - log Ka
pKa = -log (1.8 x 10x-5 M) = -(-4.74) = 4.74
An example and calculation of a weak base reacting with water
Consider the reaction of ammonia with water:
The value of Kb for ammonia is 1.75 x 10-5 M
pKb = - log Kb = -log (1.75 x 10x-5 M) = -(-4.76) = 4.76
What is a buffer?
A substance that resists a change in pH, an acidic buffer maintains pH sub 7 and a basic buffer does the opposite.
Buffering capacity in relationship to acid/base conjugates.
More conjugate acid/base available means more buffering capacity.
State the Henderson-Hasselbach equation.
pH =pKa + log ([A-]/[HA])
