(L6) Module 1 L6 - Weak acids and bases Flashcards
What is a weak acid?
An acid that undergoes incomplete dissociation in water into [H3O]
Basically they reach equilibrium
Equilibrium lies to the left (reactant)
What is Ka?
Acid dissociation constant.
What is acid dissociation constant (Ka) a measure of?
Measure of acid strength. Large vallue of Ka means more extensive dissociation at equilibrium and the stronger the acid
how to find pKa?
pKa = -log (Ka)
what is Kb?
base dissociation constant
What is base dissociation constant (Kb) a measure of?
Measure of base strength. Large value Kb means more formation of OH- at equilibrium and the stronger the base.
How to find pKb?
pKb = -log (Kb)
What does smaller Kb/Ka mean?
Weaker the acid/base
What does larger pKa or pKb mean?
Weaker the acid/base
What’s the relationship between Kb and Ka?
Ka * Kb = Kw
What’s the relationship between pKb and pKa?
pkA + pKb = 14
This relationship means, stronger the base, weaker the conj acid.
stronger the acid, weaker the conj base.
The conjugate base of a strong acid is…
The conjugate acid of a strong base is…
Very weak
What is strength of conj acid/base of weak acid/base?
they’ll both be weak.
it’s difficult to generalise. however you can not say that the conj base of a weak acid will be strong.
Ka assumptions during calculations:
first put in Ka = products/reactants
1. assume reactant have equal conc. due to 1:1 stochiometry
2.assume products at eqbm approx. equal initial products due to reactants being a weak acid/Ka being small.. so it barely disssociates
How can we rearrange Ka equations using the assumptions?
Does Kb requires different equations?
No! use the same Ka method and equations for Kb
What is a Lewis base?
The proton donator
What is a Lewis acid?
Proton acceptor
What type of molecule is a lewis base?
Ligands
What type of molecule is a lewis acid?
Transition metal cations
What are transition metal complexes?
Transition metal complexes are a class of molecules where molecules called ligands (lewis base’s) form bonds to a transition metal cation (lewis acid’s).
When the ligands are water molecules, the resulting complex can act as a weak acid.
How does positive charge of metal cation affect pKa?
pKa decreases…
complex becomes more acidic..
as positive charge on
the metal ion increases.
What’s the activation of coordinated water molecules importal to?
Activation of coordinated water molecules is central to the function of many enzymes.
carboxypeptidase is an enzyme that has zinc in it. it reacts quickly with OH- or slowly with water.
How can this reaction be sped up?
Bond the water to the zinc ion in the enzyme, turning water –> OH-, which speeds up the reaction.
