- [Instructor] Let's say the HA
represents a generic weak acid
and B represents a generic weak base.
If our weak acid donates a proton to our weak base,
that would form A minus and HB plus.
To identify conjugate acid base pairs
remember there's only one proton or one H plus difference
between an acid and it's conjugate base.
So when HA donates its proton and turns into A minus,
A minus must be the conjugate base to HA.
So there's one H plus difference between HA and A minus.
They are a conjugate acid base pair.
When B accepts a proton,
it turns into HB plus therefore HB plus must be
the conjugate acid to B.
Since there's one H plus difference between B and HB plus,
they are a conjugate acid base pair.
Let's look at the situation where the equilibrium constant
for this reaction is greater than one.
If K is greater than one, that means that at equilibrium,
there are more products than reactants
therefore, the equilibrium favors the formation
of the products.
For acid-base reactions the equilibrium
always favors the side with the weaker acid
and the weaker base.
Therefore, since the equilibrium favors the products,
the weaker acid and the weaker base,
are on the product side
and the stronger acid and the stronger base
are on the reactant side.
So when comparing our two acids,
HA and HB plus, HA is the stronger acid of the two.
And when comparing our two bases B and A minus,
B is the stronger base of the two.
When the equilibrium constant is less than one
that means at equilibrium,
there are more reactants than there are products.
Therefore, this time,
the equilibrium favors the formation of the reactants.
And the equilibrium always favors the side
with the weaker acid and the weaker base.
Since the equilibrium favors the reactants,
the weaker acid and the weaker base,
are on the reactant side
and the stronger acid and the stronger base
are on the product side.
So comparing our two acids, HA and HB plus
this time, HB plus is the stronger acid of the two
and for our bases B and A minus
this time, A minus is the stronger base of the two.
Let's look at an example of a weak acid, weak base reaction.
The Hydrogen Sulfate anion
we'll react with the Carbonate anion
to form the Sulfate anion
and the Hydrogen Carbonate anion.
Looking at the reaction, the Hydrogen Sulfate anion
is donating its proton so that must be the acid
and the Carbonate anion is accepting a proton
so that must be the base.
After the Hydrogen Sulfate anion donates it's proton
it turns into the Sulfate anion, SO four, two minus.
Therefore the Sulfate anion must be the conjugate base
to HSO four minus.
So one conjugate acid base pair is HSO four minus
and SO four, two minus.
And when the Carbonate anion and accepts the proton,
it turns into the Hydrogen Carbonate anion.
Therefore the Hydrogen Carbonate anion,
must be the conjugate acid to the Carbonate anion.
So our other conjugate acid-base pair
consists of the Carbonate anion,
and the Hydrogen Carbonate anion.
The equilibrium constant for this reaction
at 25 degrees Celsius is greater than one,
therefore the equilibrium favors
the formation of the products.
And because the equilibrium favors
the formation of the weaker acid and the weaker base,
we know that the weaker acid and the weaker base
must be on the product side
and the stronger acid and the stronger base
are on the reactant side.
Therefore the Hydrogen Sulfate anion is a stronger acid
than the Hydrogen Carbonate anion,
and the Carbonate anion in as a stronger base
than the Sulfate anion.
Let's look at another example
of a weak acid, weak base reaction.
In this case,
Hydrofluoric acid reacts with Ammonia
to form the Ammonium ion an NH four plus,
and the Fluoride anion, F minus.
Because Hydrofluoric acid donates a proton, it's an acid
and because Ammonia accepts a proton
Ammonia functions as a base.
And when HF donates a proton,
it turns into F minus the Fluoride anion,
therefore the Fluoride anion is the conjugate base to HF.
And when Ammonia accepts a proton,
it turns into NH four Plus the Ammonium ion,
therefore the Ammonium ion is the conjugate acid
to NH three.
Let's say that we mix equal moles of our weak acid HF
with our weak base NH three
and our goal is to figure out if the resulting solution
is acidic, basic or neutral.
First, we need to determine if the reactants or products
are favored at equilibrium.
And to help us determine that
here we have the Ka and Kb values
for our acids and bases.
So here's the Ka value for Hydrofluoric acid
at 25 degrees Celsius,
so all of these are at 25 degrees Celsius.
Here's the Kb value for Ammonia,
the Ka value for the Ammonium ion
and the Kb value for the Fluoride anion.
So looking at our two acids that was Hydrofluoric acid
and the Ammonium cation,
the Ka for Hydrofluoric acid is a higher value
than the Ka for the Ammonium cation.
Therefore Hydrofluoric acid is the stronger of the two,
the stronger acid of the two.
And for the two bases,
we have Ammonia and we also have the Fluoride anion,
looking at the Kb values,
for Ammonia the Kb value is higher
than the Kb value for the Fluoride anion,
therefore Ammonia is the stronger base of the two.
So we have the stronger acid and the stronger base
on the reactant side
and the weaker acid and the weaker base
on the product side.
We know the equilibrium favors the side
with the weaker acid and the weaker base.
Therefore the equilibrium
favors the formation of the products.
So we could go ahead and write the equilibrium constant
for this reaction is greater than one.
And at equilibrium
we're going to have more of our products
than we do of our reactants.
So to determine at the solution as acidic, basic or neutral,
we have to think about the Ammonium cation
and the Fluoride anion in aqueous solution.
Since the Ka value for the Ammonium cation
is greater than the Kb value for the Fluoride anion,
the Ammonium cation is better at producing Hydronium ions
in aqueous solution
than the Fluoride anion is at producing Hydroxide anions
in aqueous solution.
So the concentration of Hydronium ions
will be greater than the concentration of Hydroxide anions
and therefore the resulting solution will be acidic.