Video Transcript
The ability of a base to accept
protons could be deduced from its base dissociation constant 𝐾 𝑏. The base dissociation constant of
some bases is listed in the table below. Which of the bases listed in the
table is the weakest?
It is useful for chemists to be
able to quantify the strength of bases. We can do this using the base
dissociation constants. The base dissociation constant,
abbreviated as 𝐾 𝑏, is the equilibrium constant for the reaction of a base with
water. The general equation for this
reaction can be written as base, B, plus water. The base and water react reversibly
to produce the conjugate acid, BH+, and hydroxide ions.
We can write the base dissociation
constant for this reaction as the concentration of the products multiplied together
and divided by the concentration of the reactants multiplied together. We do not write water into the
expression as it is a liquid and not an aqueous solution with a concentration.
A weak base does not completely
dissociate in water. So, the equilibrium lies to the
left. This means that at equilibrium, the
concentration of the ions will be low, while the concentration of the base is
high. This results in a small 𝐾 𝑏
value. The weaker the base, the farther
the equilibrium lies to the left. The lower the concentration of the
ions at equilibrium, the lower the 𝐾 𝑏 value. The weakest base from the bases
listed in the table will have the smallest 𝐾 𝑏 value.
As we look at the values in the
table, it is worth mentioning that while no units are listed, 𝐾 𝑏 values are
measured by the unit moles per liter. The smallest 𝐾 𝑏 value listed in
the table is 4.3 times 10 to the negative 10, which corresponds to base E. Therefore, of the bases listed in
the table, the weakest is base E.
