Let’s get onto bases. So, we’ve talked
about acids, we’ve talked about the dissociation
of acids into conjugate base and H+. Now,
let’s talk about bases.
So, there are a number of different bases.
These can be charged bases, such as hydroxide,
or these can be uncharged bases, for example,
like ammonia and amine derivatives.
What happens in the case of a base, as we
said before, is that it can attract H+ or
a proton resulting in the formation, as you
can see from this particular equation on the
board, of BH+ and OH-. Okay? BH+ being the
conjugate acid in this particular case as
opposed to the conjugate base as we saw in
the case of acidity.
As with the acids, the strength of the base
can be shown by the equilibrium constant,
in this case Kb. And this results in pKb also
being able to be determined. However, and
this is something to bear in mind, more commonly,
you’ll find that, in the interest of clarity,
a base strength is actually measured by pKa.
Now, this is not, contrary to understanding,
the loss of a hydrogen or proton from the
base. This is the loss of the H+ from the
formed conjugate acid. So, this equation here,
as you can see on the board, this shows that
BH+, which is our conjugate acid dissociating
to give us our free base and our H+, correlates
actually to a dissociation or acidity constant
pKa. The equation for this is shown on the
Weak bases, therefore, have very low pKa values
and strong bases have very high pKa values.
And when we consider any acid or base, we
need to know or should be aware that pKa +
pKb = 14.0.
Strong bases include things like sodium hydroxide.
Indeed, mostly every hydroxide is regarded
as a strong base. And as we said before in
case... in the case of hydrochloric acid,
sulphuric acid and nitric acids, they’re
considered to be almost completely dissociated
in water. And so, therefore, in the case of
our NaOH, we see that sodium ion and hydroxide
ion are formed in solution.
Now, of course, what you’ll be talking about
and may wonder is, well, we were talking about
pH being potenz Hydrogen, we’re talking
about H+. So, where does OH- come into this?
We’re not measuring OH- when we’re looking
at pH, we’re measuring H+. And this is because
OH- will bind with any residual or large
amount of residual H+ in water and convert
it back into water because of the equilibrium
of H+... OH- in water and the preference
for it to exist as non-ionised water. This
decreases the amount of H+ in solution and
therefore, increases the pH, making it more
Amines are also examples here of weak bases.
Remember, we had weak acids. We can also get
weak bases. And here we see the pKa. Remember,
that is the equilibrium constant associated
with the dissociation of the conjugate acid
being 9.25. Methylamine is 10.66 pKa. And
the equilibrium in this particular case for
ammonia, and this is how you would form ammonium
hydroxide solution, is where we have our base
NH3, which is a neutral base, abstracting
a proton from H2O to give us NH4+, a complex
cation otherwise known as the ammonium ion
and OH-, hydroxide ion.