In the previous module, we discussed the structure
of the atom, specifically how electrons in
order to achieve a full outer shell can either
be handed off, collected or indeed shared
forming ionic or covalent bonds respectively.
Acidity relates specifically to the concentration
of H+; H+, of course, being an atom of hydrogen,
which has lost a single electron. Of course,
given your understanding of the structure
of the hydrogen atom, this results in H+,
otherwise being equivalent to a single proton.
This proton is often considered as H3O+ in
water, where the H+ is transferred onto H2O.
An acid is a substance which can lose one
or more equivalents of H+ and a base is a
substance which can pick up H+.
If we look at the equation just below on the
board, as you can see, we’ve shown a general
formula for an acid which is written as AH
or, sometimes, as HA. In the case of B, which
is our example base here, you can see that
H+ is transferred from the acid onto the base.
This results in the formation of BH+, shown
in blue and the conjugate base A-, shown
Bear in mind that, as with all things chemical,
matter is conserved, which means what is one
side of the equation must also be replicated
on the other side of the equation and that
includes the balance of charges.
When an acid AH or HA reacts with a base,
H+ is transferred. And when we’re considering
acidity, we look at the concentration of H+,
the concentration of protons, in a given solution.
Water, which is a universal solvent for the
vast majority of ionic reactions, as we’ll
see in the next lecture, is considered amphoteric.
The term “amphoteric” basically means
that a substance or system or compound can
act either as a proton donor or as a proton
acceptor. And water is an example of an amphoteric
compound. There are, of course, others which
are ionic and we may touch upon them a little
But, for now, what you should be content with
is that HA, our model acid here, can protonate
H2O to give rise to H3O+, okay; otherwise
known as the hydronium ion. B, a base, can
also react with H2O to give rise to the hydroxide
ion shown in red on the second part of that
equation. It can also react with itself and
ionise itself, forming a mixture of hydroxide
and, of course, H3O+.
As most acid-base reactions, as I’ve said,
take place in water, it is common not to include
water directly in the equation. So, you’ll
often see acid-base equilibria, acid-base
equations, written as the following where
HA is shown to separate or lyse into A-
and H+. And a base, shown here as a neutral
species, although sometimes they are ionic
and negatively charged, reacts with H+ to
give BH+. So, these are the two types of compounds
or types of equations I’d like to introduce
to you in this lecture.