Our learning intention is that we will understand the difference between strong and weak acids and
bases. So in this video today we are going to use the Bronsted-Lowry definition of an acid
which is that "an acid is a proton donor" and a base therefore is a proton acceptor.
When an acid dissolves in water a proton, or a hydrogen ion,
is transferred to a water molecule to produce a hydronium ion, that is H3O+, in addition to
a negative ion. The identity of the negative ion is dependent on what acid you are starting from.
In the general case, our formula is that HA, denoting the identity of an acid,
plus H2O creates an equilibrium reaction with H3O+, that is our hydronium ion, and a negative
ion. So let's start thinking about what actually is a strong acid and what is a weak acid and how
are we able to identify them. So generally these reactions where an acid is dissolved into water
are reversible, but in some cases the acid is actually so good at giving away the hydrogen ions
that we can think of the reaction as being irreversible or that it has gone
to completion. When this happens the acid is virtually a hundred percent ionised,
meaning that the equilibrium lies very much to the right hand side of the above equation. This means
that all of the acid molecules have separated into ions. For example when hydrogen chloride dissolves
in water to make hydrochloric acid, so little of the reverse reaction happens that we can write
HCl gives Hydrogen ions and chloride ions. You'll see here that we didn't use an equilibrium symbol
we used an arrow denoting that this chemical reaction goes to completion and does not set up
an equilibrium. But remember that the water is still involved and when we see the H+
here it's actually representing a hydronium ion, the H3O+ and that is because the H+ has
been donated to the water molecule. So going back to a hydrochloric acid example, because all of or
almost all of the hydrogen chloride reacts to produce the hydronium and chloride ions,
hydrogen chloride is therefore described as a strong acid. Now let's look at weak acids. A weak
acid is one which only partially ionises when it is dissolved in water. For example, ethanoic acid
reacts with water to produce hydronium ions and ethanoate ions as shown in the following equation.
Since the reaction doesn't go to completion and it is reversible, it sets up an equilibrium.
However the reverse reaction in this case is favoured over the forward reaction.
This means that the ions react very readily very easily with each other to reform
the acid and the water. In fact, for ethanoic acid at any one time only about one percent
of the ethanoic acid molecules have converted into ions. Most molecules will remain as ethanoic acid
molecules. So how do we identify strong versus weak acids if we have an unknown substance and
we want to know well is it is it a strong acid or a weak acid? Here are some ways we can identify
them. So the first is the pH value. A strong acid will have a pH which is close to 1 while a weak
acid will have a pH between 2 to 6, remembering that around 7 is our neutralization point.
The other way we can identify strong versus weak acids is by looking at the pKa value.
Now the pKa is a number that communicates how strong or weak an acid is and it's used based
on how many hydrogen ions are in solution. So a strong acid will have a pKa value close to zero
while a weak acid will have a high pKa value. How do you determine what the pKa value is? So
we use our dissociation constant Ka where we multiply the concentration of the products then
divide that by the concentration of the reactants. And then to convert the Ka to the pKa we take the
minus log of the Ka value. And lastly the other way we can identify them is trying to memorize
which ones are strong acids. There are actually only six acids that are considered to be strong
and any other acid is considered to be a weak acid, so our strong
acids are shown here. We've got hydrochloric acid, sulfuric acid,
nitric acid, perchloric acid, hydrobromic acid and hydroiodic acid.
Let's look now at comparing strong and weak bases. A strong base is similar to a strong
acid in that it readily and almost completely or almost 100 percent ionizes in solution.
A strong base is one in which readily accepts protons.
A weak base is similar to a weak acid in that it only partially ionizes in solution
and a weak base does not readily accept protons. Now how can we identify whether something is a
strong or weak base? So we can also use our pH value. A strong base will have a pH which is
close to 14 remember that's the uppermost end of our pH scale. While a weak base will have a
pH between 8 to 13. The pKa value that we use for acids is similar to what we use in bases,
but we call it the pKb value, denoting that we are talking about the base.
So the base dissociation constant is interpreted in the same way. So examples of strong bases
are our sodium hydroxide and calcium hydroxide, and an example of a weak base would be ammonia.