Balancing RedOx reactions.
At the first glance, the equation representing
the equation of zinc metal with silver
ions and might appear to be balanced. Why
not? We have zinc on one side, we have zinc
on the other. We have silver on one side,
we have silver on the other. But, note, the
number of electrons, in this particular case,
is not being made consistent. We have a positive
charge on one side, but we have two positive
charges on the other. This cannot be acceptable.
So, a balanced equation must also have charge
balance as well as mass balance.
As mentioned before, we can split the reaction
into two half reactions. So, let’s do that.
We will balance the RedOx reaction using the
oxidation number method.
So, we’ve see the previous equation and now,
it’s up to you to try and balance it. The
way in which we balance it is first, by assigning
oxidation number to all elements in the reaction,
bearing in mind that in the case of zinc,
we have a 2+ oxidation state and silver, we
can only have a 1+ oxidation state. Identify
the species that are being oxidised or reduced
and then compute the number of electrons lost
in the oxidation process and gained in the
reduction process, where necessary drawing
lines between the two pairs including the
number of electrons which have been lost and
the number of electrons being gained.
What we then do is multiply one or both reactions
so that both numbers match, using these factors
as balancing coefficients. And finally, balance
any other species that were not involved directly
in that electron exchange to ensure that we
have the correct numbers of each component
on either side of the reaction.
Now, let’s bring on to quantitative analysis.
Analytical chemistry, which is again one part
of this, deals with the determination of the
composition of materials, that is the analysis
of materials. What I’d like to draw your attention
to is gravimetric analysis.
Gravimetric analysis is a type of quantitative
analysis in which the amount of species in
the material is determined by converting the
species into a product that can be isolated
and weighed. There are many, many different
types of analytical methodology that are used,
not just in terms of inorganic chemistry,
but also in terms of organic chemistry. But,
that is really beyond our terms of reference
for this. Instead, in the context of... context
of molarity, I’d like us to discuss here how
you can determine the amount of something
which is in solution by precipitating it out
and then weighing it. Precipitation reactions
are often used in gravimetric analysis and
the precipitate from the reactions can then
be filtered, dried and weighed.
Consider for a moment the problem of determining
the amount of lead in a sample of drinking
water. If you add sodium sulphate to a sample
of soluble lead, maybe it’s a lead nitrate
or some such, it will precipitate out insoluble
lead sulphate as per the reaction here. Indeed,
many lead compounds, lead sulphate, lead chlorides
and oxides are not particularly soluble in
water and therefore, it is a useful reaction
to carry out when trying to precipitate out
those relatively insoluble transition metal
Sodium sulphate, shown here as Na2SO4, plus
iron gives sodium and iron and lead sulphate.
Lead sulphate can then be filtered, dried
and weighed. It’s also important to bear in
mind, and I go back to what I was saying in
the previous module, the Pb, you may think,
doesn’t have any correlation to lead. It’s
actually derived from the Latin, Plumbum.
Hence, the name Pb as the chemical symbol