>> Welcome to a screencast on an introduction to oxidation-reduction reactions.
And let's start by considering the following reaction.
If we take some hydrochloric acid, HCl aqueous, and add to that some zinc metal, zinc solid,
what we see happening pretty much right away is a lot of bubbling and fizzing occurs,
and that is the production of hydrogen gas, H2.
And then over time, the amount of zinc gets less and less.
There's less visible zinc, and if we either didn't add too much zinc
or had sufficient hydrochloric acid, eventually, all of the solid zinc disappears.
We no longer see any solid, and what has been forming,
in this case, is zinc II chloride, ZnCl2.
Now, we can write the equation for the reaction that takes place.
Zinc solid plus hydrochloric acid, HCl, forms zinc II chloride,
ZnCl2 plus hydrogen gas diatomic, H2, and then, of course, we can balance the chemical equation,
and we have a correct complete chemical equation describing the reaction that takes place.
Now, we can also note that we have chloride ion in HCl,
and we have chloride ion in zinc II chloride.
Both are aqueous, so the ions are floating around,
and the hydrogen ion actually changed to hydrogen gas.
The zinc solid changed to zinc II plus ions, but the chloride ions were just floating
around at the start and at the end as chloride ions, so they're spectator ions,
and so we can also write this reaction as a net ionic equation.
Zinc solid plus two hydrogen ions make zinc II plus ions plus hydrogen, H2, gas,
and the advantage of this is the usual advantage of net ionic equation is,
it's only showing the things that actually react and is not cluttered
up with ions that are just spectating.
Now, let's look in a little more detail at this reaction, and the key thing to note here is
that the zinc solid we started with reacts
to make zinc-two-plus ions, so the zinc lost electrons.
And the chemical term for a process where electrons are lost is oxidation,
and we would say that the zinc is oxidized in this reaction.
Now, if we look at the hydrogen, it starts as hydrogen ion.
It forms hydrogen, the element, diatomic molecules of hydrogen.
So hydrogen ended up gaining electrons, and the chemical term for a process
where electrons are gained is reduction.
And so we say that hydrogen ion gains electrons and is reduced.
Now, these don't maybe sound like the most sensible terms to come up with
for gaining and losing electrons.
There's a historical reason for these terms oxidation and reduction, or being oxidized
or being reduced, but to chemists in general, oxidizing means losing electrons.
Reducing means gaining electrons, or the process of oxidation is losing electrons.
The process of reduction is gaining electrons.
Now, one other thing to note with this reaction is that the zinc loses two electrons
to form the zinc II plus ion, and two hydrogen ions --
each of the hydrogen ions actually gains one electron, and since there's two hydrogen ions,
then there's a total of two electrons gained to form the hydrogen gas, H2.
And this is always going to be the case in an oxidation-reduction process.
The number of electrons, quote-unquote, "lost" and the number of electrons,
quote-unquote, "gained" have to be equal.
We can't have electrons disappearing or just appearing out of thin air.
Electrons are actually being transferred.
That's why the "lost" and "gained" are in quotes there because,
really, the zinc didn't lose electrons.
It simply transferred the electrons to the hydrogen ion,
and that's how the hydrogen ion gained electrons is from the zinc.
Now, another thing to note is that the zinc is oxidized, and by being oxidized,
by losing electrons itself, it provides electrons for the hydrogen ion to gain.
And so zinc is called the reducing agent
because it helps the other substance, the hydrogen ion, be reduced.
And then similarly, the hydrogen ion, which is, in fact, itself reduced, helps the other thing,
the zinc, be oxidized, and so it's called the oxidizing agent.
And this is true in general.
Whichever substance is oxidized is the reducing agent.
Whichever substance is reduced is the oxidizing agent.
And that's it for an introduction to oxidation-reduction reactions.