Video Transcript
In this video, we will learn how to
identify redox reactions and write ionic equations to describe electron transfer
between oxidants and reductants. We will only look at redox
reactions of atoms and simple ions, and not covalent compounds or polyatomic
ions. This is an atom of hydrogen. An atom of hydrogen typically has
one proton and no neutrons in its nucleus and a single electron in its electron
cloud. To help us keep track of the
electrons, we can use an electron shell diagram, with one electron in the first
electron shell. An atom of hydrogen can gain one
electron and become a hydride ion. It’s the same number of protons,
but one extra electron.
Alternatively, a hydrogen atom may
lose an electron to form a hydrogen ion. It can help to simplify this
diagram by writing H+ instead. When atoms or ions gain or lose
electrons, they change their chemical behavior, so it’s quite important to keep
track. One of the ways we can do this is
by talking about reduction and oxidation. Reduction is the process of gaining
electrons. It can be one, two, or any
number. So we say that a hydrogen atom is
reduced to form a hydride ion because it gains one electron. The opposite process is called
oxidation. Oxidation is the process of losing
electrons. For example, we say that a hydrogen
atom is oxidized to form a hydrogen ion, losing one electron.
It’s not immediately obvious from
the words which one means what. So there are a couple of mnemonics
that may come in handy. These are OIL RIG, or you can use
LEO the lion says GER. It doesn’t matter which you
use. They both mean the same thing. OIL RIG expands to oxidation is
loss and reduction is gain. And LEO the lion says GER means
that the loss of electrons is oxidation and the gain of electrons is reduction. These definitions of reduction and
oxidation are useful for atoms and monatomic ions, where we can see the charges. For polyatomic ions and for
covalent compounds, we need to use oxidation numbers instead, but that’s beyond the
scope of this video.
But there’s something slightly
strange. Where did these words come
from? It doesn’t seem to make sense for
reduction to involve adding something. Well, it all comes down to the fact
that oxidation used to be much more specific a word. It used to mean reaction with
oxygen. When we react magnesium with
oxygen, we form magnesium oxide, so we have oxidized magnesium. If we find the right chemical, we
might be able to remove the oxygen and form magnesium again. And historically, we call this
reduction, removing oxygen from an oxide or peroxide.
Now let’s have a closer look at
magnesium in its pure form and in magnesium oxide. The atoms in pure magnesium have
zero overall charge; they’re neutral. But magnesium oxide is an ionic
compound. We have magnesium two plus instead
of magnesium atoms. So when we oxidize magnesium, we’re
taking away electrons from the magnesium atoms. And when we reduce magnesium oxide,
we’re adding them back. So reacting with oxygen does
exactly the same thing as what we’ve described before, oxidation being the process
whereby electrons are removed. And it’s the same for
reduction. When we remove oxygen from
magnesium oxide, the net effect for magnesium is that it gains electrons. And this is where the word
reduction comes from, removing oxygen or reducing the amount of oxygen.
This leads to a neat trick. When we see a substance reacting
with oxygen to form some kind of oxide, that’s oxidation and electrons are being
removed. And if we see some kind of oxide
losing oxygen, then a reduction is taking place and electrons are being added. Be very careful to remember that
it’s not the oxygen itself we’re looking at when we’re looking at adding or taking
away electrons. It’s whatever it’s bonding to or
leaving. So in this case, we’re just looking
at electrons being added or taken away from magnesium or magnesium ions.
There’s a neat feature of chemistry
that we can rely on. Electrons rarely add to or leave an
atom or ion spontaneously. The reason they move is usually
because there are other reacting chemicals around it. For example, let’s imagine we have
a stable arrangement of hydrogen molecules and a lump of lithium. When they get too close, the
electrons will be transferred and will form lithium hydride, which is made of
lithium ions and hydride ions. At some point during the reaction,
for each lithium atom, the outer electron has jumped over to a hydrogen atom. So the hydrogen atoms were reduced;
they gained electrons. And the lithium atoms were
oxidized; they lost electrons.
When reduction of one species
happens at the same time as the oxidation of another, it’s called a redox
reaction. Since electrons generally don’t
move around on their own, we can be pretty sure that if we see a reduction taking
place for one entity, there’ll be another entity in the same reaction that’s being
oxidized. We now have the tools we need to
describe redox reactions. Here we have the reaction of copper
with silver nitrate, forming copper nitrate and silver. We start off with copper in its
pure form, copper atoms bonded together with metallic bonds. And we have a solution containing
silver ions and nitrate ions, Ag+ and NO3−.
When we add the copper to the
silver nitrate solution, there’s a beautiful reaction. Crystals of silver grow on the
surface of the copper, and the solution starts to turn blue. Since copper can form ions with
different charges, we have to use the II notation to indicate we’re dealing with
copper two plus ions. The copper atoms we started with
lost electrons, forming copper two plus ions. At the same time, these silver plus
ions from solution gained electrons, forming silver atoms. The copper atoms lost electrons, so
they were oxidized, while the silver atoms gained electrons and they were
reduced. So we can see which species was
reduced and which was oxidized. But it could also be helpful to
talk about the species that does the reduction or does the oxidation.
For this, it might be helpful if we
write the net ionic equation. We can start by writing out the
full ionic equation, where we’ve separated out the salts into their cations and
anions. The nitrate anions don’t
participate in the reaction. We can see they’re unchanged,
identical on the reactant and product side of the equation. If we remove spectator ions and get
the net ionic equation, it’ll be easier to see where electrons are moving from and
where they’re moving to. Each copper atom in the reaction is
donating two electrons, one to each of two silver plus ions. One way of expressing this is that
Ag+ is reduced by copper. This is because it’s copper that’s
giving up the electrons that are received by the Ag+ ions.
But simultaneously, since it’s the
silver plus ions that are receiving the electrons from the copper, we can say that
the copper is being oxidized by the silver plus. We can look at this another way and
say that copper is the reducing agent, the component of the reaction that’s
responsible for reducing another species. Likewise, we can say that silver
plus is the oxidizing agent because its presence causes another species to be
oxidized. A reducing agent or a reductant is
a substance that reduces another substance during a redox reaction. As a result of this reaction, the
reducing agent is oxidized, while an oxidizing agent or oxidant is a substance that
oxidizes another substance during a redox reaction. And an oxidizing agent, because it
oxidizes something else, is reduced as a result.
So in this example, copper will be
oxidized and silver plus will be reduced. Copper will be the reducing agent,
and silver plus will be the oxidizing agent. And the copper two plus ions will
be the product of oxidation, and the silver atoms will be the product of
reduction. Now that we’ve looked at the
mechanics of reduction and oxidation and redox reactions, let’s have some
practice.
Consider the equation shown. Cl2 aqueous plus 2Br− aqueous react
to form Br2 aqueous plus 2Cl− aqueous. (a) Which arrow shows
reduction? (b) Which arrow involves a loss of
electrons? And (c) which arrow involves the
gain of electrons?
The arrow in all three questions
refers to either A or B. In the equation, we can see the
chlorine molecule Cl2 reacting with the bromide ion Br−, forming bromine Br2, which
dissolves readily in water producing a memorable orange color, and chloride ions
Cl−. To make it easier, you can separate
the individual processes. So A refers to Cl2 transforming
into two Cl− and process B refers to two Br− transforming into Br2. Here, I’m ignoring the state
symbols.
In process A, we see a molecule of
chlorine composed of two chlorine atoms, transforming into two chloride ions, each
with a charge of one minus. Therefore, this process reflects
the addition of two electrons, one to each of the chlorine atoms. And for B, we have two negatively
charged bromide ions, transforming into neutral bromine atoms in a bromine
molecule. Therefore, B corresponds to the
process of removing two electrons.
Question (a) refers to
reduction. Reduction is the process of gaining
electrons. This is easy to remember if we use
OIL RIG. Oxidation is loss; reduction is
gain of electrons. Therefore, the answer to part (a)
is arrow A. In arrow A, chlorine is being
transformed to chloride by the addition of electrons. This is reduction.
Part (b) asks us to indicate the
arrow that involves a loss of electrons. This is arrow B. In process B, we have two bromide
ions being transformed into bromine, losing two electrons. And finally, the answer to part (c)
is arrow A. In reaction A, chlorine is gaining
electrons to form chloride.
Next, let’s look at a question
involving ionic equations.
Which of the following statements
about the following ionic equation is incorrect? Zn solid plus 2Ag+ aqueous react to
form Zn2+ aqueous plus 2Ag solid. (A) Zinc atoms becoming zinc ions
is oxidation. (B) Silver ions becoming silver
atoms is reduction. (C) Zinc acts as a reducing
agent. (D) Silver ions gain electrons. Or (E) zinc atoms becoming zinc
ions is reduction.
In this reaction, we have zinc in
its elemental solid form reacting with silver ions in solution. And our products are zinc ions in
solution and solid silver. Our job is to examine the
statements and find the incorrect statement. The first statement is that zinc
atoms becoming zinc ions is oxidation. We can use our mnemonic LEO the
lion says GER to tell us what oxidation and reduction are. LEO expands to the loss of
electrons is oxidation and GER expands to the gain of electrons is reduction. From the equation, we can see zinc
is being transformed into zinc two plus. This corresponds to the removal of
two electrons.
We can also see silver plus ions
transforming into silver atoms. This particular process involves
two electrons being added. We can see that zinc atoms in this
case are becoming zinc ions and losing electrons. This statement is true. Therefore, it can’t be our correct
answer. Next, we have silver ions becoming
silver atoms is reduction. We can see from our equation that
the silver ions are gaining electrons forming silver atoms. Therefore, this is reduction and
this statement is true and therefore not our correct answer.
The third statement requires a
little bit more insight than we’ve got thus far. Zinc acts as a reducing agent. What’s a reducing agent? A reducing agent reduces something
else by giving it electrons. We can see from the equation that
each zinc atom is giving up two electrons, one each to two silver plus ions. Since it’s giving up electrons,
it’s acting as a reducing agent. So this statement is also true. And since we’ve already discussed
that it’s the silver ions that receive the electrons, we know that silver ions gain
electrons is a true statement and therefore not our correct answer.
The last statement suggests that
zinc atoms becoming zinc ions is reduction. But we know that in this process,
the zinc atoms are losing rather than gaining electrons. So this is not reduction. This is oxidation. So the five statements given about
the ionic equation, the only one that’s incorrect is that zinc atoms becoming zinc
ions is reduction.
Finally, let’s finish up with the
key points. Reduction is a process where a
substance gains electrons, while oxidation is the reverse, the process where a
substance loses electrons. At higher levels, we may move away
from counting electrons to looking at what we call oxidation numbers. A redox reaction is a type of
reaction in which reduction of one substance and the oxidation of another substance
occurs simultaneously. The reducing agent is the substance
in a redox reaction doing the reducing of something else. As a result of this, it is
oxidized, while the oxidizing agent is the substance in a redox reaction that is
oxidizing something else and that is in turn reduced.
