Video Transcript
In this video, we will learn how to
identify and analyze oxidization and reduction reactions. We will learn how to define
oxidization, reduction, oxidizing agent, and reducing agent in terms of gain and
loss of oxygen, hydrogen, and electrons.
Oxidization and reduction are two
chemical processes that can be thought of as opposite to each other. Both can be explained in a variety
of ways. One of the simplest examples is the
burning of carbon. When charcoal, a form of carbon, is
burned, carbon is oxidized, forming carbon dioxide. Based on this, we can describe
oxidization as the addition of oxygen. So the opposite reaction would be
the loss or removal of oxygen. We can describe this process as
reduction.
Oxygen can be removed from copper
oxide to produce copper metal using hydrogen. As copper oxide has lost oxygen, we
say that the copper oxide has been reduced. Therefore, we could describe this
reaction as a reduction. However, the hydrogen has gained
oxygen, forming water. So we say that hydrogen has been
oxidized. So we can also describe this
reaction as an oxidization.
In this reaction, copper oxide has
been reduced and hydrogen gas has been oxidized. As the hydrogen gas is the agent
that caused the reduction of the copper oxide, we can call it the reducing
agent. Similarly, as the copper oxide is
the agent that caused the oxidization of hydrogen, we call it the oxidizing
agent. Therefore, the reducing agent gets
oxidized and the oxidizing agent gets reduced.
We’ve looked at oxidization and
reduction in terms of gain or loss of oxygen, but now let’s look at it in terms of
gain or loss of hydrogen. Let’s take a look at the reaction
between ammonia and bromine gas. Ammonia loses hydrogen, forming
nitrogen gas. Bromine reacts with the hydrogen
that ammonia loses, forming hydrogen bromide. As ammonia has lost hydrogen, we
say that it’s oxidized or has undergone oxidization. We can define oxidization as a
chemical process that increases the oxygen percentage or decreases the hydrogen
percentage in a substance.
As bromine gains hydrogen, forming
hydrogen bromide, we say that it’s been reduced or undergone reduction. We can define reduction as a
chemical process that decreases the oxygen percentage or increases the hydrogen
percentage in a substance. Although bromine is itself reduced,
it also causes the oxidization of ammonia. So it is known as an oxidizing
agent. An oxidizing agent provides oxygen
or removes hydrogen from a substance during a chemical reaction.
Oxidization and reduction reactions
occur in pairs, so if something is reduced, then something else will be
oxidized. And if there is an oxidizing agent,
there will also be a reducing agent. As ammonia causes the reduction of
bromine, it is a reducing agent. A reducing agent provides hydrogen
or removes oxygen from a substance during a chemical reaction.
Up until this point, we have
considered oxidization in terms of the addition of oxygen or loss of hydrogen. However, some reactions involve
oxidization and reduction but don’t contain oxygen or hydrogen. It’s also possible to define
oxidization and reduction in terms of the gain or loss of electrons.
Let’s look at the combination of
sodium and chlorine as an example. Sodium and chlorine are in their
elemental form and are not charged. So, first of all, let’s calculate
the electronic configurations for an atom of sodium and an atom of chlorine. If we look in the periodic table,
we see that sodium has an atomic number of 11. This means that a neutral atom of
sodium has 11 electrons. The first shell contains a maximum
of two electrons. The second contains a maximum of
eight electrons. And as there are 11 electrons in
total, the remaining electron goes in the third shell. So sodium’s initial electronic
configuration is 2,8,1.
If we find chlorine in the periodic
table, we see that it has an atomic number of 17. So a neutral atom of chlorine has
17 electrons. Therefore, its electronic
configuration is 2,8,7. When the two species react, sodium
donates an electron to chlorine. The sodium atom becomes a sodium
ion. It has a one-plus charge and an
electron configuration of 2,8. As the chlorine atom gains an
electron, it becomes a chloride ion. It has a one-minus charge and an
electron configuration of 2,8,8.
As the sodium atom has lost an
electron, we say that it is oxidized or has undergone oxidization. Oxidization is defined as a
reaction that involves the loss of electrons from a substance. The chlorine atoms have gained
electrons. So we say that chlorine is reduced
or has undergone reduction. Reduction is defined as a reaction
that involves the gain of electrons in a substance. As sodium is the agent that causes
the reduction of chlorine, it is called the reducing agent. And as chlorine causes the
oxidization of sodium, it is called the oxidizing agent.
It’s important to note that
oxidization and reduction are concurrent processes. This means that they both occur at
the same time. The oxidization of a substance
cannot occur without the reduction of another.
Let’s look at another example of
oxidization and reduction in terms of electrons. If magnesium reacts with copper
sulfate, it can displace the copper, producing magnesium sulfate and copper
metal. This is due to magnesium being
higher than copper in the reactivity series. Copper sulfate and magnesium
sulfate are both ionic species. A sulfate ion has a two-minus
charge. So, to ensure an overall neutral
charge, both copper and magnesium have a two-plus charge.
If we just look at magnesium, we
see that it starts off neutrally charged and ends up with a two-plus charge. So magnesium has lost two
electrons. We represent that magnesium has
lost two electrons by placing those electrons on the right-hand side of the
equation. This means that the charges on both
sides of the equation are now balanced. This type of equation is called a
half equation. And as it shows us that magnesium
has lost electrons, we can clearly see that magnesium has been oxidized.
Half equations explicitly show how
many electrons are lost or gained. So they make it considerably
clearer as to which species are oxidized and which are reduced. Let’s now write a half equation for
copper. Copper initially has a two-plus
charge. It then forms neutral copper
metal. To do this, it must gain two
electrons. This half equation clearly shows us
that copper two plus has been reduced.
We can actually combine these two
half equations to give an overall ionic equation for this chemical reaction. We first need to consider the
number of electrons. Fortunately, the number of
electrons is the same in both half equations. So we can add the equations without
making any adjustments. First, we need to add together the
species on the left side of the equation, and then we need to add the terms on the
right. As the number of electrons on each
side of the equation is the same, they can be excluded.
As we can now consider oxidization
and reduction in terms of electrons, we can also define oxidizing and reducing
agents in terms of electrons. An oxidizing agent is a substance
that can gain electrons from another substance during a chemical reaction. In the example we’ve just looked
at, copper two plus has gained electrons so is itself reduced but is also described
as an oxidizing agent.
A reducing agent is a substance
that can lose electrons to another substance during a chemical reaction. So, for this example, magnesium
loses electrons, so it is itself oxidized but is also described as a reducing
agent.
We’ve now looked at quite a lot of
different definitions. So before we move on to some
questions, let’s do a quick summary. The definition of oxidization in
terms of oxygen and hydrogen is a chemical process that increases the oxygen
percentage or decreases the hydrogen percentage in a substance. A reduction, on the other hand,
decreases the oxygen percentage or increases the hydrogen percentage. An oxidizing agent provides oxygen
or removes hydrogen from a substance during a chemical reaction, whereas a reducing
agent provides hydrogen or removes oxygen.
We can also define these words in
terms of electrons, where oxidization is a reaction that involves the loss of
electrons from a substance. Reduction, on the other hand,
involves gain of electrons. An oxidizing agent can be defined
as a substance that can gain electrons from another substance during a chemical
reaction, whereas a reducing agent loses electrons to another substance.
There is another definition we can
use for oxidizing agent and reducing agent, but it’s more general. An oxidizing agent can be defined
as a substance that oxidizes another chemical species while being reduced itself,
whereas a reducing agent reduces another chemical species while being oxidized
itself.
Oxidization, reduction, oxidizing
agent, and reducing agent were originally described in terms of oxygen and
hydrogen. The definitions in terms of
electrons came later. It’s important to note that even
though not all reactions involving oxidation and reduction can be described in terms
of oxygen and hydrogen, they can all be described in terms of electrons.
Now that we know how to identify
and analyze oxidization and reduction reactions and how to define oxidization,
reduction, oxidizing agent, and reducing agent, let’s look at a few examples.
Consider the reaction between
sodium and chlorine: Cl2 gas plus two Na solid react to produce two NaCl solid. Which chemical species is oxidized
in the reaction? (A) The sodium atoms, (B) the
sodium ions, (C) the chlorine atoms, or (D) the chlorine ions.
If a chemical species has been
oxidized, it means it has undergone oxidization, where oxidization is a reaction
that involves the loss of electrons from a substance. Since we want to find out which
species has been oxidized, we want to find out which species has lost electrons. Our starting materials are chlorine
and sodium in their elemental form. Neither of these species are
charged. The product is sodium chloride,
commonly known as salt. It is an ionic species made of
sodium ions and chloride ions. More specifically, the ions are Na+
and Cl−.
Chlorine started out as an
uncharged species. But upon reaction with sodium, it
became negatively charged. This means that chlorine gained
electrons. Therefore, chlorine has not
undergone oxidization; it has undergone reduction. Reduction can be defined as a
reaction that involves the gain of electrons in a substance. Sodium, on the other hand, existed
as a neutral starting material. But in the product, it is
positively charged. Sodium atoms have to lose electrons
to become positively charged sodium ions. So the sodium atoms undergo
oxidization and are therefore oxidized.
So the answer to the question
“Which chemical species is oxidized in the reaction?” is (A) the sodium atoms.
Which of the following descriptions
can be used for an oxidizing agent? (1) Loses electrons, (2) gains
electrons, (3) donates oxygen, (4) removes hydrogen, (5) donates hydrogen or removes
oxygen. (A) 1, 2, and 5; (B) 1, 3, and 4;
(C) 2 and 5; (D) 2, 3, and 4; or (E) 1 and 5.
An oxidizing agent can be generally
defined as a substance that oxidizes another chemical species while being reduced
itself. But we can define oxidizing agent,
and therefore also reducing agent, oxidization, and reduction, in multiple ways,
either in terms of gain or loss of oxygen, hydrogen, or electrons. In terms of oxygen and hydrogen, an
oxidizing agent provides oxygen or removes hydrogen from a substance during a
chemical reaction. As it provides oxygen, we see that
(3), donates oxygen, is correct. And as it removes hydrogen, we see
that (4), removes hydrogen, is also correct.
Description (5), which is “donates
hydrogen or removes oxygen,” is the exact opposite to descriptions (3) and (4). So this is the description of a
reducing agent, not an oxidizing agent. Therefore, we can rule out
description (5) as it cannot be used for an oxidizing agent.
Now that we’ve addressed all
options concerning oxygen and hydrogen, let’s look at the description of an
oxidizing agent in terms of electrons. It is defined as a substance that
can gain electrons from another substance during a chemical reaction. As it can gain electrons, we can
see that description (2), gains electrons, can be used to describe an oxidizing
agent. Option (1), however, loses
electrons, is the opposite of this so is used to describe a reducing agent, not an
oxidizing agent.
As we’ve determined that options
(2), (3), and (4) are the correct descriptions used for an oxidizing agent, we can
select option (D) 2, 3, and 4 as the correct answer to this question.
Now let’s look over the key points
of this video. Oxidization can be considered the
gain of oxygen, loss of hydrogen, or loss of electrons. Reduction can be considered the
loss of oxygen, gain of hydrogen, or gain of electrons. Oxidizing agents are substances
that can oxidize another substance while being reduced themselves, whilst reducing
agents are substances that can reduce another substance while being oxidized
themselves.
The electrons lost by the substance
being oxidized are gained by the oxidizing agent. And the electrons gained by the
substance being reduced are lost by the reducing agent. Lastly, oxidization and reduction
are concurrent processes. This means that they happen at the
same time.