In this explainer, we will learn how to identify and analyze oxidization and reduction reactions.
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 to produce carbon dioxide. The following equation shows the oxidization of carbon:
Based on this, we can describe oxidization as simply the addition of oxygen.
The opposite reaction would be the loss or removal of oxygen. We can describe this process as reduction.
When a stream of hydrogen gas is passed over a heated sample of copper oxide, the hydrogen gas removes oxygen from the copper oxide to produce copper metal.
The chemical equation for this reaction is
As copper oxide has lost oxygen, we could describe this reaction as a reduction.
However, the hydrogen has gained oxygen, and so we could also describe this reaction as an oxidization.
In this reaction, copper oxide has been reduced and hydrogen gas has been oxidized.
We can also describe each substance in the reaction above using the terms “oxidizing agent” and “reducing agent.”
For the reaction between copper oxide and hydrogen, we can consider the copper oxide as an oxidizing agent because it oxidizes the hydrogen gas.
We can also consider the hydrogen as a reducing agent because it reduces the copper oxide to copper metal.
From the diagram above, we can see that an oxidizing agent gets reduced and a reducing agent gets oxidized.
We can also explain oxidization and reduction in terms of the gain or loss of hydrogen.
In the following reaction diagram, we can see that the ammonia gas loses hydrogen and becomes oxidized, while the bromine gains hydrogen and becomes reduced.
We can also define oxidization as the loss of hydrogen and reduction as the gain of hydrogen.
Let’s clarify our understanding with some definitions.
Oxidization is a chemical process that increases the oxygen percentage or decreases the hydrogen percentage in a substance.
Reduction is a chemical process that decreases the oxygen percentage or increases the hydrogen percentage in a substance.
Example 1: Naming the Chemical Process that Describes the Increase in Percentage of Oxygen in a Compound
What is the name of the chemical process in which the percentage of oxygen in a substance increases?
In this question, we need to correctly name the process in which the percentage of oxygen in a substance increases.
In a reduction reaction, the percentage of oxygen decreases.
Neutralization describes a reaction between acids and bases.
Combustion reactions involve burning a substance in the presence of oxygen. While this may increase the percentage of oxygen, this is not always the case.
In a substitution reaction, a more active metal replaces a less active metal in a compound.
In an oxidization reaction, the percentage of oxygen in the substance increases, which matches the description in the question. Therefore, the correct answer is E, oxidization.
As we have seen, oxidization and reduction reactions occur in pairs. In reactions, we can identify the oxidizing and reducing agents.
Definition: Oxidizing Agent
An oxidizing agent provides oxygen or removes hydrogen from a substance during a chemical reaction.
Definition: 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, it is also possible to define oxidization and reduction in terms of the gain or loss of electrons.
Describing oxidization and reduction in terms of the gain or loss of electrons allows us to describe reactions that do not involve oxygen or hydrogen.
Consider a simple combination reaction in which sodium metal reacts with chlorine:
The sodium metal has an electronic configuration of 2, 8, 1, whereas the chlorine gas has an electronic configuration of 2, 8, 7.
During a chemical reaction, sodium and chloride ions are formed.
The atom of sodium gives an electron to the chlorine atom to form a positive sodium ion with an electronic configuration of 2, 8.
The chlorine atom receives the electron and becomes a chloride ion with an electronic configuration of 2, 8, 8.
The sodium atom has lost an electron, and the chlorine atom has gained an electron. We can define oxidization and reduction in similar terms.
Oxidization is the loss of electrons.
In the chemical reaction above, the sodium atoms lose electrons and become oxidized, with the chlorine atoms acting as an oxidizing agent.
Conversely, the chlorine atoms gain electrons and become reduced, with the sodium atoms acting as a reducing agent.
Oxidization is a reaction that involves the loss of electrons from a substance.
Reduction is a reaction that involves the gain of electrons in a substance.
Example 2: Identifying the Oxidized Species in the Reaction between Sodium Metal and Chlorine Gas
Consider the reaction between sodium and chlorine:
Which chemical species is oxidized in the reaction?
- The sodium atoms
- The sodium ions
- The chlorine atoms
- The chloride ions
Oxidization and reduction can be defined in terms of the gain or loss of electrons.
Oxidization involves the loss of electrons, and so we need to determine which chemical species loses electrons during the chemical reaction.
When sodium reacts as a metal, it tends to form positive ions as a result of losing electrons from its valence shell. Conversely, chlorine atoms gain electrons and form chloride ions as part of the final product, .
As the sodium atoms have lost electrons to form sodium ions, then they have been oxidized. The correct answer is therefore A.
In another example, we can see the displacement of copper from copper sulfate by the more active magnesium metal:
To better understand the movement of electrons, we can use half-equations to focus on the important parts of the chemical reaction.
In the example involving magnesium and copper sulfate, the magnesium atoms lose electrons and become oxidized. We can show this using the following half-equation:
This half-equation explicitly shows the number of electrons involved. As magnesium can form ions, then it loses two electrons.
Now, let’s consider only the copper ions, which form part of the ionic compound copper sulfate. We can see that they initially exist as ions with a charge.
During the chemical reaction, the copper ions gain two electrons and so become reduced. We can represent this by the following half-equation:
Notice how the number of electrons in both of these half-equations is the same.
We can combine these two half-equations to give us an overall ionic equation for this chemical reaction:
Finally, as we can consider oxidization and reduction in terms of electrons, we can also define oxidizing and reducing agents in terms of electrons.
Definition: Oxidizing Agent
An oxidizing agent is a substance that can gain electrons from another substance during a chemical reaction.
Definition: Reducing Agent
A reducing agent is a substance that can lose electrons to another substance during a chemical reaction.
Example 3: Describing an Oxidizing Agent in Multiple Ways
Which of the following descriptions can be used for an oxidizing agent?
- 1, 2, and 5
- 1, 3, and 4
- 2 and 5
- 2, 3, and 4
- 1 and 5
It is possible to define oxidization and reduction in multiple ways. For example, we can define them in terms of the gain or loss of oxygen, hydrogen, or electrons.
Similarly, oxidizing and reducing agents can be defined in the same ways.
An oxidizing agent oxidizes another chemical and during the process lose electrons itself. These electrons are gained by the oxidizing agent, and so option 2 is part of our correct answer.
Oxidization could also be an increase in the percentage of oxygen in a substance, which would require an oxidizing agent to donate oxygen. Therefore, we can consider option 3, the donation of oxygen, as also correct.
The final way of defining oxidization is through the loss of hydrogen. For an oxidizing agent, this would mean removing hydrogen from another chemical species. Therefore, option 4 is also correct.
We have now established that options 2, 3, and 4 can describe an oxidizing agent. Therefore, the correct answer is D.
Oxidization and reduction are concurrent processes (i.e., they both occur at the same time). The oxidization of a substance cannot occur without the reduction of another.
Example 4: Describing the Relationship between Oxidization and Reduction Reactions
Which of the following statements about oxidization and reduction is correct?
- Oxidization can happen independently of reduction.
- Reduction happens before oxidization.
- Oxidization happens before reduction.
- Oxidization and reduction happen concurrently.
This question is asking us to identify the correct statement about oxidization and reduction reactions.
We can describe oxidization and reduction reactions in terms of the transfer of electrons, oxygen atoms, or hydrogen atoms between reactants.
When one chemical gains an oxygen atom, another chemical must have lost that same oxygen atom.
Similarly, if a substance loses hydrogen atoms, then those hydrogen atoms must have been gained elsewhere.
Finally, if a substance gains or loses electrons, then those electrons must have come from or gone to another chemical.
All three statements above show that oxidization and reduction depend on each other. For one of them to occur, the other must also occur.
For oxidization to occur, reduction must also occur in the same reaction. In other words, they occur concurrently. Therefore, the correct answer is D.
Let’s consider one final reaction.
Iron(III) chloride reacts with hydrogen gas to form iron(II) chloride and hydrochloric acid:
Let’s just focus on what happens to the iron during this reaction.
In the reactant, , iron exists as a ion, whereas in the product, , iron exists as a ion. The iron ion has gained an electron to become an iron ion, and so it has been reduced.
This electron is gained from a hydrogen atom, making hydrogen a reducing agent. As a reducing agent, the hydrogen loses an electron and is oxidized to a hydrogen ion. This hydrogen ion then forms part of the product, the hydrochloric acid.
The chloride ions remain unchanged during the course of the reaction and are neither oxidized nor reduced.
We can write half-equations for the oxidization and reduction processes for this reaction.
For the reduction of , we can write
For the oxidization, we can write
We can represent this reaction as an ionic equation, without the chloride ions:
- 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.
- 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.
- The electrons gained by the substance being reduced are lost by the reducing agent.
- Oxidization and reduction are concurrent processes.