In this explainer, we will learn how to identify redox reactions and write ionic equations to describe electron transfer between oxidants and reductants.
A redox reaction is a combination of a reduction and an oxidation.
Definition: Redox Reaction
A redox reaction is a chemical reaction that involves the transfer of electrons between chemical species.
A reduction involves gaining electrons, while an oxidation involves losing electrons. In a redox reaction, electrons are transferred from one species to another. One species is oxidized and loses electrons, which then are accepted by the other species, which is reduced.
Definition: Reduction
A reduction is any chemical reaction where a chemical species gains electrons.
Definition: Oxidation
An oxidation is any chemical reaction where a chemical species loses electrons.
The word “oxidation” comes from the word “oxygen,” but not all oxidation reactions involve oxygen. Antoine Lavoisier first discovered oxygen-consuming reactions, which he called “oxidations.” Later, chemists realized that consuming oxygen to form an oxide involved a loss of electrons, so the term “oxidation” was expanded to include any reaction that involves a loss of electrons.
An example of an oxidation reaction that includes oxygen is the formation of calcium oxide, also known as quicklime, by exposing calcium metal to oxygen:
The product, calcium oxide, is an ionic compound made up of a ion and an ion.
During this reaction, the neutral solid calcium becomes a ion, meaning it has lost electrons to make its charge more positive. On the other hand, each oxygen atom in oxygen gas turns into a ion, meaning that they have each gained two electrons to make their charges more negative. Since calcium is losing electrons, it is being oxidized, and since oxygen is gaining electrons, it is being reduced.
Viewed from the perspective of gaining or losing oxygen, we can see that the calcium atom gains an oxygen atom to form calcium oxide, and therefore it is oxidized. The oxygen atoms begin bonded to one another, but each loses the other oxygen to bond with a calcium atom, and so they are being reduced.
When oxygen is involved in a redox reaction, it is frequently the element that is reduced, while the atom that gains an oxygen is oxidized. However, since not all redox reactions involve oxygen, we cannot rely on this definition alone.
Example 1: Determining Whether a Compound Is Oxidized or Reduced during a Reaction
A piece of magnesium oxide is heated with hydrogen until the magnesium is recovered. Is the magnesium oxide being oxidized or reduced?
Answer
This question is asking us to determine whether magnesium oxide is oxidized or reduced when heated with hydrogen. In other words, is the magnesium ion in magnesium oxide losing or gaining electrons to form solid magnesium?
We can write a chemical equation for the reaction of magnesium oxide with hydrogen:
Magnesium oxide is an ionic compound consisting of an ion and an ion. The product, solid elemental magnesium, is a neutral atom.
We can see that the charge on the oxygen atom in both and remains .
The charge on the magnesium atom goes from in to 0 in . To go from a charge of to 0, magnesium must have gained two electrons along the way.
Reduction involves gaining electrons, so the magnesium was reduced.
We could have also used the definition of reduction as the loss of oxygen to determine that the magnesium is reduced.
In order to learn about reduction and oxidation in the absence of oxygen, let’s consider the reaction between zinc chloride and magnesium:
In this reaction, a strip of magnesium metal is dipped into a solution of zinc chloride. The formation of a dark-colored layer on the magnesium strip, in addition to the formation of bubbles, suggests that a reaction has occurred. Now consider the ionic equation for this reaction:
In aqueous solution, zinc exists as a ion. However, the product of the reaction is solid zinc, with a charge of 0. To go from a ion to a neutral atom, it must gain two electrons during the reaction. Since it is gaining electrons, zinc is being reduced.
The opposite is true for magnesium. As a reactant, the magnesium is in its solid, elemental form with a charge of 0. However, the product of the reaction is an aqueous solution containing magnesium ions with a charge of . To go from a charge of 0 to a charge of requires the loss of two electrons. Since it is losing electrons, magnesium is being oxidized.
The chloride ions remain unchanged from the beginning to the end of the reaction. Ions that do not change over the course of the reaction are called spectator ions. Since their charge does not change, they are neither oxidized nor reduced. The complete ionic equation above includes the spectator ions, but we can also write the net ionic equation of this reaction, which omits them:
To help us distinguish between oxidation and reduction, we can use a couple of mnemonic devices. One phrase to remember is “OIL RIG,” or “Oxidation Involves Loss (of electrons), Reduction Involves Gain (of electrons).”
Another option is the phrase “LEO the lion says GER,” or “Lose Electrons Oxidation, Gain Electrons Reduction.”
When an atom or ion gains electrons, its charge becomes more negative. When a neutral atom turns into a negative ion, it is gaining electrons and being reduced. In addition, when a positive ion’s charge decreases, for example, from to or from to 0, it is gaining electrons and being reduced.
When an atom or ion loses electrons, its charge becomes more positive. When a neutral atom turns into a positive ion, it is losing electrons and being oxidized. Furthermore, when a negative ion’s charge increases, for example, from to or zero, it is losing electrons and being oxidized.
Example 2: Identifying the Reduction Reaction in a Chemical Equation
Consider the equation 
- Which arrow shows reduction?
- Which arrow involves the loss of electrons?
- Which arrow involves the gain of electrons?
Answer
Part 1
Reduction is a process that involves a substance gaining electrons. When a substance gains electrons, its charge decreases or becomes more negative.
To answer this question, we need to determine the charges of the reactants and products. Putting this information in a table can be a helpful visualization. As a reminder, the charge of a neutral atom or molecule is zero.
| Arrow | Species | Reactant Charge | Product Charge | Change in Charge |
|---|---|---|---|---|
| A | Chlorine | 0 | ||
| B | Bromine | 0 |
From the chemical equation and the table above, we can see that for arrow B, the charge on bromine changes from to 0 over the course of the reaction. This change is an increase in charge resulting from the loss of electrons. A loss of electrons means that bromine has been oxidized.
On the other hand, the table shows arrow A corresponds to chlorine changing from a neutral ion to a negative molecule. Its charge changes from 0 to . This change is a decrease in charge from gaining electrons. A gain of electrons means that chlorine has been reduced.
The arrow that shows reduction, where electrons are gained and the charge decreases, is arrow A.
Part 2
If an atom or element loses negatively charged electrons, it is being oxidized and its charge becomes more positive, or it increases.
Arrow A shows the charge of chlorine decreasing from 0 to . This corresponds to a gain of electrons, so arrow A is incorrect.
Arrow B shows the charge on bromine increasing from to 0. The increase in charge results from the loss of electrons, and hence arrow B is the correct answer.
Part 3
When an ion or atom gains negatively charged electrons, its charge will decrease.
Arrow B shows the charge on bromine increasing from to 0. The increase in charge results from the loss of electrons, and hence arrow B is incorrect.
Arrow A shows the charge of chlorine decreasing from 0 to . This corresponds to a gain of electrons and so arrow A is the correct answer.
Sodium chloride is an ionic compound made from sodium and chloride ions. Since sodium atoms lose electrons to go from a neutral atom to a ion, they are being oxidized. Conversely, chlorine atoms gain electrons to go from a neutral atom to a ion, and so they are being reduced.
So far, we have defined oxidation as the loss of electrons. Another way to view this process is that oxidation involves the giving of an electron to another chemical species. In the example above, sodium gives an electron to chlorine to form an ionic bond. Because of this, sodium causes chlorine to be reduced by giving it an extra electron. As such, we can call sodium the “reducing agent” as it causes chlorine to be reduced. The reducing agent itself is then oxidized, as it loses the electrons it donates to the other species.
Conversely, chlorine accepts an electron from sodium, causing sodium to oxidize. We call chlorine the “oxidizing agent.” The oxidizing agent itself is reduced, as it gains the electrons donated by the other species.
Definition: Reducing Agent
A reducing agent is a chemical species that reduces another chemical species while being oxidized itself.
Definition: Oxidizing Agent
An oxidizing agent is a chemical species that oxidizes another chemical species while being reduced itself.
If we look again at the formation of quicklime, we can determine the oxidizing agent and reducing agent:
Calcium oxide is an ionic compound consisting of ions and ions. Calcium starts as a neutral atom and ends as a ion. Calcium’s charge increases and so it is being oxidized.
Oxygen starts as a neutral atom and ends as a ion. Oxygen’s charge decreases, and so it is being reduced.
During the reaction, the calcium atom gives up its electron to the oxygen atom. As the oxygen atom is being reduced, then calcium is the reducing agent. The calcium atom is then itself oxidized.
The oxygen atom receives the electrons from the calcium atom, which allows the calcium to be oxidized. This makes oxygen the oxidizing agent, which is then reduced.
Example 3: Identifying the Characteristics of an Oxidized Species
Which of the following is a substrate that is oxidized?
- Cationic species
- Anionic species
- Neutral species
- Reducing agent
- Oxidizing agent
Answer
This question is asking us to identify which of the chemical species is oxidized. As oxidation refers to the loss of electrons, a substrate that is oxidized will be able to lose electrons during a reaction.
Since the oxidized substrate will give electrons to the other species to reduce it, we can also refer to the oxidized species as the “reducing agent.” A reducing agent is a chemical species that reduces another chemical species while being oxidized itself. This matches the description given in choice D.
The correct answer is therefore choice D, reducing agent.
Example 4: Determining Whether a Substance Is an Oxidizing or Reducing Agent from a Chemical Equation
Consider the following equation:
Fill in the blank: Nickel could be described as .
- a reducing agent because lead ions gain electrons
- an oxidizing agent because the nickel atoms gain electrons
- an oxidizing agent because lead ions gain electrons
- a reducing agent because the nickel atoms gain electrons
Answer
This question is asking us to determine whether nickel is an oxidizing agent or a reducing agent during the given reaction and why.
In a redox reaction, a reducing agent is a chemical species that reduces another chemical species while being oxidized itself. An oxidizing agent is a chemical species that oxidizes another chemical species while being reduced itself. So the key to answering this question is to determine how the charge on nickel changes during the course of the reaction.
As a reactant, nickel is in its elemental solid form with no charge. By the end of the reaction, nickel has formed the ionic compound nickel chloride in aqueous solution. In this solution, there are chloride ions and nickel ions.
We can see that the charge on nickel has increased from 0 to . The increase in charge suggests that nickel has lost electrons during the reaction. Nickel has therefore been oxidized. By being oxidized, nickel is therefore likely to be a reducing agent, and so the correct answer must be choice A or choice D.
To distinguish between these two choices, we must determine whether lead ions or nickel atoms gain electrons during the reaction.
We have already determined that nickel’s charge becomes more positive and has therefore lost electrons. On the other hand, lead begins as a ion in solution and ends as a neutral solid with a charge of 0.
The decrease in charge for lead suggests it has been reduced and therefore gained electrons.
The correct answer is choice A.
Key Points
- A “redox” reaction is a reaction that involves the transfer of electrons from one molecule, element, or ion to another.
- In a redox reaction, one substance is oxidized while another one is reduced.
- Oxidation involves a loss of electrons. Reduction involves a gain of electrons. We can remember these definitions with the acronym “OIL RIG.”
- We can observe the change in the charge of a molecule, atom, or ion from the reactants to the products to determine whether it gained or lost electrons.
- A species that reduces another species is known as a reducing agent. A reducing agent is itself oxidized.
- A species that oxidizes another species is known as an oxidizing agent. An oxidizing agent is itself reduced.
