DoAssignment study guide

F3.1 · Explain redox with electron transfer and oxidation numbers

Learn to explain redox with electron transfer and oxidation numbers through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Electrochemistry

How to track electrons and recognise oxidation and reduction

A piece of iron left in moist air can develop a reddish-brown coating. That visible change is connected to changes among particles: iron atoms lose electrons, while particles containing oxygen gain electrons. The electron transfer is not visible, but a chemical equation and oxidation numbers help us track it. In this lesson, you will use these tools to explain redox reactions.

What you will learn

Explain oxidation and reduction as electron transfer.
Assign oxidation numbers using common rules.
Use oxidation-number changes to identify what is oxidized and reduced.
Identify the oxidizing agent and reducing agent in a redox reaction.

1. From particles to redox

A chemical reaction rearranges atoms and particles. In some reactions, electrons move from one reactant particle to another. A reaction that involves electron transfer is an oxidation-reduction reaction, usually called a redox reaction.

Oxidation and reduction always occur together in a redox reaction. Oxidation is the loss of electrons. Reduction is the gain of electrons. A short memory aid is “OIL RIG”: oxidation is loss, reduction is gain. The words describe electron changes, not whether a substance has oxygen in its name.

For example, a magnesium atom can transfer two electrons to a copper(II) ion. Magnesium becomes a positively charged ion, and the copper(II) ion becomes a neutral copper atom. The charges show that electrons have moved, even though electrons are not written as reactants in the overall equation.

An ion is an atom or group of atoms with a net electric charge. A positive ion has lost electrons; a negative ion has gained electrons. This connection between charge and electron number is a useful prerequisite for understanding redox.

oxidation: loss of electrons\qquadreduction: gain of electrons

Oxidation means loss of electrons.
Reduction means gain of electrons.
Oxidation and reduction happen together in a redox reaction.

2. Using oxidation numbers

An oxidation number is a bookkeeping value assigned to an atom in a substance. It helps show whether electrons have been lost or gained during a reaction. It is not always the same as the atom’s actual charge. For a monatomic ion, however, its oxidation number equals its ion charge.

Use these common rules to assign oxidation numbers at this level. An element by itself has an oxidation number of zero, as in solid copper or oxygen gas. A monatomic ion has an oxidation number equal to its charge. In a neutral compound, the oxidation numbers add to zero. In a polyatomic ion, they add to the ion’s overall charge.

For many compounds, hydrogen is assigned an oxidation number of +1 and oxygen is assigned −2. These are common rules for the examples in this lesson. When a problem gives a different value or a special case, follow that information rather than forcing the common value.

Compare oxidation numbers before and after a reaction. An increase in an atom’s oxidation number means oxidation: the atom has lost electrons, or has been assigned a greater share of positive charge. A decrease means reduction: the atom has gained electrons, or has been assigned a greater share of negative charge.

The oxidation-number change lets you identify redox even when the reaction does not show free electrons moving between simple ions. In this lesson, use the change as a tracking tool: identify the element whose value changes, then connect the increase or decrease to oxidation or reduction.

\sum \text{oxidation numbers}=\text{overall charge}

A free element has oxidation number zero.
A monatomic ion’s oxidation number equals its charge.
Oxidation numbers sum to zero in a neutral compound and to the net charge in an ion.
An increase indicates oxidation; a decrease indicates reduction.

3. Identifying the agents

The reducing agent is the reactant that causes another reactant to be reduced by donating electrons. It is itself oxidized. The oxidizing agent is the reactant that causes another reactant to be oxidized by accepting electrons. It is itself reduced.

These names can feel reversed at first. Name each agent by what it does to the other reactant, not by the change it undergoes. The reducing agent gives electrons and becomes oxidized. The oxidizing agent takes electrons and becomes reduced.

To explain a redox reaction clearly, state the electron transfer, identify which substance is oxidized and which is reduced, and name the reducing and oxidizing agents. Oxidation numbers provide a second way to confirm the electron-transfer account.

When a reaction equation includes states, the state symbols describe the physical form of each substance: for example, solid, aqueous (dissolved in water), or gas. They do not change the oxidation-number rules.

The reducing agent donates electrons and is oxidized.
The oxidizing agent accepts electrons and is reduced.
Use both electron transfer and oxidation-number changes to support a redox explanation.

Worked example

Magnesium transfers electrons to copper(II) ions

For the reaction shown, identify the electrons transferred, the species oxidized and reduced, their oxidation-number changes, and the oxidizing and reducing agents.

Write the balanced particle equation
The equation represents one magnesium atom reacting with one copper(II) ion. The total charge is the same on both sides, so both atoms and net charge are conserved.
$\mathrm{Mg(s)+Cu^{2+}(aq)\rightarrow Mg^{2+}(aq)+Cu(s)}$
Track the electrons
A neutral magnesium atom becomes a magnesium ion with a charge of +2, so it loses two electrons. A copper(II) ion with charge +2 becomes neutral copper, so it gains those two electrons. The electron changes balance: two are lost and two are gained.
$\mathrm{Mg(s)\rightarrow Mg^{2+}(aq)+2e^-}\qquad\mathrm{Cu^{2+}(aq)+2e^-\rightarrow Cu(s)}$
Compare oxidation numbers
Magnesium is an element by itself on the left, so its oxidation number is zero. In the product ion, it is +2. Its oxidation number increases, so magnesium is oxidized. Copper starts as a monatomic ion with oxidation number +2 and ends as an element with oxidation number zero. Its value decreases, so copper is reduced.
$\mathrm{Mg}:0\rightarrow +2\qquad\mathrm{Cu}:+2\rightarrow 0$
Name the agents
Magnesium donates electrons and causes copper(II) ions to be reduced. Therefore magnesium is the reducing agent. Copper(II) ions accept electrons and cause magnesium to be oxidized, so copper(II) ions are the oxidizing agent.

Answer: Magnesium is oxidized and is the reducing agent. Copper(II) ions are reduced and are the oxidizing agent. Two electrons move from each magnesium atom to a copper(II) ion.

Check: The left side has net charge +2, and the right side has net charge +2. Each side contains one magnesium atom and one copper atom.

Common mistakes and how to avoid them

Saying oxidation means gaining oxygen in every case.

Correction: For redox, define oxidation as loss of electrons. Use oxidation-number increase to track it.

Calling a substance the reducing agent because it is reduced.

Correction: The reducing agent donates electrons and is itself oxidized. The oxidizing agent accepts electrons and is itself reduced.

Treating an oxidation number as the actual charge on every atom.

Correction: Oxidation number is a bookkeeping value. For a monatomic ion, it does equal the ion charge.

Checking only atoms when balancing a reaction with ions.

Correction: Check both the number of each kind of atom and the total charge on each side.

Lesson summary

Redox reactions involve electron transfer.
Oxidation is electron loss and an increase in oxidation number.
Reduction is electron gain and a decrease in oxidation number.
The reducing agent donates electrons and is oxidized; the oxidizing agent accepts electrons and is reduced.

Check your understanding

Question 1

A particle changes from oxidation number −1 to +3. What happened to it?

It was oxidized because its oxidation number increased.
It was reduced because its oxidation number increased.
It was oxidized because its oxidation number decreased.
It was neither oxidized nor reduced.

Show answer and explanation

It was oxidized because its oxidation number increased.

An increase in oxidation number indicates oxidation, which corresponds to electron loss.

Question 2

In a reaction, a reactant accepts electrons. What is its role?

It is reduced and acts as the oxidizing agent.
It is oxidized and acts as the oxidizing agent.
It is reduced and acts as the reducing agent.
It is oxidized and acts as the reducing agent.

Show answer and explanation

It is reduced and acts as the oxidizing agent.

Accepting electrons is reduction. The electron acceptor causes the other reactant to be oxidized, so it is the oxidizing agent.

Question 3

What is the oxidation number of sulfur in neutral sulfur dioxide, using oxygen as −2?

+4
−4
+2
0

Show answer and explanation

The two oxygen atoms total −4. The compound is neutral, so sulfur must be +4 to make the sum zero.

Key terms

Redox reaction: A reaction involving oxidation and reduction through electron transfer.
Oxidation: Loss of electrons; an atom’s oxidation number increases.
Reduction: Gain of electrons; an atom’s oxidation number decreases.
Oxidation number: A bookkeeping value used to track changes related to electron transfer.
Reducing agent: The reactant that donates electrons and is itself oxidized.
Oxidizing agent: The reactant that accepts electrons and is itself reduced.

Continue through SCH4U

View the complete SCH4U Ontario Grade 12 Chemistry curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation F3.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

Official curriculum reference

Report a correction or ask a question