Oxidation and reduction in terms of electrons

Introduction

Oxidation and reduction can also describe the loss and gain of electrons, remembered using the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
If a magnesium atom loses two electrons to form a Mg²⁺ ion, it has been oxidised; if it gains those electrons back, it has been reduced.

Oxidation and Reduction: Two Meanings

One meaning of oxidation is the gain of oxygen, whilst reduction is the loss of oxygen (e.g. aluminium gaining oxygen to form aluminium oxide is oxidation).
Oxidation and reduction also refer to the loss and gain of electrons respectively.
The mnemonic OIL RIG helps remember this: Oxidation Is Loss, Reduction Is Gain (of electrons).

2. Oxidation and Reduction: Two Meanings AQA

Notes

One meaning of oxidation is the gain of oxygen, whilst reduction is the loss of oxygen (e.g. aluminium gaining oxygen to form aluminium oxide is oxidation).
Oxidation and reduction also refer to the loss and gain of electrons respectively.
The mnemonic OIL RIG helps remember this: Oxidation Is Loss, Reduction Is Gain (of electrons).

Electron Transfer in Redox Reactions

A magnesium atom losing two electrons to form a Mg²⁺ ion is an example of oxidation.
In the reaction of magnesium with dilute acid, Mg atoms lose two electrons (oxidation) whilst H⁺ ions gain those electrons and are reduced to hydrogen gas.
The substance that loses electrons is oxidised, and the substance that gains electrons is reduced.

3. Electron Transfer in Redox Reactions AQA

Notes

A magnesium atom losing two electrons to form a Mg²⁺ ion is an example of oxidation.
In the reaction of magnesium with dilute acid, Mg atoms lose two electrons (oxidation) whilst H⁺ ions gain those electrons and are reduced to hydrogen gas.
The substance that loses electrons is oxidised, and the substance that gains electrons is reduced.

Ionic Equations

Ionic equations show only the particles that actually change during a reaction, making it easier to see what is really happening.
Ions that remain unchanged throughout the reaction are called spectator ions and are removed from the ionic equation.
For example, in the reaction between calcium and iron sulfate solution, the sulphate ions (SO₄²⁻) are spectator ions and are not included in the ionic equation.

4. Ionic Equations AQA

Notes

Ionic equations show only the particles that actually change during a reaction, making it easier to see what is really happening.
Ions that remain unchanged throughout the reaction are called spectator ions and are removed from the ionic equation.
For example, in the reaction between calcium and iron sulfate solution, the sulphate ions (SO₄²⁻) are spectator ions and are not included in the ionic equation.

Writing Half Equations

Half equations show the loss or gain of electrons for each individual element involved in a redox reaction separately.
For the oxidation of calcium, the half equation is written as: Ca → Ca²⁺ + 2e⁻, showing that two electrons are lost.
For the reduction of iron, the half equation is written as: Fe²⁺ + 2e⁻ → Fe, showing that two electrons are gained.
Electrons are placed on the right-hand side of a half equation when they are lost (oxidation) and on the left-hand side when they are gained (reduction).

5. Writing Half Equations AQA

Notes

Half equations show the loss or gain of electrons for each individual element involved in a redox reaction separately.
For the oxidation of calcium, the half equation is written as: Ca → Ca²⁺ + 2e⁻, showing that two electrons are lost.
For the reduction of iron, the half equation is written as: Fe²⁺ + 2e⁻ → Fe, showing that two electrons are gained.
Electrons are placed on the right-hand side of a half equation when they are lost (oxidation) and on the left-hand side when they are gained (reduction).

Balancing Half Equations

To check a half equation is correct, the overall charge on both sides of the equation must be equal.
In the iron half equation, the left-hand side has a 2+ charge from Fe²⁺ and a 2- charge from the two electrons, giving an overall charge of zero, which matches the neutral iron atom on the right.
If the charges on each side do not balance, the electrons have likely been placed on the wrong side of the equation.

6. Balancing Half Equations AQA

Notes

To check a half equation is correct, the overall charge on both sides of the equation must be equal.
In the iron half equation, the left-hand side has a 2+ charge from Fe²⁺ and a 2- charge from the two electrons, giving an overall charge of zero, which matches the neutral iron atom on the right.
If the charges on each side do not balance, the electrons have likely been placed on the wrong side of the equation.

Summary / Key Terms

Oxidation is the loss of electrons (or gain of oxygen), whilst reduction is the gain of electrons (or loss of oxygen).
A redox reaction involves both oxidation and reduction occurring at the same time.
Spectator ions are ions that do not participate in the reaction and are excluded from ionic and half equations.

7. Summary / Key Terms AQA

Notes

Oxidation is the loss of electrons (or gain of oxygen), whilst reduction is the gain of electrons (or loss of oxygen).
A redox reaction involves both oxidation and reduction occurring at the same time.
Spectator ions are ions that do not participate in the reaction and are excluded from ionic and half equations.

AQA student objectives

All · Most · Some

Key knowledge

Introduction

Notes

Oxidation and Reduction: Two Meanings

Notes

Electron Transfer in Redox Reactions

Notes

Ionic Equations

Notes

Writing Half Equations

Notes

Balancing Half Equations

Notes

Summary / Key Terms

Notes