Redox Reactions · Lesson 1 of 8

Oxidation and Reduction Always Occur Together

“Learn to recognise paired chemical changes through oxygen, hydrogen, and the elements exchanged between substances.”

Learning Objectives

• Recognise oxidation and reduction using the classical definitions. • Explain why the definitions extend beyond reactions involving oxygen. • Identify the substances oxidised and reduced in a reaction. • Connect familiar redox processes with changes in the composition of matter.

Two changes inside one reaction

When magnesium burns, its brilliant light makes the change easy to notice. The chemical explanation is that magnesium combines with oxygen to form magnesium oxide. Burning a fuel, extracting a metal, and operating a battery may look very different, yet they belong to a shared family of changes called oxidation–reduction, or redox, reactions. The central idea is a partnership: whenever one substance is oxidised, another is reduced.

Redox processes appear in the use of fuels, biological energy transformations, manufacture of chemicals, agriculture, corrosion, and the extraction of reactive elements. Hydrogen used as a fuel and changes involving atmospheric ozone also involve redox chemistry. We will first recognise these reactions by following the elements that substances gain or lose. Later, electron transfer will explain why these apparently different descriptions fit together.

Oxidation began as the addition of oxygen

Many elements combine with the oxygen in air. The resulting compounds are oxides, which helps explain why many elements occur naturally in combined forms rather than as free elements. The original meaning of oxidation was simply the addition of oxygen to a substance.

Definition

Oxidation: classical description

Oxidation is the addition of oxygen or another more electronegative element to a substance, or the removal of hydrogen or another more electropositive element from it.

An electronegative element attracts shared electrons strongly in a bond; an electropositive element tends more readily to lose electrons. For now these terms help us compare the elements being added or removed. Oxygen, fluorine, and chlorine commonly provide the electronegative partner in the examples below.

Magnesium gains oxygenLaTeX

2 Mg (s) + O_{2} (g) \to 2 MgO (s)

The symbol (s) means solid and (g) means gas. Magnesium is oxidised as it becomes part of magnesium oxide.

Sulphur behaves similarly in $S + O_{2} \to S O_{2}$ . Combustion of methane is a more involved example: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ . Carbon starts bonded to hydrogen and ends bonded to oxygen. Following only the changing substance is enough to recognise oxidation, but identifying its partner will give a complete redox description.

The definition expands beyond oxygen

A useful definition should connect reactions with the same underlying behaviour. Magnesium also combines with fluorine, chlorine, and sulphur. Calling only its reaction with oxygen oxidation would hide the similarity between these changes. We therefore extend oxidation to include addition of a more electronegative element.

Reaction	Classical reason for oxidation
$Mg + F_{2} \to Mg F_{2}$	Fluorine is added to magnesium.
$Mg + C l_{2} \to MgC l_{2}$	Chlorine is added to magnesium.
$Mg + S \to MgS$	Sulphur is added to magnesium.
$2 H_{2} S + O_{2} \to 2S + 2 H_{2} O$	Hydrogen is removed from the sulphur-containing substance.

The last reaction shows another way to recognise oxidation. In $H_{2} S$ , sulphur is combined with hydrogen. When sulphur is produced, that hydrogen has been removed. The removal of hydrogen is therefore included in the classical definition of oxidation. Removal of another electropositive element can play the same role.

Example — Removing an electropositive element

Problem
Explain why the conversion of potassium ferrocyanide to potassium ferricyanide is described as oxidation in $2 K_{4} [Fe (CN)_{6}] + H_{2} O_{2} \to 2 K_{3} [Fe (CN)_{6}] + 2KOH$ .

1.Compare the two complex salts. The formula changes from $K_{4} [Fe (CN)_{6}]$ to $K_{3} [Fe (CN)_{6}]$ .
2.In the classical bookkeeping description, electropositive potassium has been removed from the original salt. This brings the change within the expanded definition of oxidation.
3.This comparison describes the change in composition. It does not claim that a potassium atom physically leaves an intact molecule as a separate elementary reaction step.

Reduction reverses these kinds of change

Heating mercury(II) oxide can release mercury and oxygen. The mercury-containing substance loses oxygen, so this is the reverse of the original oxidation pattern. The term reduction extends in a matching way to the loss of electronegative elements and the gain of hydrogen or other electropositive elements.

Definition

Reduction: classical description

Reduction is the removal of oxygen or another more electronegative element from a substance, or the addition of hydrogen or another more electropositive element to it.

Mercury oxide loses oxygenLaTeX

2 HgO (s) Δ 2 Hg (l) + O_{2} (g)

The triangle over the arrow indicates heating; (l) means liquid. Mercury oxide is reduced.

Change	Why it represents reduction
$2FeC l_{3} + H_{2} \to 2FeC l_{2} + 2HCl$	The iron chloride loses part of its chlorine content.
$C H_{2} = C H_{2} + H_{2} \to C H_{3} - C H_{3}$	Ethene gains hydrogen to form ethane.
$2HgC l_{2} + SnC l_{2} \to H g_{2} C l_{2} + SnC l_{4}$	Mercury(II) chloride becomes a form containing more electropositive mercury relative to chlorine.

In the last reaction, the same change can be described as chlorine being transferred from the mercury compound to the tin compound. Mercury is reduced, while tin(II) chloride is oxidised to tin(IV) chloride. This immediately suggests why oxidation and reduction cannot be treated as unrelated changes.

Find both partners

The word redox joins reduction and oxidation. If oxygen is transferred from one substance to another, the substance gaining it is oxidised and the substance losing it is reduced. If hydrogen or another element is transferred, the same paired reasoning applies. Always ask two questions: what is oxidised, and what is reduced?

Example — Hydrogen sulphide and chlorine

Problem
Identify the substance oxidised and the substance reduced in $H_{2} S + C l_{2} \to 2HCl + S$ .

1.Sulphur in $H_{2} S$ loses its hydrogen partner as elemental sulphur forms. Hydrogen sulphide is oxidised.
2.Chlorine gains hydrogen as hydrogen chloride forms. Chlorine is reduced.
3.Both descriptions refer to the same overall reaction; one change supplies the counterpart required by the other.

Example — Extracting iron with aluminium

Problem
Identify the paired changes in $3F e_{3} O_{4} + 8Al \to 9Fe + 4A l_{2} O_{3}$ .

1.Aluminium gains oxygen and becomes aluminium oxide, so aluminium is oxidised.
2.The iron oxide loses oxygen and produces iron metal, so the iron oxide is reduced.
3.The oxygen removed from the iron oxide appears in aluminium oxide. It has not disappeared.

Example — A case that needs electronegativity

Problem
Why is $2Na + H_{2} \to 2NaH$ a redox reaction even though oxygen is absent?

1.Hydrogen is more electronegative than sodium. Combining sodium with hydrogen is addition of a more electronegative element to sodium, so sodium is oxidised.
2.Hydrogen gains an electropositive partner, sodium, so hydrogen is reduced.
3.The familiar shortcut “hydrogen is added, therefore reduction” must be applied to the substance receiving hydrogen and interpreted with electronegativity. The next lesson will make this case clearer using electrons.

Common mistake

Do not identify oxidation by the presence of oxygen alone. A reaction without oxygen may be redox, and a reaction involving an oxygen-containing compound may be non-redox. Follow the chemical change and identify both partners.

Quick check

Which change is oxidation according to the classical description?

Quick check

In $CuO + H_{2} \to Cu + H_{2} O$ , what happens to copper oxide?

Quick check

Why does the oxidation of one substance require a reduction partner?

Quick check

In $2FeC l_{3} + H_{2} \to 2FeC l_{2} + 2HCl$ , which substance is reduced?

Quick check

What is the best classical explanation for sodium oxidation in $2Na + H_{2} \to 2NaH$ ?

Practice Problems

For $2Mg + O_{2} \to 2MgO$ , identify both redox partners using the classical descriptions.
Explain how methane combustion fits both oxygen-addition and hydrogen-removal descriptions of oxidation.
For $2HgC l_{2} + SnC l_{2} \to H g_{2} C l_{2} + SnC l_{4}$ , explain why the changes of mercury and tin are paired.
Write one oxygen-free redox reaction from this lesson and justify its classification.
A student says that reduction always means removing oxygen. Use two different examples to improve this definition.
Compare the roles of oxygen in extracting iron from its oxide and in burning magnesium.

Key Takeaways

Oxidation includes gain of oxygen or an electronegative element, and loss of hydrogen or an electropositive element. Reduction includes the reverse kinds of change. Electronegativity makes the expanded definitions meaningful. Oxidation and reduction are paired parts of one redox process. Combustion, extraction of metals, batteries, and corrosion connect this idea to familiar changes.

Start of chapter

Next · Lesson 2

Electron Transfer and Competition for Electrons

Two changes inside one reaction

Oxidation began as the addition of oxygen

The definition expands beyond oxygen

Reduction reverses these kinds of change

Find both partners

Quiz

Practice Problems

Key Takeaways