Oxidizing and Reducing Agents

Oxidation without oxygen

Lesson 1.7.2 called a reaction with oxygen an oxidation, and hydrogen pulling oxygen away from copper(II) oxide a “reduction” back to copper. By that gain-or-loss-of-oxygen idea, is sodium burning in chlorine to make sodium chloride an oxidation? No oxygen is involved at all. Chemists use oxidation numbers to give oxidation and reduction a broader definition.

From oxygen to oxidation numbers

Look at hydrogen reducing copper(II) oxide:

\overset{+2}{\ce{Cu}}\ce{O} + \overset{0}{\ce{H2}} \xrightarrow{\Delta} \overset{0}{\ce{Cu}} + \overset{+1}{\ce{H2}}\ce{O}

CuO loses oxygen and the oxidation number of Cu falls from +2 to 0. H₂ gains oxygen and the oxidation number of H rises from 0 to +1. So:

When an element’s oxidation number rises, it undergoes oxidation氧化: it is oxidized.
When an element’s oxidation number falls, it undergoes reduction还原: it is reduced.
A reaction in which oxidation numbers change is a redox reaction氧化还原反应. Oxidation and reduction always happen together.

By this definition, 2Na + Cl₂ → 2NaCl is a redox reaction: Na rises from 0 to +1 and Cl falls from 0 to −1, with no oxygen in sight.

What is really happening: electron transfer

Why do oxidation numbers change? When sodium reacts with chlorine, each Na atom loses one electron to become Na⁺ and each Cl atom gains one to become Cl⁻. When hydrogen and chlorine form HCl, no electron is completely transferred, but the shared pair shifts toward Cl, so H’s oxidation number rises and Cl’s falls.

So redox is, at heart, electron transfer, and the change in oxidation number is how you spot it: losing electrons raises the oxidation number (oxidation); gaining electrons lowers it (reduction). A common memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

The species that gains electrons (its oxidation number falls) is the oxidizing agent氧化剂; the species that loses electrons (its oxidation number rises) is the reducing agent还原剂.

	Oxidizing agent	Reducing agent
Electrons	gains	loses
Oxidation number	falls	rises
Itself	is reduced	is oxidized

Notice the twist: the oxidizing agent causes something else to be oxidized, and is itself reduced.

In every redox reaction, electrons lost = electrons gained. One way to show this is to draw an arrow from each element on the reactant side to the same element on the product side and label it with the electrons lost or gained. (Chinese textbooks call this the “double-bridge” diagram, 双线桥.)

Common oxidizing agents include O₂, Cl₂, HNO₃, concentrated H₂SO₄, KMnO₄, and Fe³⁺. Common reducing agents include active metals (Na, Al, Zn, Fe), H₂, C, and CO. From lesson 2.13.1: an element at its highest oxidation number can only be an oxidizing agent (N in HNO₃), one at its lowest can only be a reducing agent (N in NH₃), and one in between can be either.

In some reactions, part of one element’s atoms rise in oxidation number while the rest fall. This is called disproportionation歧化反应. In Cl₂ + H₂O ⇌ HCl + HClO, some Cl goes from 0 down to −1 (in HCl) and some goes up to +1 (in HClO), so Cl₂ is both the oxidizing agent and the reducing agent. The decomposition of H₂O₂ into H₂O and O₂ is another example: some O goes from −1 down to −2, and some goes up to 0.

Here are eight reactions. Pick the oxidizing and reducing agents (choose “none” for both if a reaction isn’t redox), then check to see the electron arrows. Arrows above the equation show oxidation numbers rising (electrons lost); arrows below show them falling (electrons gained):

Try it: find the oxidizing and reducing agents and trace the electrons

Oxidizing agent

Reducing agent

Redox and reaction types

Redox is a different way of sorting reactions from the types in lesson 2.7.1:

Single-replacement reactions are always redox: an element becomes part of a compound and an element in a compound is set free.
Double-replacement reactions are never redox: ions just trade partners.
Combustion reactions are always redox.
Synthesis and decomposition reactions are redox when an element is involved, such as 2H₂ + O₂ → 2H₂O, and usually not otherwise, such as CaO + H₂O → Ca(OH)₂.

Some redox reactions fit none of these types, for example Fe₂O₃ + 3CO → 2Fe + 3CO₂ in a blast furnace.

How Chinese textbooks differ

Chinese textbooks track 化合价 (valence) going up and down rather than oxidation numbers, and use the mnemonic 升失氧，降得还: “up, loses, oxidized; down, gains, reduced”.
They routinely draw electron-transfer arrows: the double bridge (双线桥) described above, or a single bridge (单线桥) running from the reducing agent’s element to the oxidizing agent’s element. US textbooks show the same bookkeeping with half-reactions instead, such as Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu (lesson 2.13.3).
They also name the oxidation product (氧化产物) and the reduction product (还原产物). Oxidizing and reducing agents are also called oxidants and reductants.

Example 1: hydrogen reduces copper(II) oxide

For CuO + H₂ → Cu + H₂O (heated), which element is oxidized and which is reduced? What are the oxidizing and reducing agents? How many electrons are transferred?

Step-by-step solution

Oxidation numbers. Cu: +2 → 0 (down); H: 0 → +1 (up). O stays −2.
The links to remember. Oxidation number up → electrons lost → oxidized → that substance is the reducing agent. Oxidation number down → electrons gained → reduced → that substance is the oxidizing agent. (“OIL RIG”: oxidation is loss, reduction is gain.)
Oxidized and reduced. H loses electrons and is oxidized; Cu gains electrons and is reduced.
Agents. CuO is the oxidizing agent and H₂ is the reducing agent.
Electrons. One Cu goes from +2 to 0, gaining 2 electrons; in one H₂, each H loses 1, for 2 in total. 2 electrons are transferred per CuO.
Check. Electrons lost = electrons gained: 2 = 2.
A common mistake. Mixing up “reduced” and “reducing agent.” CuO is reduced, so it is the oxidizing agent; H₂ is oxidized, so it is the reducing agent.

Example 2: making chlorine in the lab

Chlorine is made by heating MnO₂ with concentrated hydrochloric acid: MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O. (a) What are the oxidizing and reducing agents? (b) What fraction of the HCl is oxidized? (c) How many electrons are transferred per Cl₂ formed?

Step-by-step solution

Oxidation numbers. Mn: +4 → +2 (down). Cl: −1 → 0 for the Cl that ends up in Cl₂ (up); the Cl in MnCl₂ stays −1.
Agents. Mn goes down, so MnO₂ is the oxidizing agent; Cl goes up, so HCl is the reducing agent.
Fraction oxidized. Of the 4 HCl, only 2 have their Cl turned into Cl₂; the other 2 end up in MnCl₂ unchanged, acting as an acid:
$\frac{2}{4} = \boxed{\tfrac{1}{2}}$
Electrons. Forming one Cl₂ takes 1 electron from each of 2 Cl atoms: 2 electrons.
Check. Mn going from +4 to +2 gains exactly 2, so lost equals gained.
A common mistake. Assuming all 4 HCl are oxidized and counting 4 electrons. Check whether each atom’s oxidation number actually changes.

Practice: what role does the species play?

CO in Fe₂O₃ + 3CO → 2Fe + 3CO₂
Fe₂O₃ in the same reaction
H₂O in 2Na + 2H₂O → 2NaOH + H₂
Na in the same reaction
H₂SO₄ in Zn + H₂SO₄ → ZnSO₄ + H₂
Cl₂ in Cl₂ + H₂O ⇌ HCl + HClO
H₂O in the same reaction
H₂O₂ in 2H₂O₂ → 2H₂O + O₂
MnO₂ in MnO₂ + 4HCl → MnCl₂ + Cl₂ + 2H₂O

Common mistakes

Mistake 1: “The oxidizing agent is oxidized.” It’s the other way around. The oxidizing agent gains electrons and is itself reduced; the reducing agent loses electrons and is itself oxidized.

Mistake 2: “Any reaction involving an element is redox.” In 3O₂ → 2O₃ an element reacts and an element forms, but O stays at 0 throughout, so it isn’t redox.

Mistake 3: “The more electrons a species loses, the stronger a reducing agent it is.” Strength depends on how easily electrons are lost, not how many. Na loses only 1 electron and Al loses 3, but Na gives up its electron more easily, so it is the stronger reducing agent.

Quick check

Which statement about 2Na + Cl₂ → 2NaCl is correct?

Which reaction is a redox reaction?

Summary

Oxidation is a rise in oxidation number (loss of electrons); reduction is a fall (gain of electrons). They happen together, and redox is at heart electron transfer: OIL RIG.
The oxidizing agent gains electrons and is reduced; the reducing agent loses electrons and is oxidized. Electrons lost = electrons gained.
Single replacements and combustions are always redox; double replacements never are; synthesis and decomposition depend on whether oxidation numbers change.

On to AP (for later)

AP topic 4.7 sorts reactions into acid–base, precipitation, and redox, and topic 4.9 asks you to identify what is oxidized and reduced from oxidation numbers, write the half-reactions, and state how many electrons are transferred. Unit 9 applies all of this to batteries and electrolysis.

Oxidizing and Reducing Agents氧化剂与还原剂

Learning goals

Before you start

Oxidation without oxygen

From oxygen to oxidation numbers

What is really happening: electron transfer