AP Chemistry Redox Reactions

How to decide whether a reaction is redox, what is oxidized and reduced, and how many electrons move.

Most redox questions come down to one move: write the oxidation numbers on both sides and see whether any of them changed. Everything else, including balancing with half-reactions and building electrochemical cells, builds on being fast and accurate at that first step. This page practices it on four reactions that look nothing alike, including two that are not redox, since telling those apart is half the skill. The oxidation numbers guide covers the assignment rules.

How to Identify Oxidation and Reduction in Five Steps

Assign oxidation numbers to every atom in every reactant and product.
Compare each element before and after. If nothing changes, the reaction is not redox. If anything changes, it is.
Label the changes. An oxidation number that increases means the element was oxidized (it lost electrons). One that decreases means it was reduced (it gained electrons). The memory aid OIL RIG says the same: Oxidation Is Loss, Reduction Is Gain.
Count the electrons. Multiply the change in oxidation number by the number of atoms of that element that changed.
Check the balance. The electrons lost by what was oxidized must equal the electrons gained by what was reduced.

Worked Example 1: A Metal Displacing Another Metal (Zn and Cu²⁺)

For Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), identify what is oxidized and what is reduced, and the electrons transferred.

Zinc: 0 → +2. The oxidation number increased, so zinc is oxidized, losing 2 electrons per atom.
Copper: +2 → 0. The oxidation number decreased, so copper is reduced, gaining 2 electrons per atom.
Check: one Zn atom loses 2 e⁻ and one Cu²⁺ gains 2 e⁻, so the transfer is balanced at 2 electrons.

A free element always has oxidation number 0, so any reaction that turns an element into an ion, or an ion into a free element, has to be redox.

Worked Example 2: Combustion as a Redox Reaction (Methane)

For CH₄ + 2 O₂ → CO₂ + 2 H₂O, find what is oxidized, what is reduced, and the electrons transferred.

Carbon: −4 in CH₄ (hydrogen is +1) → +4 in CO₂ (oxygen is −2). Oxidized, losing 8 electrons.
Oxygen: 0 in O₂ → −2 in both products. Reduced, gaining 2 electrons per atom. The equation uses 4 oxygen atoms (2 O₂), so 4 × 2 = 8 electrons gained.
Hydrogen: +1 on both sides, so it is unchanged.
Check: 8 electrons lost = 8 electrons gained. ✓

Every combustion of a hydrocarbon in oxygen is redox for the same reason: carbon ends up bonded to oxygen, a more electronegative element, and oxygen gains electrons. The equation itself is balanced as in balancing equations.

Worked Example 3: Reactions That Are Not Redox (Precipitation and Neutralization)

Decide whether each reaction is redox: (a) AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq); (b) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).

(a) Ag is +1 on both sides, N is +5, O is −2, Na is +1, Cl is −1. Nothing changes, so it is not redox. The ions simply swapped partners and one pair precipitated.
(b) H is +1, Cl is −1, Na is +1, O is −2 throughout. Nothing changes, so it is not redox. A proton moved from HCl to OH⁻, but no atom's oxidation number changed.

These are the precipitation and acid-base reaction types, which the Unit 4 review separates from redox. The test is always the same: if no oxidation number changes, no electrons were transferred.

Worked Example 4: One Element Both Oxidized and Reduced (Hydrogen Peroxide)

For 2 H₂O₂ → 2 H₂O + O₂, identify what is oxidized and what is reduced.

Oxygen in the peroxide H₂O₂ is −1 (the peroxide exception to the usual −2). In H₂O it is −2, and in O₂ it is 0.
Reduced: −1 → −2, gaining 1 electron per atom, for the 2 oxygen atoms that end up in water: 2 electrons gained.
Oxidized: −1 → 0, losing 1 electron per atom, for the 2 oxygen atoms that end up in O₂: 2 electrons lost.
Check: 2 lost = 2 gained. ✓ Hydrogen stays +1 throughout.

The same element is both oxidized and reduced. This kind of reaction, called disproportionation, is unusual, but it shows why you must track each atom by its oxidation number rather than label a whole substance as “the one being oxidized.”

Which Redox Terms the AP Exam Requires

You need to identify what is oxidized and what is reduced from oxidation number changes, to state the number of electrons transferred, and to balance redox equations with the half-reaction method. The course does not assess the meanings of the terms “oxidizing agent” and “reducing agent,” a limit the Unit 4 review also notes, though many classes and textbooks use them, so learn them if your teacher does. Redox then returns in Unit 9, where it powers galvanic cells.

Five Redox Practice Questions With Answers

For Mg + 2 HCl → MgCl₂ + H₂, is it redox? What is oxidized and reduced?
Show answer
Redox. Mg goes 0 → +2 (oxidized); H goes +1 → 0 (reduced). Cl stays −1.
For Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂, what is oxidized and what is reduced?
Show answer
Fe goes +3 → 0 (reduced); C goes +2 → +4 (oxidized). O stays −2.
Is CaCO₃ → CaO + CO₂ a redox reaction?
Show answer
No. Ca stays +2, C stays +4, and O stays −2 throughout, so no oxidation number changes.
For Cl₂ + 2 NaBr → 2 NaCl + Br₂, what is oxidized and reduced?
Show answer
Cl goes 0 → −1 (reduced); Br goes −1 → 0 (oxidized). Na stays +1.
How many electrons are transferred in 2 Al + 3 Cu²⁺ → 2 Al³⁺ + 3 Cu?
Show answer
6. Two Al atoms each lose 3 electrons (2 × 3 = 6), and three Cu²⁺ each gain 2 (3 × 2 = 6).

Common Redox Mistakes

Reversing oxidized and reduced. An increase in oxidation number is oxidation, even though a “plus” number sounds like gaining. OIL RIG fixes it.
Forgetting the peroxide exception. Oxygen in H₂O₂ is −1, which changes the whole analysis.
Counting electrons per atom, not per equation. Multiply the change by the number of atoms that change, using the coefficients.
Calling a reaction redox because it “looks” like it exchanges something. Always compare oxidation numbers; precipitation and neutralization are not redox.
Labeling a whole compound oxidized. It is a particular element within the compound whose oxidation number changed.

Once identification is automatic, balancing redox equations and predicting cell voltage follow the same bookkeeping. If you want to see how these topics add up in a score, the AP Chem Score Calculator converts practice results into an estimate.

Frequently Asked Questions

What is a redox reaction?

A redox (oxidation-reduction) reaction is one in which electrons are transferred between species. It always involves two linked changes: one species is oxidized and another is reduced, and the electrons lost equal the electrons gained.

How do you tell whether a reaction is a redox reaction?

Assign oxidation numbers to every atom on both sides of the equation. If any element's oxidation number changes, the reaction is redox. If every oxidation number stays the same, it is not, as in precipitation and acid-base reactions.

What do oxidation and reduction mean?

Oxidation is the loss of electrons, shown by an increase in oxidation number. Reduction is the gain of electrons, shown by a decrease in oxidation number. The memory aid OIL RIG stands for Oxidation Is Loss, Reduction Is Gain.

Is combustion a redox reaction?

Yes. In the combustion of a hydrocarbon, carbon is oxidized and oxygen is reduced. In methane burning, carbon goes from -4 to +4 and oxygen goes from 0 to -2.

Does AP Chemistry require the terms oxidizing agent and reducing agent?

The course does not assess the meanings of those two terms. You are expected to identify what is oxidized and what is reduced from changes in oxidation number, and to balance redox equations using half-reactions.

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