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General Chemistry I · Aqueous Reactions

Oxidation–Reduction Reactions

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

An oxidation–reduction (redox) reaction is one in which electrons are transferred between substances. Oxidation is the loss of electrons (and an increase in oxidation state); reduction is the gain of electrons (and a decrease in oxidation state) — summarized as OIL RIG (Oxidation Is Loss, Reduction Is Gain). Oxidation and reduction always occur together. The substance that is oxidized (loses electrons) is the reducing agent, and the substance that is reduced (gains electrons) is the oxidizing agent.

Why this matters

Redox chemistry powers batteries, fuel cells, corrosion, combustion, respiration, photosynthesis, and metal refining. When your phone battery discharges, lithium is oxidized and cobalt is reduced; when iron rusts, Fe is oxidized by O₂. Recognizing what is oxidized and what is reduced is the first step to understanding energy storage, electroplating, and biological energy transfer.

The college version

Key Ideas

OIL RIG

  • Oxidation Is Loss (of electrons).
  • Reduction Is Gain (of electrons).
  • Oxidation state goes up on oxidation, down on reduction.

Oxidation-state rules (priority order)

  1. Free element (e.g., Na, O₂, Cl₂) → oxidation state 0.
  2. Monatomic ion → oxidation state equals its charge (Na⁺ = +1, Cl⁻ = −1).
  3. Fluorine is always −1; oxygen is usually −2 (except in peroxides, where it is −1, and in OF₂); hydrogen is +1 with nonmetals and −1 with metals.
  4. Sum of oxidation states in a neutral compound = 0; in a polyatomic ion = the ion's charge.

Agents

  • Reducing agent = the species that is oxidized (it causes reduction by donating electrons).
  • Oxidizing agent = the species that is reduced (it causes oxidation by accepting electrons).

Equations and Variables

  • Oxidation state change signals redox: if any element's oxidation state changes, the reaction is redox.
  • Half-reaction (example): Zn → Zn²⁺ + 2 e⁻ (oxidation); Cu²⁺ + 2 e⁻ → Cu (reduction).
  • Net redox: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).

How It Works (Problem-Solving Method)

  1. Assign oxidation states to every element using the rules.
  2. Compare oxidation states of each element between reactants and products.
  3. If any change, the reaction is redox.
  4. Identify what is oxidized (state increased) and what is reduced (state decreased).
  5. Name the agents: the oxidized species is the reducing agent; the reduced species is the oxidizing agent.

Worked Example

Analyze Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).

  1. Oxidation states: Zn = 0 (free element); Cu²⁺ = +2; Zn²⁺ = +2; Cu = 0.
  2. Zn: 0 → +2 (increased → oxidized). Cu: +2 → 0 (decreased → reduced).
  3. Redox reaction (electrons transferred from Zn to Cu²⁺).
  4. Zn is the reducing agent (it loses electrons and is oxidized).
  5. Cu²⁺ is the oxidizing agent (it gains electrons and is reduced).

Non-redox check: NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq) — no element changes oxidation state, so it is not a redox reaction (it is a precipitation reaction).

Common Confusions

  • "Oxidation requires oxygen." — Historically named that way, but oxidation is any loss of electrons (e.g., Zn → Zn²⁺ with no oxygen involved).
  • "The oxidizing agent is the substance that gets oxidized." — Backwards. The oxidizing agent causes oxidation by being reduced itself.
  • "Hydrogen and oxygen always have fixed oxidation states." — Usually +1 and −2, but H is −1 in metal hydrides (NaH) and O is −1 in peroxides (H₂O₂).
  • "A reaction with ions is automatically redox." — Only if oxidation states change; precipitation and neutralization are usually not redox.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Picture two kids passing electrons like trading cards. The kid who hands over a card "loses" it — that's oxidation (loss). The kid who receives a card "gains" it — that's reduction (gain). The catch: names feel backwards at first. The kid who loses the card is called the "reducing agent" because by giving a card away, he lets the other kid gain one (get reduced). So the reducing agent is the one who gets oxidized. Remember "OIL RIG" and it stops being confusing.

Key takeaways

  • OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
  • Oxidation → oxidation state increases; reduction → it decreases.
  • Free elements have oxidation state 0; monatomic ions equal their charge.
  • Oxygen is usually −2; hydrogen usually +1 (with nonmetals).
  • Sum of oxidation states = 0 (compound) or ion charge (polyatomic ion).
  • Reducing agent is oxidized; oxidizing agent is reduced.
  • OIL RIG: oxidation = loss of e⁻, reduction = gain of e⁻.
  • Assign oxidation states with the priority rules.
  • Oxidation state up = oxidized; down = reduced.
  • Reducing agent is oxidized; oxidizing agent is reduced.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

This lesson has no separate scored set. Practice draws from the subject’s question bank.

Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define oxidation and reduction using the OIL RIG mnemonic.
  • Assign oxidation states using the standard rules.
  • Identify the oxidizing and reducing agents in a redox reaction.
  • Recognize redox reactions from changes in oxidation state.

Sources & references

  1. Chemistry LibreTexts, "4.4: Oxidation-Reduction Reactions."
  2. Chemistry LibreTexts, "5.4: Oxidation-Reduction: Some General Principles."
  3. Chemistry LibreTexts, "7.9: Oxidation–Reduction Reactions."

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

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