General Chemistry
Redox Reactions
Electron transfer between species — half oxidation, half reduction
A redox (reduction-oxidation) reaction involves transfer of electrons between species. Oxidation: loss of electrons. Reduction: gain of electrons. They always happen together — one species is oxidized (oxidation state increases), another reduced (decreases). Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain). Examples: combustion (C + O₂ → CO₂; C oxidized, O reduced), corrosion (Fe → Fe³⁺), batteries, photosynthesis, respiration. Oxidation state tracks electron ownership. Half-reactions show each step separately.
- OxidationLoss of electrons (oxidation state increases)
- ReductionGain of electrons (oxidation state decreases)
- MnemonicOIL RIG (Oxidation Is Loss, Reduction Is Gain)
- Oxidation states0 for elements; vary by compound
- Half-reactionsShow oxidation and reduction separately
- ExamplesCombustion, batteries, corrosion, respiration
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Why redox matters
- Energy. Combustion, respiration, photosynthesis.
- Batteries. Electrochemical energy storage.
- Metallurgy. Smelting, refining.
- Biology. Aerobic respiration; photosynthesis.
- Corrosion. Oxidation of metals; major economic impact.
- Chemistry analysis. Titrations, indicators.
- Industrial. Bleaching, cleaning, etching.
Common misconceptions
- Oxidation requires oxygen. Originally yes; now any electron loss.
- Reduction means smaller. Originally yes; now electron gain.
- One can occur alone. Always paired.
- Oxidation state = ionic charge. Hypothetical; doesn't equal real charge in covalent.
- O is always -2. Several exceptions.
- Oxidizing agents oxidize themselves. They reduce themselves while oxidizing others.
Frequently asked questions
What's the difference between oxidation and reduction?
Oxidation: loss of electrons; oxidation state increases. Example: Na → Na⁺ + e⁻ (Na oxidized 0 → +1). Reduction: gain of electrons; state decreases. Example: Cl₂ + 2e⁻ → 2Cl⁻ (Cl reduced 0 → -1). Always occur together — electrons must come from somewhere and go somewhere.
How do you determine oxidation state?
Rules. (1) Pure element: 0 (e.g., O₂, Fe, Na). (2) Monatomic ion: equal to charge (Na⁺ = +1, Cl⁻ = -1). (3) O usually -2 (exceptions: peroxides O = -1, OF₂ O = +2). (4) H usually +1 (exception: metal hydrides H = -1). (5) Sum = total charge of compound. Example: in H₂SO₄, H = +1, O = -2, so S = +6.
What's an oxidizing agent?
Species that causes oxidation by being reduced itself. Common: O₂, F₂, Cl₂, H₂O₂, KMnO₄, K₂Cr₂O₇. Strong oxidizers have high tendency to gain electrons. Strong: F₂ (most), then O₃ > H₂O₂ > Cl₂ > O₂ > Br₂ > I₂. Reverse for reducing agents (cause reduction by being oxidized; want to lose electrons): Li, Na, Mg, Zn, H₂.
How are redox reactions balanced?
Half-reaction method. (1) Split into oxidation and reduction half-reactions. (2) Balance atoms (except H, O). (3) Balance O with H₂O. (4) Balance H with H⁺ (acidic) or OH⁻ (basic). (5) Balance charge with e⁻. (6) Multiply to equal e⁻ in both halves. (7) Add halves. Example: oxidation Fe → Fe²⁺ + 2e⁻; reduction Cl₂ + 2e⁻ → 2Cl⁻. Total: Fe + Cl₂ → Fe²⁺ + 2Cl⁻ → FeCl₂.
What's combustion?
Rapid redox with oxygen. Fuel oxidized; O₂ reduced. Releases energy as heat. CH₄ + 2O₂ → CO₂ + 2H₂O. C: -4 → +4 (oxidized); O: 0 → -2 (reduced). Energy from forming stronger O=O → C=O, O-H bonds. Powers: heating, transportation, electricity (combustion engines, power plants).
What's the difference between metabolism and combustion?
Both are redox of organic molecules with O₂, producing CO₂ and water. Difference: combustion is rapid, uncontrolled (fast O₂ + fuel → flames). Metabolism is slow, controlled (enzymes, multiple steps, energy captured as ATP). Same overall stoichiometry: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Energy released similarly.
How are batteries redox?
Spontaneous redox that produces electricity. Anode: oxidation occurs. Cathode: reduction. Electrons flow through external circuit (current). Example: Zn-Cu cell (Daniell cell): Zn oxidized → Zn²⁺; Cu²⁺ reduced → Cu. Voltage: ~1.1 V. Modern batteries use various redox couples (Li-ion, lead-acid, alkaline).