4.9 Oxidation-Reduction (Redox) Reactions

A redox reaction is any chemical reaction where electrons are transferred—one species loses electrons (oxidation) and another gains them (reduction). On the AP, you’ll show these with oxidation and reduction half-reactions (CED LO 4.9.A). The quickest way to tell a redox reaction is happening: assign oxidation states to each element before and after the reaction. If any element’s oxidation number changes, electrons moved and it’s redox. The species that’s oxidized is the reducing agent; the species reduced is the oxidizing agent. For full AP-style work, write separate half-reactions, balance mass and charge (add H+, H2O in acidic or OH– and H2O in basic), and balance electrons between half-reactions before combining. On electrochemistry problems, connect reduction potentials to identify anode (oxidation) and cathode (reduction). For more step-by-step examples and practice, see the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and lots of practice problems (https://library.fiveable.me/practice/ap-chemistry).

Quick method you can use every time: 1. Assign oxidation numbers to each element in the reactants and products (use rules: free elements 0, O usually –2, H usually +1, ions equal charge, etc.). 2. Compare numbers: the element whose oxidation number increases is oxidized (lost electrons); the one whose oxidation number decreases is reduced (gained electrons). 3. Identify agents: the species that’s reduced is the oxidizing agent; the species that’s oxidized is the reducing agent. 4. To write half-reactions (CED 4.9.A): separate the oxidation and reduction half-reactions, balance atoms (H2O, H+, OH–) and charge by adding electrons, then multiply to cancel electrons and combine. For acidic vs basic solutions, balance with H+ or add OH– as needed. This is exactly what AP asks you to do on redox problems—practice doing oxidation-state changes and writing balanced half-reactions (see the Topic 4.9 study guide for step-by-step examples: https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa). For extra practice, try the many AP-style problems at https://library.fiveable.me/practice/ap-chemistry.

Oxidation = loss of electrons; reduction = gain of electrons. Quick ways to keep them straight: think “OIL RIG” (Oxidation Is Loss; Reduction Is Gain) or “LEO the lion says GER” (Loss Electrons = Oxidation, Gain Electrons = Reduction). In redox, the species that’s oxidized increases its oxidation state and is the reducing agent (it gives e−). The species that’s reduced decreases its oxidation state and is the oxidizing agent (it accepts e−). On the AP, you must be able to split a reaction into oxidation and reduction half-reactions, balance mass and charge (add e−, H+, H2O in acidic or add OH− in basic), and combine them so electrons cancel (CED LO 4.9.A). In electrochemical cells, reduction happens at the cathode and oxidation at the anode. Want step-by-step practice and examples? Check the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and use Fiveable’s AP practice problems (https://library.fiveable.me/practice/ap-chemistry).

Think of a half-reaction as one side of a tug-of-war for electrons—it shows only the oxidation part (loss of e–) or the reduction part (gain of e–). To use them on the AP exam (LO 4.9.A), you: 1) split the overall redox into two half-reactions (one oxidation, one reduction); 2) balance atoms (add H2O, H+ in acidic solution or add OH– in basic); 3) balance charge by adding electrons; and 4) multiply so the electrons lost = electrons gained, then add the half-reactions and cancel common species. Remember: the species that’s reduced is the oxidizing agent, the one oxidized is the reducing agent; reduction happens at the cathode and oxidation at the anode. Practice balancing electrons and mass/charge—that’s what AP free-response often checks. For a step-by-step guide and examples, see the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa). More unit review and 1000+ practice problems are at (https://library.fiveable.me/ap-chemistry/unit-4) and (https://library.fiveable.me/practice/ap-chemistry).

Step-by-step half-reaction method (good for AP 4.9.A): 1. Identify oxidation and reduction changes by assigning oxidation states. 2. Split the overall reaction into two half-reactions (one oxidation, one reduction). 3. For each half-reaction, balance all atoms except H and O. 4. Balance O by adding H2O; balance H by adding H+ (acidic). For basic solutions, add OH- to both sides to neutralize H+. 5. Balance charge by adding electrons (e-) to the more positive side so charge is equal. 6. Multiply each half-reaction by an integer so the total electrons gained = electrons lost. 7. Add the half-reactions, cancel identical species (including e-), then simplify (combine waters, H+, OH-). 8. Check mass and charge balance and that oxidation numbers changed appropriately. On the AP exam, show your half-reactions, electron balancing, and final mass/charge check (CED 4.9.A.1). For extra practice, review the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and do problems at (https://library.fiveable.me/practice/ap-chemistry).

You separate redox reactions into half-reactions because redox is about electron transfer, not just atoms. Writing an oxidation half-reaction (loss of e-) and a reduction half-reaction (gain of e-) lets you balance mass and charge separately, then combine them so electrons cancel. That ensures charge conservation and shows exactly how many electrons move—which species is the reducing agent and which is the oxidizing agent. Half-reaction method also handles acidic vs. basic media cleanly (add H+, OH−, and H2O as needed) and works when different atoms change oxidation states by different numbers of electrons. The AP CED explicitly expects you to “represent a balanced redox reaction equation using half-reactions” (4.9.A), so practicing this method is required for exam problems. Want guided walkthroughs and practice? Check the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa), the unit overview (https://library.fiveable.me/ap-chemistry/unit-4), and Fiveable’s AP Chem practice problems (https://library.fiveable.me/practice/ap-chemistry).

Saying “electrons are transferred” means one species loses electrons (oxidation) and another gains them (reduction). In practice that means you can split a redox reaction into two half-reactions: an oxidation half (shows electrons as products) and a reduction half (shows electrons as reactants). The substance that loses electrons is the reducing agent (it’s oxidized); the one that gains electrons is the oxidizing agent (it’s reduced). You track these changes by assigning oxidation states and making sure electrons lost = electrons gained when you balance the half-reactions (mass and charge balanced; add H+, OH−, H2O as needed in acid or base). On the AP exam you’re expected to represent balanced redox equations using half-reactions (CED 4.9.A). If you want step-by-step practice and worked examples, check the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and other Unit 4 resources (https://library.fiveable.me/ap-chemistry/unit-4)—Fiveable also has 1000+ practice questions (https://library.fiveable.me/practice/ap-chemistry) to help you master balancing electrons.

Think of oxidation numbers as bookkeeping for electrons so you can spot electron transfer in redox. Quick rules: - Free elements = 0 (O2, Fe(s) = 0). - Monatomic ion = its charge (Na+ = +1, S2- = -2). - H is usually +1 (except with metals/hydrides), O is usually -2 (except in peroxides where it’s -1). - Sum of oxidation numbers in a neutral compound = 0; in a polyatomic ion = ion charge. What they tell you: - If an element’s oxidation number increases → it’s oxidized (loses electrons) and is the reducing agent. - If it decreases → it’s reduced (gains electrons) and is the oxidizing agent. On the AP exam you’ll use these to write oxidation and reduction half-reactions, balance mass and charge (add H2O, H+, and e−; in basic solution add OH−), and make sure electrons cancel before combining (CED keywords: half-reaction, electron transfer, balance electrons, acidic/basic balancing, oxidizing/reducing agent, cathode/anode). For a short refresher and practice, see the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and the AP Chem practice problems (https://library.fiveable.me/practice/ap-chemistry).

Look at electron flow. Oxidation is the half-reaction where electrons are produced (lost by a species); reduction is the half-reaction where electrons are consumed (gained by a species). Quick steps you can use on AP problems: - Assign oxidation states to each element. The element whose oxidation number increases is oxidized; the one whose oxidation number decreases is reduced. - Write two half-reactions: one showing electrons on the product side (oxidation) and one with electrons on the reactant side (reduction). - Balance mass and charge by adding H2O, H+ (acidic) or OH– (basic), and electrons so the electrons lost = electrons gained (CED 4.9.A). - For electrochemical cells, the half-reaction with the more positive standard reduction potential is the one that will be reduced (cathode); the other is oxidized (anode). Review worked examples in the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and practice many problems (https://library.fiveable.me/practice/ap-chemistry)—AP free-response often asks you to represent balanced redox by half-reactions, so get fluent at identifying electrons and balancing charge.

When you balance redox reactions in acidic solution you use half-reactions and two simple moves: add H2O to fix oxygen atoms and add H+ to fix hydrogen atoms. Why? Each half-reaction must be balanced for mass (atoms) and charge. If O is missing, add H2O so oxygen atoms match; that introduces extra H, so add H+ to balance the hydrogens. Then add electrons to balance charge. (In basic solution you’d neutralize H+ with OH− instead.) This is exactly what the CED expects for Topic 4.9: construct balanced redox equations from half-reactions (4.9.A). Practice doing the steps: split into oxidation/reduction, balance atoms (O with H2O, H with H+), balance charge with e−, then multiply and combine so electrons cancel. For step-by-step examples and practice, check the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and the AP Chem practice sets (https://library.fiveable.me/practice/ap-chemistry).

You balance redox reactions the same way (split into oxidation and reduction half-reactions, balance atoms and electrons), but the solvent gives you different bookkeeping tools. In acidic solution you may add H2O and H+ to balance O and H; in basic solution you can’t introduce free H+—instead you add OH− and H2O. Practically you balance as if acidic, then neutralize H+ by adding OH− to both sides (H+ + OH− → H2O). That changes where H2O and OH− appear and can change coefficients, so the final balanced equation looks different even though electron transfer is the same. For AP Chem, show balanced half-reactions with mass and charge balanced and note whether you used acidic or basic conditions (CED 4.9.A). For a step-by-step example, check the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa). More practice problems are at (https://library.fiveable.me/practice/ap-chemistry).

Think of LEO = Loss of Electrons → Oxidation, and GER = Gain of Electrons → Reduction. Memorize it as: when a species loses electrons its oxidation number increases (it’s oxidized) and it’s the reducing agent; when a species gains electrons its oxidation number decreases (it’s reduced) and it’s the oxidizing agent. On AP Chem problems (half-reactions, balancing electrons, acid/basic solutions) always label which side loses/gains electrons, then balance mass and charge—that tells you anode (oxidation) vs cathode (reduction) and which agent does the oxidizing/reducing (CED keywords: oxidation half-reaction, reduction half-reaction, electron transfer, oxidizing/reducing agent). For quick practice and review, use the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and Unit 4 overview (https://library.fiveable.me/ap-chemistry/unit-4). For lots of problems, see Fiveable practice (https://library.fiveable.me/practice/ap-chemistry).

In a redox reaction electrons are transferred from one chemical species to another—they don’t vanish. The species that loses electrons is oxidized (the reducing agent) and the species that gains electrons is reduced (the oxidizing agent). We represent this with half-reactions: an oxidation half shows electrons as products, a reduction half shows electrons as reactants; you balance the overall equation by making the electrons cancel (CED keywords: oxidation half-reaction, reduction half-reaction, balance electrons). In solution those electrons are taken up by the oxidizing species (for example Ag+ + e– → Ag). In an electrochemical cell the electrons physically travel through an external wire from the anode (where oxidation occurs) to the cathode (where reduction occurs); ions in the salt bridge complete the charge balance. For AP tasks you’ll often write half-reactions, balance mass and charge, and identify oxidizing/reducing agents (Topic 4.9; useful practice: the Unit 4 redox study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and 1000+ practice problems (https://library.fiveable.me/practice/ap-chemistry)).

See a simple one in action: a copper–zinc “galvanic” reaction (like in a classic voltaic cell). Overall: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s). Half-reactions: - Oxidation (anode): Zn(s) → Zn2+(aq) + 2 e− - Reduction (cathode): Cu2+(aq) + 2 e− → Cu(s) Zn is the reducing agent (it’s oxidized, loses electrons); Cu2+ is the oxidizing agent (it’s reduced, gains electrons). You balance electrons between half-reactions (here both transfer 2 e−), then combine to give a mass-and-charge balanced equation—exactly the skill AP wants in 4.9.A. Where you might see it: a simple electroplating demo, a homemade lemon battery, or the chemistry inside AA batteries (different metals but same redox idea). For practice writing and balancing half-reactions and more real examples, check the Topic 4.9 study guide (https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa) and the AP Chem practice problems (https://library.fiveable.me/practice/ap-chemistry).

Do the usual half-reaction method: make each half-reaction mass-and-charge balanced, then multiply whole half-reactions so the electrons lost = electrons gained, and add them so electrons cancel. Steps (short): 1. Write oxidation and reduction half-reactions and balance atoms except H and O. 2. Balance O with H2O and H with H+ (if basic, add OH– to both sides after). 3. Balance charge by adding electrons. 4. Multiply one or both half-reactions by integers so the number of electrons is the same. 5. Add the half-reactions and cancel identical species (including electrons). Quick example: Fe2+ → Fe3+ + e– and Cu2+ + 2 e– → Cu. Multiply the Fe half by 2 so 2 e– cancel: 2 Fe2+ → 2 Fe3+ + 2 e–; add to Cu half → balanced overall. On the AP exam you’ll be asked to “represent a balanced redox reaction using half-reactions” (CED LO 4.9.A; see Topic 4.9 study guide for examples: https://library.fiveable.me/ap-chemistry/unit-4/oxidation-reduction-redox-reactions/study-guide/43xfitnkAe6lhVeXtDXa). For extra practice, Fiveable has many practice problems (https://library.fiveable.me/practice/ap-chemistry).