Redox and Electrochemistry Study Guide
Redox is electron bookkeeping: assign oxidation numbers, spot who lost and who gained, and balance the transfer. That one skill unlocks batteries, corrosion, electroplating, and electrolysis. The classic exam traps are the anode/cathode sign flip between galvanic and electrolytic cells, forgetting that E° values don't scale with coefficients, and assuming cell potential equals E° at nonstandard concentrations. This guide builds the method step by step and flags every trap.
Key formulas
Oxidation = loss of electrons; reduction = gain of electrons (OIL RIG)
Oxidation number rules: free element = 0; monatomic ion = its charge; O = −2 (peroxide −1); H = +1 (hydride −1); F = −1; sum equals total charge
E°cell = E°cathode − E°anode, both as standard reduction potentials; E°cell > 0 means spontaneous
E = E° − (RT/nF) ln Q (Nernst equation); at 25 °C, E = E° − (0.0592/n) log Q
Moles of electrons = (I x t) / F, where I is current in amperes, t is time in seconds, F = 96485 C/mol
Anode = oxidation, cathode = reduction (always); galvanic: anode (−), cathode (+); electrolytic: anode (+), cathode (−)
Topic checklist
Oxidation versus reduction
Oxidation = loss of electrons; reduction = gain of electrons (mnemonic: OIL RIG).
Oxidation and reduction always occur together in a redox reaction — one species' loss is another's gain.
Track electron transfer by assigning oxidation numbers before and after.
Oxidation number rules
Free elements: 0. Monatomic ions: equal to the charge. Oxygen: usually −2 (except peroxides −1, with F positive).
Hydrogen: +1 with nonmetals, −1 with metals (hydrides). Fluorine: always −1.
The sum of oxidation numbers equals the species' total charge.
Oxidizing and reducing agents
The oxidized species is the reducing agent (it gives electrons to something else).
The reduced species is the oxidizing agent (it takes electrons from something else).
Say it precisely: 'Zn is oxidized, so Zn is the reducing agent.'
Balancing redox equations
Balance atoms first (half-reaction method), then balance charge with electrons.
In acidic solution, balance oxygen with H2O and hydrogen with H+; in basic solution, neutralize H+ with OH−.
Check both mass and charge balance on the final equation.
Galvanic versus electrolytic cells
Galvanic (voltaic) cells: spontaneous redox produces electricity; anode is negative, cathode is positive.
Electrolytic cells: external electricity drives nonspontaneous redox; anode is positive, cathode is negative.
Anode = oxidation always; cathode = reduction always — the signs flip, the roles don't.
Cell potential
E°cell = E°cathode − E°anode using standard reduction potentials for both half-reactions.
Positive E°cell means spontaneous (galvanic); negative means nonspontaneous.
The Nernst equation extends cell potential to nonstandard concentrations.
Common mistakes
Mistake: Flipping anode/cathode signs between galvanic and electrolytic cells
Fix:
Memorize the roles, not the signs: anode is always oxidation, cathode is always reduction. Then attach the signs: galvanic anode (−)/cathode (+); electrolytic anode (+)/cathode (−).
Mistake: Balancing atoms but forgetting to balance charge with electrons
Fix:
Split into half-reactions, balance atoms, then add electrons to balance charge — do charge and mass as separate, deliberate steps.
Mistake: Multiplying E° by the stoichiometric coefficient when scaling a half-reaction
Fix:
E° values are intensive: do NOT multiply them when you multiply a half-reaction. Only the electron count n changes when scaling.
Mistake: Assuming the cell potential stays at E° when concentrations differ from 1 M
Fix:
Apply the Nernst equation: E = E° − (RT/nF) ln Q. Only at standard concentrations (all 1 M) does E equal E°.
Exam tips
Drill redox-practice for balancing speed — the half-reaction method must be automatic before exam day.
Use electrochemistry-practice for cell-potential calculations and Nernst equation setups.
Draw every cell with labeled anode, cathode, electron flow, and ion flow; the diagram prevents sign errors.
When asked for the oxidizing agent, name the species that gets reduced — exams test this reversal constantly.
For electrolysis problems, convert current x time to coulombs to moles of electrons with F = 96485 C/mol.