Unit 15
Redox and electrochemistry
Rusting iron, a discharging battery, electroplating and the chlor-alkali industry all move electrons from one substance to another. This unit starts from oxidation-number bookkeeping and balancing redox equations, then uses one table of standard electrode potentials to compute a cell’s voltage, free energy and equilibrium constant, shows how concentration and pH shift potentials and how electrolysis pushes reactions uphill, and finally redraws the whole table as Latimer, Frost and Pourbaix diagrams.
- 1 Oxidation states: bookkeeping for electrons Burning methane involves no ions, yet electrons still change hands. A set of priority rules gives every atom a formal charge, and who is oxidised and who is reduced becomes plain. 25 min
- 2 Balancing redox equations: the half-reaction method The equation for MnO₄⁻ oxidising Fe²⁺ conjures up 8 H⁺ and 4 H₂O. Split the reaction into halves, balance atoms and charge step by step, and electron conservation tells you the rest. 30 min
- 3 Galvanic cells and standard electrode potentials Separate the two halves and make the electrons take a wire, and chemical energy becomes electrical. One table of standard potentials gives the voltage, ΔG° and equilibrium constant of any cell. 30 min
- 4 The Nernst equation: concentration and pH shift potentials Every tenfold change in concentration shifts a potential by 59/n mV, and every H⁺ in the half-reaction makes it fall with rising pH. pH meters and concentration cells run on this logarithm. 30 min
- 5 Electrolysis: driving reactions with electricity Apply a large enough voltage and a reaction runs against its spontaneous direction. What reacts at each electrode depends on potentials and overpotentials; how much forms follows Faraday’s laws. 30 min
- 6 Overpotential and electrode kinetics Why does evolving hydrogen on mercury cost more than a volt beyond platinum? Draw a Tafel plot from Tafel’s original 1905 data, extract the transfer coefficient and exchange current density, and use the Butler–Volmer equation to see how potential controls the rate of an electrode reaction. 30 min
- 7 Batteries: chemistry that stores charge Voltage comes from Gibbs energy and capacity from Faraday’s law: compute the theoretical capacities of graphite, LiCoO₂ and LiFePO₄ from their formulas, see how a lead–acid battery consumes its acid, and explain the flat discharge plateau of LiFePO₄ with a lattice-gas model. 30 min
- 8 Maps of oxidation states: Latimer, Frost and Pourbaix diagrams Why is there no Cu⁺ in water? Why does chlorine turn into bleach in base? Why does iron rust while a copper roof lasts centuries? Redraw one table of potentials three ways and the answers appear. 35 min