Template:HideTOC 12.1: Oxidation States - Electron Bookkeeping Redox reactions are all about electrons being transferred from one substance to another, so it would be useful if we had a system for keeping track of what gains and what loses electrons, and how many electrons are involved. We do - our record-keeping system is called Oxidation Numbers. You may remember from earlier chemistry classes something called electronegativity. 12.2: Oxidation and Reduction- Some Definitions "Redox" is short for "oxidation and reduction", two complimentary types of chemical reactions. The term oxidation originally referred to substances combining with oxygen, as happens when an iron bar rusts or a campfire log burns. We often refer to these two examples as corrosion and combustion. Reduction originally referred to the process of converting metal ores to pure metals, a process that is accompanied by a reduction in the mass of the ore. 12.3: Balancing Oxidation-Reduction Equations Oxidation–reduction reactions are balanced by separating the overall chemical equation into an oxidation equation and a reduction equation. In oxidation–reduction reactions, electrons are transferred from one substance or atom to another. We can balance oxidation–reduction reactions in solution using the oxidation state method, in which the overall reaction is separated into an oxidation equation and a reduction equation. 12.4: Voltaic (or Galvanic) Cells- Generating Electricity from Spontaneous Chemical Reactions A galvanic (voltaic) cell uses the energy released during a spontaneous redox reaction to generate electricity, whereas an electrolytic cell consumes electrical energy from an external source to force a reaction to occur. Electrochemistry is the study of the relationship between electricity and chemical reactions. The oxidation–reduction reaction that occurs during an electrochemical process consists of two half-reactions, one representing the oxidation process and one the reduction process. 12.5: Standard Reduction Potentials Redox reactions can be balanced using the half-reaction method. The standard cell potential is a measure of the driving force for the reaction. \(E°_{cell} = E°_{cathode} − E°_{anode} \] The flow of electrons in an electrochemical cell depends on the identity of the reacting substances, the difference in the potential energy of their valence electrons, and their concentrations. The potential of the cell under standard conditions is called the standard cell potential (E°cell). 12.6: Batteries- Using Chemistry to Generate Electricity Commercial batteries are galvanic cells that use solids or pastes as reactants to maximize the electrical output per unit mass. A battery is a contained unit that produces electricity, whereas a fuel cell is a galvanic cell that requires a constant external supply of one or more reactants to generate electricity. One type of battery is the Leclanché dry cell, which contains an electrolyte in an acidic water-based paste. 12.7: Electrolysis- Driving Non-spontaneous Chemical Reactions with Electricity In electrolysis, an external voltage is applied to drive a nonspontaneous reaction. Electrolysis can also be used to produce hydrogen and oxygen gas from water. Electroplating is the process by which a second metal is deposited on a metal surface. The amount of material consumed or produced in a reaction can be calculated from the stoichiometry of an electrolysis reaction, the amount of current passed, and the duration of the electrolytic reaction. 12.8: Corrosion- Undesirable Redox Reactions Corrosion is a galvanic process that can be prevented using cathodic protection. The deterioration of metals through oxidation is a galvanic process called corrosion. Protective coatings consist of a second metal that is more difficult to oxidize than the metal being protected. Alternatively, a more easily oxidized metal can be applied to a metal surface, thus providing cathodic protection of the surface. A thin layer of zinc protects galvanized steel. Sacrificial electrodes can also be attached