Redox Reactions — Class 11 Chemistry

Redox reactions, in which oxidation and reduction occur simultaneously, form a major class of chemical change underlying combustion, metallurgy, batteries and corrosion. The unit develops three parallel ways of viewing them. The classical idea defines oxidation as the addition of oxygen or an electronegative element, or the removal of hydrogen or an electropositive element, with reduction as the reverse. The electronic idea defines oxidation as loss of electrons and reduction as gain of electrons, so the oxidising agent accepts electrons (and is itself reduced) while the reducing agent donates them. Competitive electron-transfer experiments, such as zinc displacing copper from copper sulphate, establish a relative order of reactivity. The oxidation-number concept assigns oxidation states by a fixed set of rules and redefines oxidation as an increase, and reduction as a decrease, in oxidation number; this allows redox reactions to be classified into combination, decomposition, displacement and disproportionation types. Two systematic methods balance redox equations — the oxidation-number method and the half-reaction (ion-electron) method — applicable in both acidic and basic media, and redox titrations use self-indicators such as permanganate. Finally redox is linked to electrode processes through the redox couple, the Daniell cell and the standard electrode potential, the standard hydrogen electrode being assigned a value of exactly zero volts.