For a closed system with one specified chemical reaction, the extent of reaction records progress from through . Here is the amount of substance of species , and the signed stoichiometric vector has negative entries for reactants and positive entries for reaction products. For the water chemical equation in the order , it is . The normalization of depends on the chosen coefficients. See IUPAC Gold Book, extent of reaction.
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The extent of reaction is a concept used in chemical thermodynamics and reaction kinetics to quantify the progress of a chemical reaction. It is often denoted by the symbol \( \xi \) (xi) and represents the amount by which a reaction has proceeded relative to its stoichiometry. Mathematically, the extent of reaction is defined as a variable that describes the change in the number of moles of reactants and products during a reaction.