What is the extent of reaction (ξ) defined as?

The extent of reaction, denoted by the symbol ξ (xi), is a parameter that quantifies the progression of a chemical reaction as it moves from reactants to products. Specifically, it represents the amount of reactants that have been converted into products.

Defining ξ in terms of the change in the quantity of reactants highlights its role in stoichiometry, as it allows for the calculation of the concentrations of reactants and products at any point in the reaction. The extent of reaction is particularly useful in thermochemical calculations, as it relates to both the quantities involved and the associated changes in energy.

When considering other choices, while heat absorbed, total energy released, and volume of gas produced may be relevant concepts in thermodynamics and physical chemistry, they do not accurately describe the extent of reaction. ξ does not directly measure heat changes or energy release; instead, it focuses on the quantitative progress of reactants being transformed into products over the course of the reaction. Thus, defining the extent of reaction as the change in the quantity of reactants is indeed the most precise representation.