The term refers to the relationship between the amounts of substances that react together in a particular chemical reaction, and the amounts of products that are formed. The general @S06021@ equation: \[a\text{A}\,+\,b\text{B}\,+\,...\,\rightarrow\,...\,+\,y\text{Y}\,+\,z\text{Z}\] provides the information that a moles of A reacts with b moles of B to produce y moles of Y and z moles of Z. The stoichiometry of a reaction may be unknown, or may be very complex. For example, the thermal @D01547@ of acetaldehyde yields mainly methane and carbon monoxide, but also a variety of minor products such as ethane, acetone and diacetyl. The @S06021@ equation:
is therefore only an approximate one. Even when the overall stoichiometry of a reaction is well defined, it may be time-dependent in that it varies during the course of a reaction. Thus if a reaction occurs by the mechanism \(\text{A}\rightarrow \text{X}\rightarrow \text{Y}\), and X is formed in substantial amounts during the course of the process, the relationship between the amounts of A, X and Y will vary with time, and no one @S06021@ equation can represent the reaction at all times.
PAC, 1996, 68, 149. (A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)) on page 187 [Terms] [Paper]