The transformation of a substance by physical, chemical, or biochemical means into other substances with the release or consumption of energy. Single reactions are rare in practice but instead are generally complex resulting in several products, intermediates, and undesirable products. In parallel reactions, the concentration level of desired product is key to the control of the product distribution. For reactions in series, the mixing of the different compositions is the key to the formation of an intermediate. Series-parallel reactions are seen from the point of view of the constituents. The thermodynamic feasibility of a chemical reaction can be determined from the free energy while the kinetics of the reaction determines the rate at which the reaction takes place, and therefore determines the size of the reactor required to contain the reaction. The choice of reactor, such as plug (p. 318) flow reactor or continuous stirred-tank reactor, is dependent on the heating and cooling requirements of the reaction. Heats of reaction can be determined from heats of combustion and heats of formation of the individual components in the reaction.