The source of chemical species due to reaction, , is computed as the sum of the reaction sources over the reactions that the species may participate in:

(19–8)

where	is the mole mass of species
	is the stoichiometric coefficient of the th species in the th reaction
	is the molar reaction rate of reaction

Note that in Equation 19–8,

	for the reactants of the forward reaction
	for the products of the forward reaction

The individual reaction rates are themselves functions of the form

(19–9)

where is a rate "constant", the terms are the molar concentrations of the reactant species , and are a set of exponential powers.

The values of the rate constant and the exponential orders are dictated by a reaction mechanism that is an elementary step-by-step description of a chemical interaction.

The exponential orders may be different from the stoichiometric coefficients of the reaction itself.

For a given reaction mechanism, the influence of temperature on the rate constant can be expressed in terms of an Arrhenius type relationship involving the universal gas constant and the absolute temperature :

(19–10)

where		is a pre-exponential factor
		is a temperature constant
		is the activation energy

A change in temperature, however, may modify the reaction mechanism. Equation 19–10 holds as long as a change in the system does not change the reaction mechanism. It is a subtle fact that a change in composition can also change the reaction mechanism.

Considering an unknown rate constant at any temperature , and a reference rate constant at a reference temperature , the pre-exponential factor can be eliminated from Equation 19–10 to yield

(19–11)

represents a shift in temperature. It is set to 0 by default.

This relationship has proven useful to relate to from appropriate reference values and . The temperature can also be expressed in a relative scale in conjunction with the absolute temperature with respect to the zero of the relative scale ().

Chemical reactions are either exothermic or endothermic (that is, they either generate or consume energy). Energy (positive or negative) associated with the chemical reaction acts as a source term of the energy equation as follows:

(19–12)

where is the heat of reaction .

where is the heat of reaction . Note that the temperature of the system strongly affects the rate of reaction; as a general rule, a change in temperature of about ten degrees changes the reaction rate by a factor of two.