Use the Reaction node () to enter reversible or irreversible reactions that do not involve electrons (for example, for gas phase reactions). Enter the Formula for the reaction into the field and then fill in the appropriate information about the reaction.

COMSOL Multiphysics automatically ensures that the Reaction selection is the same as the Diffusion and Migration selection.

See the Reaction Formula settings for the Electron Impact Reaction node. Note that the Reaction type can be Reversible or Irreversible unlike the Electron Impact Reaction.

To activate the Arrhenius Parameters section and make the kinetic expressions use these parameters, select the Use Arrhenius expressions check box. In the Arrhenius Parameters section, A denotes the frequency factor, n the temperature exponent, E the activation energy (SI unit: J/mol), and Rg the gas constant, 8.314 J/(mol·K). The pre-exponential factor, including the frequency factor A and the temperature factor Tn, is given the units (m3/mol)α − 1/s, where α is the order of the reaction (with respect to the concentrations). Text fields are available for each of the Arrhenius parameters in the equation above.

Modify the default expressions for the rate constant. This field is only available if the Use Arrhenius expressions check box is cleared. The units of the rate constant depend on the order of the reaction.

If the Reaction type is Reversible, the Reverse rate constant can either be specified directly, or by using an equilibrium constant. When Specify equilibrium constant is not active, enter a value for the Reverse rate constant. The units depend on the reaction order. When Specify equilibrium constant is active, enter a value for the Equilibrium constant. When the Calculate thermodynamic properties property is active, the Equilibrium constant is computed based on the thermodynamic properties.