To add a Reaction node () either right-click the Chemistry node or on the Chemistry toolbar click Reaction.

Enter a chemical reaction Formula. Click Apply to make the interface examine the species taking part in the model’s reactions and automatically add the associated Species features to the Model Builder.

If all species are written using their chemical formula, the stoichiometric coefficients can automatically be calculated, if the problem is well posed, by clicking the Balance button to the right of the Reaction Formula section title. This turns, for example, the formula “H2+O2=>H2O” into “2H2+O2=>2H2O”.

Examples of problems that are not well posed are, for example, “C+H2=>CH4+C2H6”, from which any ratio of CH4/C2H6 could be obtained, and “H2=>O2”, where not all elements are present on both sides.

Select the Reaction type — Reversible, Irreversible, or Equilibrium — or edit the expression directly in the Formula field. In the latter case, specify the reaction type with a delimiter separating the two sides of the equation:

Each Reaction type has its own set of reaction kinetics:

where νij is the stoichiometric coefficient.

This section is available when the Reaction type is either Reversible or Irreversible.

When Mass action law is selected (default), the rate expression is automatically derived from the stoichiometric coefficients in the reaction formula:

The deduced overall reaction order is shown in text below the respective equation in the Reaction Rate section.

If the reaction order differs from the stoichiometric coefficients, or if an arbitrary rate expressions is applicable, change Reaction Rate to User defined. An expression field r appears with the default expression being that from the mass action law. Below this there are fields to set the reaction order. For a reversible reaction the reverse reaction order may be specified in addition to the forward one. The unit of the rate constant k (or frequency factor A in the case of Arrhenius behavior), is derived from the reaction order, in SI units: (m3/mol)α − 1/s, where α equals the order with respect to volumetric species. When surface species are present — identified by their “(ads)” suffix — the unit is instead given by m3α+2β − 2/molα+β−1/s, where β is the order with respect to surface species.

This section applies for Reversible or Irreversible reactions and defines the reaction rate constants used in the reaction rates.

The Forward rate constant kf is used for both Reversible and Irreversible reactions. The Reverse rate constant kr is only used for Reversible reactions (Equation 2-99).

The SI units of the rate constants are automatically based on the order of the reaction with respect to the concentrations, as defined in the Reaction formula.

The Specify equilibrium constant check box is available for Reversible reactions. If the check box is selected the rate constants are defined in a different manner with the reverse rate constant being computed from the following expression:

Thus, in this case, the forward rate constant and equilibrium constant for the reaction are needed. The Equilibrium constant is edited in the Equilibrium Settings section.

When the Use Arrhenius expressions check box is selected the Arrhenius parameters are automatically used in predefined expressions for the forward and reverse rate constants kfand kr, respectively.

Specify the activation energy and the frequency factor in the Arrhenius expressions to account for temperature variations. The reference temperature, Tref equals 1 K. The available fields are based on the Reaction type chosen in the Reaction node. Enter values or expressions for each of the following (reverse expressions are only available for reversible reactions):

This section is available for equilibrium reactions, and for reversible reactions when the Specify equilibrium constant check box has been selected.

For an equilibrium reaction, specify the Equilibrium expression. When the Equilibrium expression is set to Automatic the following expression is used:

Select User defined from the Equilibrium expression list to instead enter a manually defined equilibrium expression.

Specify the Equilibrium constant Keq0 for an equilibrium reaction, or for a reversible reaction when the Specify equilibrium constant check box has been selected (in the Rate Constants section).

The Equilibrium constant can either be User defined, or automatically defined when set to Automatic or Thermodynamics.

Use the Automatic option to compute the equilibrium constant for an ideal system.

The Thermodynamics option is available when all reactions in the interface are equilibrium reactions, and the interface is fully coupled to a Thermodynamic System (see Species Matching). Use this setting to automatically compute the equilibrium constant for an ideal or nonideal system, dependent on the thermodynamic model applied for the coupled system.

Using Automatic or Thermodynamics, Keq0 is calculated from the Gibbs free energy of the reaction. For more details see The Equilibrium Constant and the Automatically Defined Equilibrium Constants section therein.

This section contains information about thermodynamic properties that relate to a selected reaction. Several Automatic definitions are available here.

The Enthalpy of reaction H (SI unit: J/mol) is calculated by the interface from species properties and the related stoichiometric coefficients:

The Entropy of reaction S (SI unit: J/(mol·K)) comes from a similar expression:

In Equation 2-101 and Equation 2-102, hi and si are the species’ molar enthalpy and molar entropy, respectively.

Enter these quantities in the Species Thermodynamic Expressions section for the Species node either by using the predefined polynomial or by providing a custom expression or constants.

The stoichiometric coefficients, νij, are defined as being negative for reactants and positive for products. Using Equation 2-101 and Equation 2-102 to equate the Gibbs free energy of reaction enables the equilibrium constant to be expressed according to Equation 2-101.

The Heat source of reaction (SI unit: W/m3) is automatically computed from the heat of each reaction j, given by: