### 27.2.2. Single Step

This option displays three other tabs in addition to Basic Settings. One of them is a Reactants tab, displaying a list of reactants and specifying the ratio with which they react together and the order of the reaction. A list of products is also set on the Products tab and includes the ratio with which they are produced. Reaction Rates has optional forward and backward reaction rates and third body terms can be applied.

#### 27.2.2.1. Single Step: Basic Settings

1. Optionally, select Reaction Description to enter a description for the reaction (to a maximum of 120 alphanumeric characters).

2. Optionally, specify any additional materials for this reaction using the Additional Materials List.

3. Optionally, select Reaction or Combustion to set a reaction or combustion model.

   Any settings specified here will override the choice of models selected on the Fluid Models tab of the domains form (unless the choice of models on the Fluid Models form is set to `None`).

This is implemented to enable reaction-step specific combustion modeling for multi-step reactions. For details, see Reaction-Step Specific Combustion Model Control in the CFX-Solver Modeling Guide.

#### 27.2.2.2. Single Step: Reactants

For details, see Multiphase: Reactants.

1. Option assumes the value `Child Materials` when creating a reaction involving one phase.

2. Choose the reactant(s) to add from the Materials List drop-down.

3. Enter the Stoichiometric Coefficient for the each of the selected components.

4. Optionally, specify a reaction order.

   If the reaction order is not entered, it will default to the same value as the stoichiometric coefficient.

#### 27.2.2.3. Single Step: Products

The Products tab is identical to the Reactants tab, with the only difference being that the settings here apply to the products instead of the reactants.

#### 27.2.2.4. Single Step: Reaction Rates

1. For each of Forward Reaction Rate and Backward Reaction Rate, Option defines the reaction rate dependency. Select from:

   - Arrhenius in the CFX-Solver Modeling Guide

   - Arrhenius with Temperature PDF in the CFX-Solver Modeling Guide

   - Expression in the CFX-Solver Modeling Guide

2. The Pre Exponential Factor and Temperature Exponent are required elements for the `Arrhenius` reaction type.

3. The temperature limit list (Lower Temperature and Upper Temperature) is required for the `Arrhenius with Temperature PDF` reaction type.

4. Reaction Activation enables `Activation Temperature` or `Activation Energy` to be set.

5. Some reactions require a Third Body Term to proceed.

   For details, see Third Body Terms in the CFX-Solver Theory Guide.

6. You can define a reaction in terms of a dependency, equilibrium or an expression.

   For details, see Reaction Rate Types in the CFX-Solver Modeling Guide.

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