Use this node to model a reaction where the kinetics is assumed so fast that the equilibrium condition is fulfilled at all times. The node solves for an additional degree of freedom (the reaction rate Req) to fulfill the equilibrium condition at all times in all space coordinates.

Selecting Equilibrium constant in the list, the following equilibrium condition based on the species activities and the law of mass action is used

where νi are the stoichiometric coefficients and the species activities are defined from the concentration, ci, and the unit activity concentration ca0.

Enter a value or expression for the dimensionless Equilibrium constant Keq, and the Unit activity concentration Ca0.

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

Enter a value for the stoichiometric coefficientν for all participating species. Use negative values for reactants and positive values for products in the modeled reaction. Species with a stoichiometric coefficient value of 0 are not affected by the Equilibrium Reaction node.