This keyword data block is used to define irreversible reactions that transfer specified amounts of elements to or from the aqueous solution during batch-reaction or transport calculations. REACTION steps are specified explicitly and do not depend directly on solution composition or time. Use KINETICS and RATES data blocks instead of the REACTION data block to model the rates of irreversible reactions that evolve with time and vary with solution composition.
number --Positive number to designate the following stoichiometric reaction. A range of numbers may also be given in the form m-n , where m and n are positive integers, m is less than n , and the two numbers are separated by a hyphen without intervening spaces. Default is 1.
phase name or formula --If a phase name is given, the program uses the stoichiometry of that phase as defined by PHASES input; otherwise, formula is a chemical formula to be used in the stoichiometric reaction. Additional lines can be used to define additional reactants.
relative stoichiometry --Amount of this reactant relative to other reactants, it is a molar ratio between reactants. In the example data block, the reaction contains 2000 times more NaCl than calcite. Default is 1.0.
list of reaction amounts --A separate calculation will be made for each listed amount. If INCREMENTAL_REACTIONS is false (default), example data block 1 performs the calculation as follows: the first step adds 0.25 mol of reaction to the initial solution; the second step adds 0.5 mol of reaction to the initial solution; the third 0.75; and the fourth 1.0; each reaction step begins with the same initial solution and adds only the amount of reaction specified. If INCREMENTAL_REACTIONS keyword is true, the calculations are performed as follows: the first step adds 0.25 mol of reaction and the intermediate results are saved as the starting point for the next step; then 0.5 mol of reaction are added and the intermediate results saved; then 0.75 mol; then 1.0 mol; the total amount of reaction added to the initial solution is 2.5 mol. The total amount of each reactant added at any step in the reaction is the reaction amount times the relative stoichiometric coefficient of the reactant. Additional lines may be used to define all reactant amounts.
in steps --" in" indicates that the stoichiometric reaction will be divided into steps number of steps. If INCREMENTAL_REACTIONS is false (default), example data block 2 performs the calculations as follows: the first step adds 0.25 mol of reaction to the initial solution; the second step adds 0.5 mol of reaction to the initial solution; the third 0.75; and the fourth 1.0. If INCREMENTAL_REACTIONS keyword is true, the calculations are performed as follows: each of the four steps adds 0.25 mol of reaction and the intermediate results are saved as the starting point for the next step.
The REACTION data block is used to increase or decrease solution concentrations by specified amounts of reaction. The specified reactions are added to or removed from solution without regard to equilibrium, time, or reaction kinetics. Irreversible reactions for which time evolution or concentration-dependent rates are needed must be modeled using the KINETICS and RATES keywords, however, a kinetic rate expression is needed for this type of modeling.
Example data block 1 with INCREMENTAL_REACTIONS false and example data block 2 with INCREMENTAL_REACTIONS True or False will generate the same results--solutions after a total of 0.25, 0.5, 0.75, and 1.0 mol of reaction have been added. Example data block 1 with INCREMENTAL_REACTIONS true generates results after a total of 0.25, 0.75, 1.5, and 2.5 mol of reaction have been added.
If a phase name is used to define the stoichiometry of a reactant, that phase must be defined by PHASES input in the database or in the input data file. If negative relative stoichiometries or negative reaction amounts are used, it is possible to remove more of an element than is present in the system, which results in negative concentrations. Negative concentrations will cause the calculations to fail. It is possible to "evaporate" a solution by removing H 2 O or dilute a solution by adding H 2 O. If more reaction steps are defined in the REACTION_TEMPERATURE or KINETICS data blocks than in REACTION, then the final reaction amount defined by REACTION will be repeated for the additional steps. Suppose only one reaction step of 1.0 mole is specified in a REACTION data block and two temperature steps are specified in a REACTION_TEMPERATURE data block. If INCREMENTAL_REACTIONS is false, then the total amount of reaction added by the end of step 1 and 2 is the same, 1.0 mole. However, if INCREMENTAL_REACTIONS true, the total amount of reaction added by the end of step 1 will be 1.0 mole and by the end of step two will be 2.0 mole.
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