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Keywords

### REACTION

This keyword data block is used to define irreversible reactions that transfer specified amounts of elements to or from the aqueous solution during reaction calculations.

#### Example 1

```Line 0:  REACTION 5 Add sodium chloride and calcite to solution.
Line 1a:      NaCl     2.0
Line 1b:      Calcite  0.001
Line 2:       0.25     0.5     0.75     1.0  moles
```

#### Explanation 1

Line 0: REACTION [number] [description]

REACTION is the keyword for the data block.

number--positive number to designate this reaction. Default is 1. 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.

description--optional character field that describes the reaction.

Line 1: (phase name or formula), relative stoichiometry

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 the chemical formula to be used in the irreversible reaction.

relative stoichiometry--Amount of this reactant relative to other reactants, it is a molar ratio between reactants. In the example, the reaction contains 2000 times more NaCl than calcite.

Line 2: list of reaction amounts, units

list of reaction amounts--A separate calculation will be made for each listed amount. In the example, a solution composition will be calculated after adding 0.25, 0.5, 0.75, and 1.0 mol of the reaction to the initial solution. The additions are not cumulative; each reaction step begins with the same initial solution and adds only the amount of reaction specified. The total amount of each reactant added at any step in the reaction is the reaction amount times the stoichiometric coefficient of the reactant. Thus the total amount of sodium and chloride added at each reaction step is 0.5, 1.0, 1.5, and 2.0 mol; the total amount of calcium and carbonate added at each step is 0.00025, 0.0005, 0.00075, and 0.001 mol. Additional lines may be used to define all reactant amounts.

units--units must be moles, millimoles, or micromoles. Units must follow all reaction amounts. Default is moles.

If line 2 is not entered, the default is one step of 1.0 mol.

#### Example 2

```Line 0: REACTION 5 Add sodium chloride and calcite to reaction solution.
Line 1a:     NaCl       2.0
Line 1b:     Calcite    0.001
Line 2:      1.0 moles in 4 steps
```

#### Explanation 2

Line 0: REACTION [number] [description]

Same as example 1.

Line 1: (phase name or formula), relative stoichiometry

Same as example 1.

Line 2: reaction amount [units] [in steps]

reaction amount--a single reaction amount is entered. This amount of reaction will be added in steps steps.

units--same as example 1.

in steps--"in" indicates that the reaction will be divided into steps number of steps and must be lower case. Example 2 performs exactly the same calculations as example 1, 1.0 mol of reaction is divided into 4 steps. 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 line 2 is not entered, the default is one step of 1.0 mol.

#### Notes

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 solution; ensuing calculations will probably fail. It is possible to "evaporate" a solution by removing H2O or dilute a solution by adding H2O. If more reaction steps are defined with REACTION_TEMPERATURE than in REACTION, then the final reaction amount defined by REACTION will be repeated for the additional temperature steps.

#### Example problems

The keyword REACTION is used in example problems 4, 5, 6, and 7.

#### Related keywords

PHASES and REACTION_TEMPERATURE.

Example 1
Explanation 1
Example 2
Explanation 2
Notes
Example problems
Related keywords

User's Guide to PHREEQC - 07 MAY 96
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