3.6.2. Reaction Data

Reaction Data follows the REACTIONS line and precedes an END statement that concludes the Reactions Data section. Each reaction entry line is divided into two fields. The first contains the symbolic description of the reaction path for that reaction while the second contains the Arrhenius rate coefficients. Both fields are format-free and blank spaces are ignored. Any line or portion of a line starting with an exclamation mark (!) is considered a comment and is ignored. Blank lines are also ignored.

The reaction description, given in the first field, must be composed of the species symbols, coefficients, delimiters, and any special symbols defined in Table 3.5: Reaction Data Criteria .

Table 3.5: Reaction Data Criteria

Species Symbols

 

Each species in a reaction is described with the unique sequence of characters as they appear in the species data and the thermodynamic data (e.g., H2).

Coefficients

 

A species symbol may be preceded by an integer or real coefficient. The coefficient has the meaning that there are that number of moles of the particular species present as either reactants or products; e.g. 2OH, is equivalent to OH + OH. Non-integer coefficients are allowed in Gas-phase Kinetics input, but the element balance in the reaction must still be maintained.

Delimiters

+

A plus sign is the delimiter between each reactant species and each product species.

=

An equality sign is the delimiter between the last reactant and the first product in a reversible reaction.

<=>

An equality sign enclosed by angle brackets can also be used as the delimiter between the last reactant and the first product in a reversible reaction.

=>

An equality sign with an angle bracket on the right is the delimiter between the last reactant and the first product in an irreversible reaction.

Special Symbols

+M

An M as a reactant and product stands for an arbitrary third body. An M in the reaction description indicates that a third body is participating in the reaction. In a reaction containing an M, species can be specified to have enhanced third body efficiencies, in which case auxiliary information (described below) must follow the reaction line. If no enhanced third body efficiencies are specified, then all species act equally as third bodies and the effective concentration of the third body is the total concentration of the mixture.

(+M)

An M as a reactant and product surrounded by parentheses indicates that the reaction is a pressure-dependent reaction, in which case auxiliary information line(s) (described below) must follow the reaction to identify the fall-off formulation and parameters. A species may also be enclosed in parenthesis. Here, for example, (+H2O) indicates that water is acting as the third body in the fall-off region, not the total concentration M.

E

The symbol E as a reactant and/or product is used to represent an electron. An electron is treated just like any other species, and is composed of the element E, which must be declared as element data. If an E appears in any reaction, then it must also be declared as a species in the species data.

!

An exclamation mark means that all following characters are comments on the reaction. For example, the comment may be used to give a reference to the source of the reaction and rate data.


The second field of the REACTIONS line is used to define the Arrhenius rate coefficients A i, , and E i, in that order, as given by Equation 3–5 of the Chemkin Theory Manual . At least one blank space must separate the first number and the last symbol in the reaction. The three numbers must be separated by at least one blank space, be stated in either integer, floating point, or "E" format (for example, 123, 123.0 or 123E1), and have units associated with them.

Note:  Unless modified by options specified on the REACTIONS line or in Auxiliary Reaction Keywords, the default units for A i are in cgs (cm, sec, K, mole), the exact units depending on the reaction. The factor is dimensionless. The default units for the activation energies are cal/mole.

Examples of reaction data are shown in Figure 3.4: Examples of Reaction Data .

Figure 3.4: Examples of Reaction Data

REACTIONS                          CAL/MOLE   ! these are the default units for the reaction rates
H2 + O2 = 2OH                                          1.7E13  0  47780.  !  Ref. 21
! H2 + O2 = OH + H                  1.7E13  0  47780.  !  same as previous reaction,
                                         ! commented to prevent a duplication error
H  + O2 + M = HO2 + M                                             2.0E15  0.000  -870.
!  H + O2 + M = HO2                                               2.0E15  0.000  -870.
!  H + O2 = HO2 + M                                               2.0E15  0.000  -870.
OH+ + H + E = H2O                                                        1.E19  0  0.0
O = O(*)                                                                 1.3E5  0  0
                                         ! photoactive reaction, represented without HV
0.5H2 + 0.5O2 = OH                                   ! example of real coefficients
END                                ! END statement is optional;  ! <eof> condition is equivalent

Table 3.6: Summary of the Rules for Reaction Data is a summary of the reaction data rules.

Table 3.6: Summary of the Rules for Reaction Data

Rule

Description

1

The first reaction line must start with the word REACTIONS (or REAC), and may include units definition(s).

2

The reaction description can begin anywhere on the line. All blank spaces, except those between Arrhenius coefficients, are ignored.

3

Each reaction description must have =, <=> or => between the last reactant and the first product.

4

Each reaction description must be contained on one line.

5

Three Arrhenius coefficients must appear in order (, , and ) on each Reaction line, separated from each other and from the reaction description by at least one blank space; no blanks are allowed within the numbers themselves.

6

There cannot be more than six reactants or six products in a reaction.

7

Comments are any and all characters following an exclamation mark.

8

For best results an END statement should follow the reaction input.