# 8.26: Untitled Page 215

## volume average

### area average

concentration

concentration in

concentration

at a point in

the reactor

in the exit

the reactor

For the specific system illustrated in Figure 9‐2 the assumption of perfect mixing leads to

c    c

c ,

R

 R V ,

A  H , Br , HBr

(9‐14)

A

A exit

A

A

A

2

2 Reaction Kinetics

399

Given these simplifications we can discuss the process illustrated in Figure 9‐1 in terms of local conditions for which the chemical kinetics may be illustrated by a schema of the form

k

Local chemical kinetic schema:

H

 Br

 2HBr

(9‐15)

2

2

This schema suggests that a molecule of hydrogen collides with a molecule of bromine to produce two molecules of hydrogen bromide as illustrated in Figure 9‐3. The frequency of the collisions that cause the reaction depends on Figure 9‐3. Molecular collision leading to a chemical reaction the product of the two concentrations, c

and c

, and this leads to the local

H2

2

Br

chemical reaction rate equations given by

R

  k c c

,

R

  k c c

(9‐16)

H2

H2 Br2

Br2

H2 Br2

Chemical kinetic schemata are traditionally represented in local form, as indicated in Eq. 9‐15, even when they are based on macroscopic observations as we have suggested in Figures 9‐1 and 9‐2. If we make use of the chemical reaction rate equations given by Eqs. 9‐16 and the stoichiometric equations given by Eqs. 9‐7, the local chemical reaction rate equation for the production of hydrogen bromide takes the form

Local chemical reaction rate equation:

R

 2 k c c

(9‐17)

HBr

H2 Br2

This rate equation is based on the concept of mass action kinetics which, in turn, is based on the picture illustrated by Eq. 9‐15 or the picture illustrated by Figure 9‐3.

The words associated with Eq. 9‐16 and with Eq. 9‐17 depend on what aspect of the equations we wish to emphasize. In this text we attempt to use a consistent set of phrases indicated by  400