Page 146 - Modeling of Chemical Kinetics and Reactor Design

116 Modeling of Chemical Kinetics and Reactor Design

3
1. Zero order (n = 0; k = moles/m -sec)
(–r ) = kC 0 A (3-18)
A
2. First order (n = 1; k = 1/sec)

(–r ) = kC A (3-19)
A
3
3. Second order (n = 2; k = m /moles-sec)
(–r ) = kC 2 A (3-20)
A
2
3
–1
4. Third order (n = 3; k = (m /moles) •sec )
(–r ) = kC 3 A (3-21)
A

DETERMINING THE ORDER OF REACTIONS

ZERO ORDER REACTIONS

The rate of a chemical reaction is of a zero order if it is independent
of the concentrations of the participating substances. The rate of
reaction is determined by such limiting factors as:

1. In radiation chemistry, the energy, intensity, and nature of radiation.
2. In photochemistry, the intensity and wave length of light.
3. In catalyzed processes, the rates of diffusion of reactants and
availability of surface sites.

If the rate of the reaction is independent of the concentration of
the reacting substance A, then the amount dC by which the concentra-
A
tion of A decreases in any given unit of time dt is constant throughout
the course of the reaction. The rate equation for a constant volume
batch system (i.e., constant density) can be expressed as:

− ( r A ) =− dC A = k (3-22)
dt
The negative sign indicates that the component A is removed from
3
the system, and k is the velocity constant with the units as moles/m -
sec. Assuming that at time t the concentration of A is C AO , and at
1