Principles of Reaction Kinetics

Abstract

Chemical reaction kinetics is concerned with the determination of rate laws and time laws for a reaction, for example, the oxidation of a metal sheet. Rate laws describe the reaction rate as a function of process parameters:

$$v = \frac{{dm}}{{dt}} = v(p{o_2},T,l, \ldots ).$$

((2.1))

The mass gain per time unit of the sheet can depend on the oxygen pressure, the temperature, the oxide scale thickness l and other parameters. The time law is the integration of the rate law over the reaction time and gives in our example the weight gain as a function of the time and the other parameters mentioned,

$$\Delta m\int_{{t_0}}^t {vdt} = \Delta m({p_{{O_2}}},T,l, \ldots t).$$

((2.2))

If we try to understand the reaction mechanism hidden behind the expiremental data on an atomistic scale, we have to develop a reaction model from first principles which is able to formulate theoritical rate and time laws and to compare them with the curves measured. Metal-gas interactions are complex heterogeneous chemical reactions composed of several individual partial reactions. These partial steps can precede parallel or one behind the other and they are linked together by the concentrations of intermediate reaction products. As long as only one of these steps is the bottle neck of the whole process and, therefore, rate determining the rate and time law can be determined easily. We need only the equilibrium constants of preceding and succeding patial steps and the rate law of the rate determining partial reation.