Formally-Kinetic Description of One- and Two-Step Reactions
We must accept that in order to describe the chemical system it is urgent for us to know the exact way it follows during the transformation of the reagents into the products of the reaction. Knowledge of that kind gives us a possibility to command chemica
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Formally-Kinetic Description of One- and Two-Step Reactions
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Main Concepts of Chemical Kinetics
We must accept that in order to describe the chemical system it is urgent for us to know the exact way it follows during the transformation of the reagents into the products of the reaction. Knowledge of that kind gives us a possibility to command chemical transformation deliberately. In other words, we need to know the mechanism of the chemical transformation. Time evolution of the transition of the reactionary system from the unconfigured state (parent materials) to the finite state (products of the reaction) is of great importance too, because it is information of how fast the reaction goes. Chemical kinetics is a self-contained branch of chemical knowledge, which investigates the mechanisms of the reactions and the patterns of their passing in time, and which gives us the answers to questions from above. Chemical kinetics together with chemical thermodynamics forms two corner stones of the chemical knowledge. However, classic thermodynamic approach to the description of the chemical systems is based on the consideration of the unconfigured and finite states of the system exceptionally, with the absolute abstracting from any assumptions about the methods (ways) of the transition of the system from the unconfigured to the finite state. Thermodynamics can define whether the system is in equilibrium state. If it is not, than thermodynamics states that the system would certainly pass into the equilibrium state, because the factors for such transition exist. Still, it is impossible to predict, what the dynamics of such transition would be, that is in what time the equilibrium state will come, in terms of classic thermodynamic approaches. Such problems are not in interest of thermodynamics, and the time coordinate is absolutely extraneous to the thermodynamics approach. This is the distinction of kind between thermodynamic and kinetic methods of the description of the chemical systems. The mainframe notion of the chemical kinetics is the rate of the reaction. Reaction rate is defined as the change in the quantity of the reagent in time unit, referred to the unit of the reactionary space. The concept of reactionary space differs depending on the nature of the reaction. In the homogeneous system the
V.I. Korobov and V.F. Ochkov, Chemical Kinetics with Mathcad and Maple, DOI 10.1007/978-3-7091-0531-3_1, # Springer-Verlag/Wien 2011
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1 Formally-Kinetic Description of One- and Two-Step Reactions
reaction is carried out in the whole volume, whereas in the heterogeneous system – at the phase interface. In mathematic terms: r¼
dn ðhomogeneous reactionÞ; Vdt
r¼
dn ðheterogeneous reactionÞ: Sdt
Derivative’s side here formally shows if the current substance is accumulated or consumed during the reaction. If the volume of the system in the homogeneous reaction is constant (closed system), then dn/V ¼ dC, and therefore, the rate is interrelated to the change of the molar concentration of the reagent in time: r¼
dC
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