Some fundamental ideas of kinetics

It is ordinarily observed that the majority of reactions that lead to a lower over all energy state take place readily. However, there are many reactions that lead to a decrease in energy yet the rates of the reactions are low. For example, the heat of formation of water is 285 Kj/mol, but the reaction

H2(g)+1/202(g0

Takes place very slowly, if at all, unless the reaction is started by a spark. The reason for this is that although a great deal of energy is released, there is no low energy pathway for the reaction to follow. In order for water to from, molecules of H2 and O2 must react, and their bond energies are about 435 and 490 kI/ mol, respectively.

Thermodynamics is concerned with the overall energy change for a process. If necessary , this change can result after an infinite time. Accordingly, thermodynamic does not deal with the subject of rates of reactions, at least not directly. The example above shows that the thermodynamics of the reaction favors the production of water; however, kinetically the process in unfavorable. We see here the first of several important principles of chemical kinetics. There is no necessary correlation between thermodynamics and kinetics. Some reactions that are energetically favorable take place very slowly because there is no low energy pathway for the reaction.

One of the features of the study of reaction rates is that a rate can not be calculated. Theory is not developed to the point where it is possible to calculate how fast most reactions will take place. For some very simple gas phase reaction, it is possible to calculate approximately how fast the reaction should take place, but details of the process must still be determined experimentally.

The rate of a chemical reaction is expressed as a change in concentration of some species with time. Therefore, the units of the rate must be those of concentration/time . A reaction such as has a rate which can be expressed either in terms of the disappearance of a or the appearance of B. Because the concentration of B is increasing with time, the rate is expressed as +(change in [B]/change in time) or +d [B]/dt.