Kinetic Considerations and Reaction Rate Laws

Despite this usefulness, thermodynamic considerations have limitations, and these most often are apparent in environmental systems at lower temperatures, in biological systems, and in the description of reactions at phase boundaries. Thermodynamics applies to chemical processes among large numbers (i.e., Avogadro s number) of molecules and deals with overall reactions among a set of chemical species. Strictly speaking, equilibrium thermodynamics provide no information about how a chemical system reached its current state. 

The earth s subsurface is not at complete thermodynamic equilibrium, but parts of the system and many species are observed to be at local equilibrium or, at least, at a dynamic steady state. For example, the release of a toxic contaminant into a groundwater reservoir can be viewed as a perturbation of the local equilibrium, and we can ask questions such as. What reactions will occur How long will they take and Over what spatial scale will they occur Addressing these questions leads to a need to identify actual chemical species and reaction processes and consider both the thermodynamics and kinetics of reactions. 

For any chemical reaction, whether inorganic or organic, we must choose which kinetic species to include in the elementary reactions that make up the overall process ideally, molecular or chemical information is available to guide this choice. In general, for an elementary (irreversible) reaction among species A and B, to give species C and D, in relative amounts a, b, c, and d, respectively. 

In a first-order reaction, the rate-determining step involves a transformation where one reactant reacts to give one product, that is, A — B. In first-order reactions, there is an exponential decrease in the reactant concentration, so that at any given time, the transformation rate is dependent on the corresponding concentration of the reactant at the same time. This can be expressed in the following way  

When the reaction rate is not dependent on the reactant concentration, the reaction is zero order  

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