Reaction Rate | Research & Encyclopedia Articles

Reaction Rate

The rate of a chemical reaction is typically specified as the rate of formation of a particular product of the reaction or the rate of disappearance of a particular reactant. These two rates are the same for some reactions, such as those in which the reaction of every molecule of a particular reagent leads to the production of one molecule of a particular product. Many reactions are more complex and are characterized by different rates for the different species involved in the reaction.

In order to be able to predict the rate of a reaction, the affect of changing the concentrations of each species involved in the reaction is determined experimentally. The experimental information is summarized in an equation called the rate law for the reaction. It expresses the change in concentration of a particular species as a function time in terms of the concentration, or concentrations, of species involved in the reaction. The rate law is usually written in terms of concentrations that can be determined experimentally. The rate law equation also contains one or more rate constants that are multiplied by the concentration terms. The rate constants account for the probability that the specific reaction takes place under the conditions the reaction rate are measured, the more probable and faster the reaction, the higher the value of the rate constant. The units of the rate constant are determined by the concentration terms of the rate law and the time units used.

A term that is closely related to reaction rate is the "half- life" of a reaction. The term half-life is very commonly used in reference to radioactive decay reactions, but it is a useful concept for other reactions as well. The half-life of a reaction is the time required to convert half the initial concentration of a reagent to product. The shorter the half-life, the faster the reaction rate. The half-life is given the symbol t_1/2. For a first order reaction the half- life is easily calculated: it is equal to the natural logarithm of 2 (ln₂ = 0.693) divided by the rate constant. For second or higher order reactions the calculation is considerably more complex.

To alter the rate of a reaction, the kineticist typically determines the step of a reaction that is the slowest, the rate determining step. Once the rate determining step is identified, reaction conditions can often be modified to make it proceed faster and thereby speed up the entire reaction.

One method of speeding up reactions is to use a catalyst, a species that increases the rate of a chemical reaction but is not changed by the process. Ideally, the catalyst can be recovered in its original form when the reaction is over. The catalyst increases the rate of a step by providing an alternate route, allowing the overall reaction to be accomplished without proceeding through the slow, rate-determining step.

Radioactive isotopes are typically characterized by their half-lives. Isotopes with short half-lives (hours) are often useful for medical diagnosis, while isotopes with long half-lives are useful for archaeological and geological dating and as time standards. The half-life is used to determine how many nuclear transformations occur per mole and, hence, per gram of the isotope. The Table gives the half-lives for several important radionuclides. Extensive tables of half-lives and the corresponding nuclear processes are available in reference sources such as the Handbook of Chemistry and Physics published by the Chemical Rubber Company each year.

Rates of reactions are affected by factors other than concentration: they are also often highly dependent on temperature and many reactions can be affected by catalysts. Chemical reactions require a minimum or threshold energy, called the activation energy. At higher temperature more molecules have an energy at or above this activation energy. Hence the probability that a reaction will occur, and the reaction rate, increases with an increase in temperature.

The rate of a reaction can also be increased by use of an appropriate catalyst. A catalyst provides a different pathway for a reaction that has a lower activation energy. Hence, at a particular temperature more molecules will have the lower activation energy available through the catalytic pathway to products and the reaction via the catalytic pathway will be faster. Although the catalyst must be intimately involved in the reaction as part of the activation process, it is not consumed in the process.

In complex systems in which there are many possible reactants, a catalyst for one reaction may become a reactant for another and will, therefore, be consumed by the other reaction. This situation occurs in catalytic converters in automobiles when contaminants in gasoline react with the catalyst and "poison" it, rendering the converter inefficient or useless.