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Kinetics Module Overview

The rates of chemical reactions A rate is a measure of how some property varies with time. For instance, wage is a rate that represents the amount of money someone is paid for working a certain amount of time (for instance, $10/hour). A chemical reaction also has an associated rate. Different chemical reactions typically occur at different rates. The rates at which chemical reactions take place are vital to many processes, from the metabolism of organisms to the production of medications. Many factors influence the rates of chemical reactions. The ability to control reaction rates is important for many consumer and industrial products and processes.

Definition and measurement of reaction rates The rate of a chemical reaction is a measure of how much reactant is consumed, or how much product is produced, by the reaction in a given amount of time. Reaction rates are usually determined by measuring the time dependence of some property that can be related to reactant or product amounts. For example, rates of reactions that consume or produce gaseous substances are conveniently determined by measuring changes in volume or pressure. For reactions involving one or more colored substances, rates may be monitored via measurements of light absorption.

Variability of reaction rates The reaction rate of a particular chemical reaction can vary over time. In the case of reactants and products in solution, the reaction rate typically decreases with time as the concentration of the reactants in the solution decreases. At any specific time, the rate at which a reaction is proceeding is known as its instantaneous rate. The instantaneous rate of a reaction at “time zero,” or when the reaction commences, is its initial rate. The average reaction rate over some time interval is simply the change in the concentrations of the products (or reactants) divided by the time interval.

Factors affecting reaction rates The rate of a chemical reaction is affected by several parameters. Reactions involving two phases proceed more rapidly when there is greater surface-area contact. A liquid, for instance, will react more rapidly with a finely divided solid than with a large piece of the same solid. Increasing the temperature generally increases the rates of chemical reactions. Similarly, increasing the reactant concentration usually increases the reaction rate. A catalyst can increase the rate of a reaction by providing an alternative pathway that causes the energy required to decrease.

Rate laws Rate laws provide a mathematical description of how changes in the amount of a substance affect the rate of a chemical reaction. Rate laws are determined experimentally and cannot be predicted by reaction stoichiometry. The order of reaction describes how much a change in the amount of each substance affects the overall rate. The overall order of a reaction is the sum of the orders for each substance present in the reaction. Reaction orders are typically first order, second order, or zero order, but fractional and even negative orders are possible.

Differential and integrated rate laws Differential rate laws can be determined by the method of initial rates or other methods. The values for the initial rates of a reaction are measured at different concentrations of the reactants. From these measurements, the order of the reaction for each reactant is then determined. Integrated rate laws are determined by integration of the corresponding differential rate laws. Rate constants for those rate laws are determined from measurements of concentration at various times during a reaction.

Reaction half-life The half-life of a reaction is the time required to decrease the amount of a given reactant by one-half. The half-lives of reactions vary according to the order of the reaction. For a zero-order reaction, the half-life of the reaction decreases as the initial concentration of the reactant in the reaction decreases. For a first-order reaction, the half-life is independent of concentration. In the case of a second-order reaction, the half-life decreases as the concentration increases.

Collision theory Chemical reactions require collisions between the various reactants that are present. These reactant collisions must be of proper orientation and sufficient energy in order to result in product formation. Collision theory provides a simple but effective explanation for the effect of many experimental parameters on reaction rates. The Arrhenius equation describes the relation between a reaction’s rate constant and its activation energy, temperature, and dependence on collision orientation.

Reaction mechanisms A chemical reaction usually occurs in steps. Each of these steps is called an elementary reaction. A reaction mechanism consists of the sequence of elementary reactions, with the result that the reactants are converted into products during the course of the chemical reaction. The overall rate of a reaction is determined by the rate of the slowest step, called the rate-determining step. Chemical species that are produced in one step and consumed in a subsequent step are called intermediates.

Types of elementary reactions The molecularity of an elementary reaction is the number of reactant species (atoms, molecules, or ions) involved in that reaction. A unimolecular elementary reaction involves the rearrangement of a single reactant species to produce one or more products. Unimolecular reactions have first-order rate laws. A bimolecular elementary reaction involves the collision and combination of two molecules or atoms to form an activated complex in an elementary reaction. Bimolecular reactions have second-order rate laws. By comparing the rate laws derived from a reaction mechanism to that determined experimentally, the mechanism may be deemed either incorrect or plausible.

Catalysis The activation energy (Ea) of a reaction is the energy required for the reaction to take place. A catalyst speeds up the rate of a chemical reaction by lowering the activation energy. The catalyst is not consumed, but instead is regenerated in the process. Some chemical reactions that are thermodynamically favorable can occur without a catalyst, but are so slow that the reaction can only occur at a reasonable rate if a catalyst is present. Catalysts can be homogenous (in the same phase as the reactants), or heterogeneous (a different phase than the reactants).

How to study this module Read the syllabus or schedule of assignments regularly. Understand key terms; look up and define all unfamiliar words and terms. Take notes on your readings, assigned media, and lectures. Work all problems assigned and as many additional problems as possible. Discuss topics with classmates. Review your notes routinely. Make flow charts and outlines from your notes to help you study for assessments. Complete all course assessments.

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