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946 Chapter 17 | Kinetics
reaction mechanism stepwise sequence of elementary reactions by which a chemical change takes place reaction order value of an exponent in a rate law, expressed as an ordinal number (for example, zero order for 0,
first order for 1, second order for 2, and so on)
termolecular reaction elementary reaction involving the simultaneous collision and combination of three reactant species
unimolecular reaction elementary reaction involving the rearrangement of a single reactant species to produce one or more molecules of product
Key Equations
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integrated rate law for zero-order reactions:
integrated rate law for first-order reactions:
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• •
• • •
17.1 Chemical Reaction Rates
integrated rate law for second-order reactions:
Summary
The rate of a reaction can be expressed either in terms of the decrease in the amount of a reactant or the increase in the amount of a product per unit time. Relations between different rate expressions for a given reaction are derived directly from the stoichiometric coefficients of the equation representing the reaction.
17.2 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. If temperature or reactant concentration is increased, the rate of a given reaction generally increases as well. A catalyst can increase the rate of a reaction by providing an alternative pathway that causes the activation energy of the reaction to decrease.
17.3 Rate Laws
Rate laws provide a mathematical description of how changes in the concentration 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 concentration of each substance affects the overall rate, and 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.
17.4 Integrated Rate Laws
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.
The half-life of a reaction is the time required to decrease the amount of a given reactant by one-half. The half-life
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