Chemical kinetics is the study of rates of chemical processes and includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that can describe the characteristics of a chemical reaction. Faculty Feature Dr. Maria Dittrich's research explores the processes at the biosphere-geosphere interface, focusing on the geochemical cycling of carbon, nutrients, and metals in aquatic ecosystems. Her work examines how biogeochemical and physical processes, particularly at the sediment-water interface, influence nutrient release, pollutant dynamics, and carbon burial. Using field studies, lab experiments, and modeling, she investigates the impacts of seasonal variations, climate change, and human activities. Dr. Dittrich’s findings provide critical insights into aquatic ecosystem responses to environmental changes, highlighting the relevance of chemical kinetics and reaction mechanisms in understanding complex natural systems. Read more about Dr. Maria Dittrich and her research here: https://mdbiogeochemistry.com/ 20.1: Introduction The study of chemical kinetics concerns the rate at which a reaction yields products and the molecular-scale means by which a reaction occurs. In this chapter, we will examine the factors that influence the rates of chemical reactions, the mechanisms by which reactions proceed, and the quantitative techniques used to determine and describe the rate at which reactions occur. 20.2: Chemical Reaction Rates 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. 20.3: 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. 20.4: 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, and the overall order of a reaction is the sum of the orders for each substance present in the reaction. 20.5: Integrated Rate Laws Differential rate laws can be determined by the method of initial rates or other methods. We measure values for the initial rates of a reaction at different concentrations of the reactants. From these measurements, we determine the order of the reaction in each reactant. 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. 20.6: Collision Theory Chemical reactions require collisions between reactant species. 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. 20.7: Reaction Mechanisms The sequence of individual steps, or elementary reactions, by which reactants are converted into products during the course of a reaction is called the reaction mechanism. The overall rate of a reaction is determined by the rate of the slowest step, called the rate-determining step. Unimolecular elementary reactions have first-order rate laws, while bimolecular elementary reactions have second-order rate laws. 20.8: Catalysis Catalysts affect the rate of a chemical reaction by altering its mechanism to provide a lower activation energy. Catalysts can be homogenous (in the same phase as the reactants) or heterogeneous (a different phase than the reactants). 20.9: Key Terms 20.10: Key Equations 20.11: Summary 20.12: Exercises These end-of-chapter exercises are for practice to help you both understand concepts from the course and for practice for tests and exams.