This chapter introduces the experimental study of chemical reaction rates, showing how rate laws are used to describe the dependence of reaction rates on concentration, determine reaction order, and calculate concentrations, rate constants, and half-lives. It presents several experimental methods for determining rate laws, examines the effect of temperature through the Arrhenius equation, and introduces the concepts of activation energy and temperature-dependent reaction rates. Finally, the chapter provides an introduction to collision theory and transition state theory as theoretical models that explain why chemical reactions occur at the rates observed experimentally and prepares students for the study of reaction mechanisms in the next chapter.
Define the rate of a chemical reaction and relate the rates of appearance and disappearance of reactants and products using reaction stoichiometry. Chemical reaction rates can be expressed in terms of the disappearance of reactants or the appearance of products. Because these concentration changes are related by the stoichiometry of the balanced chemical equation, they all describe the same overall reaction rate.
Determine the order of a reaction and its rate constant from experimental concentration–time data using integrated rate laws, graphical methods, the method of initial rates, or half-life analysis. Reaction orders and rate constants cannot generally be predicted from the balanced chemical equation and must instead be determined experimentally. Several complementary methods are available, each using different types of kinetic data to identify the rate law.
Write and interpret rate laws, including identifying the order with respect to individual reactants and the overall reaction order. A rate law describes how the reaction rate depends on the concentrations of reactants (and sometimes products or catalysts). The exponents in the rate law define the order with respect to each species and the overall order of the reaction.
Use integrated rate laws to calculate concentrations, reaction times, half-lives, and rate constants for zeroth-, first-, and second-order reactions. Integrated rate laws describe how reactant concentrations change over time and provide a convenient way to analyze kinetic data. They can be used to predict concentrations, determine reaction times, and calculate rate constants and half-lives.
Describe the effect of temperature on reaction rates and apply the Arrhenius equation to determine activation energies or predict changes in rate constants with temperature. Reaction rates generally increase as temperature increases because a larger fraction of molecular collisions have sufficient energy to produce a reaction. The Arrhenius equation provides a quantitative relationship between the rate constant, temperature, and activation energy.
Describe how collision theory and transition state theory account for the temperature dependence of reaction rates and the role of activation energy in chemical reactions. Collision theory and transition state theory provide molecular-level explanations for why reactions occur at the rates observed experimentally. These models introduce the concepts of effective collisions, activated complexes, and activation energy, laying the foundation for the study of reaction mechanisms in the next chapter.