Elementary Chemical Kinetics (25.1-25.2)

1. Elementary Chemical Kinetics (25.1-25.2) • Kinetics is the study of how reactions occur • Speed of reaction depends on frequency of productive collisions between molecules (concentration, temperature, nature of productive collision) • Many reactions involve more than one step, so a mechanism is used to explain how the reaction occurs • Reaction rates are measured as the speed with which a reactant is consumed or a product is created • Reaction rate is a differential equation since we are looking at a change in concentration in a given amount of time • One typically monitors either decay of one reactant or the production of a single product • Accomplished through absorbance, fluorescence, pH, etc.

2. Rate Laws and Reaction Mechanisms (25.3-25.4) • We know from experience that reaction rates often depend on concentration of reactants • Rate can be expressed as a product of reactant concentrations of certain orders • Order for each reactant is not necessarily the stoichiometric coefficient (α ≠ a) • Rate constant (k) must contain information about temperature and productive collisions • Overall order of the reaction is the sum of the orders for each reactant and must be determined experimentally • The rate can be determined by measuring the change in concentration of a reactant/product over a short range of time (tangent to curve is rate) • The order for each reactant can be obtained by changing the concentrations of a single species and monitoring the change in rate (isolation method, method of initial rates) • Reaction mechanism is a set of elementary reactions that can be used to explain a rate law • Order of reactants in an elementary rate law is the stoichiometric coefficient • Mechanism is only viable if the sum of elementary rate laws match overall rate law

3. Integrated Rate Law – First Order (25.5) • Differential form of rate equation can be combined with rate law to give a relation between concentration and time • First-order reactions (elementary) only involve a single reactant to first-order • Integrated rate law shows the concentration of A decays exponentially with time • Linearized plots can be used to determine order • Natural logarithm is used to get time out of the exponent • If a plot of ln[A] vs. time gives a line, then the reaction is first order and the slope of the line is related to the rate constant • Half-life is a measure of how long it takes for the concentration of reactant to decay to 50% and can also be used to indicate order of reaction • Half-life of first-order reaction is independent of concentration of reaction

4. Integrated Rate Law – Second Order (25.5) • One type of second-order reaction (elementary) only involves one reactant • Type I second-order reactions show that time is related to the inverse of [A] • The half-life of this reaction is dependent on the initial concentration of reactant • If a plot of 1/[A] vs. time is generated and gives a line, then the reaction is second-order and the slope is related to the rate constant • Another type of second-order reaction involves two reactants (Type II) • This integrated rate law involves concentrations of both reactants • This can be reduced to a Type I rate law if [A]0 = [B]0

5. Concentrations of Product and Reactant During Reaction

6. Determination of Reaction Rate

7. First-Order Reaction

8. Second-Order Reaction (Type I)