# Elementary Chemical Kinetics ( )

## Presentation on theme: "Elementary Chemical Kinetics ( )"— Presentation transcript:

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.

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

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

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

Concentrations of Product and Reactant During Reaction

Determination of Reaction Rate

First-Order Reaction

Second-Order Reaction (Type I)