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Elementary Chemical Kinetics (25.1-25.2) Kinetics is the study of how reactions occur – Speed of reaction depends on frequency of productive collisions.

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Presentation on theme: "Elementary Chemical Kinetics (25.1-25.2) Kinetics is the study of how reactions occur – Speed of reaction depends on frequency of productive collisions."— Presentation transcript:

1 Elementary Chemical Kinetics ( ) 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 productmonitors – Accomplished through absorbance, fluorescence, pH, etc.

2 Rate Laws and Reaction Mechanisms ( ) 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)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 Linearized – 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 reactantsecond-order reaction – 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)


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