Kinetics

Kinetics - KINETICS I. Introduction. A. Need for kinetics....

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1 KINETICS I. Introduction. A. Need for kinetics. 1. Information not available from thermodynamics or the balanced chemical equation. a. The speed of the reaction. Rate of the reaction. 1) Rate = time rate of change of the molar concentration of a reactant or product. 2) For the reaction 2NO(g) + 2H 2 (g) -----> N 2 (g) + 2H 2 O(g) Rate = [H 2 O] t = - [NO] t = - [H 2 ] t = 2 [N 2 ] t 3) Dimensions = mol/ Ls or Ms -1 4) Rate is always a positive number. b. How the reaction takes place. That is, the sequence of bond breaking and bond making by which reactants are converted into products. 1) Sequence of steps = Mechanism . 2) Written as a series of simple elementary (one step) reactions that sum to give the balanced chemical equation. 2. Kinetics = Study of the rates and mechanisms of reactions. --Rates are measured experimentally -- Mechanisms are postulated to explain the experiments. B. Instantaneous Rate. 1. For most reactions, plots of the concentrations of reactants or products versus time are not straight lines. This means that the rate changes with time. a. In most all cases the rate of a reaction will decrease as reactants are consumed and products are formed. b. Plot of concentration of a product vs. time for a reaction. Concentration time C 1 C 2 C 3 C 4 ! t ! t ! t ! t
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2 Rate 1 = ! t C 1 - 0 ; Rate 2 = ! t C 2 - C 1 ; Rate 3 = ! t C 3 - C 2 ; Rate 4 = ! t C 4 - C 3 1 > 2 3 > Rate 4 2. Instantaneous rate = rate at a given instant in time = slope of the concentration vs. time plot at that time. If for the reaction 2NO(g) + 2H 2 (g) ------> N 2 (g) + 2H 2 O(g) the rate was measured by following the appearance of H 2 O, then the instantaneous rate at a particular time will be the slope of the [H 2 O] vs. time plot at that time. L ! t " 0 im ! t ! [H 2 O] = dt d[H 2 Instantaneous Rate = dt d[H 2 C. Variation of rate with concentration - Rate Law. 1. Rate Law = mathematical equation that gives the rate as a function of concentration. Must be determined experimentally. 2. In many cases the rate law for a reaction such as aA + bB ------> dD + eE is of the rather simple form Rate = k[A] x [B] y a. x = the order of the reaction in A y = the order of the reaction in B x+y = overall order of the reaction. b. x and y are usually small positive integers, they must be determined experimentally. c. Example: Suppose the rate law was found to be of the form Rate = k[A] 2 [B] x = 2 the reaction is said to be second order in A y = 1 the reaction is said to be first order in B x+y = 3 the reaction is overall third This is a 3 rd order reaction that is 2 nd order in A and 1 st order in B. 3. k = Specific rate constant = The proportionality constant between the rate and concentrations. a. Its value depends on the particular reaction.
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3 b. It is independent of concentration. c. It increases as the temperature increases. 1) The temperature dependence of k is given empirically by the Arrhenius equation k = Ae - RT E a where A = a temperature independent constant E a = activation energy ( dimensions = J/mol; different for different reactions) R = gas constant = 8.314 J/mol K.
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This note was uploaded on 04/07/2008 for the course CHEM 1304 taught by Professor Prof.maguire during the Spring '08 term at SMU.

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Kinetics - KINETICS I. Introduction. A. Need for kinetics....

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