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Unformatted text preview: 15.1 15. Chemical Kinetics (text Ch 11) A. Introduction Chemical kinetics is the study of how quickly chemical reactions proceed and the factors that affect the rate. The rate of a chemical reaction depends on not only the nature of the reactants themselves, but also on the: o Concentration of the reactants o Temperature o Presence of a catalyst Rates of chemical reactions are usually defined by comparing the change in reactant or product concentration over time. Consider the reaction N 2 + 3 H 2 2 NH 3 We could measure the rate at which N 2 and H 2 are consumed and the rate at which NH 3 is produced. However, H 2 will be used up three times faster than N 2 , and NH 3 will be produced twice the speed that N 2 is used up. Thus, the rate can be defined as + = = = t t t ] [NH 2 1 ] [H 3 1 ] [N Rate 3 2 2 By defining rate this way, the value of rate is independent of the species being observed. 15.2 B. Concentration Effects The effect of concentration is described using rate laws , which are mathematical expressions that link the rate of a reaction with reactant concentration. Consider the reaction A + B + C products The rate of the reaction is given by the equation below, where Rate = k [A] x [B] y [C] z o concentrations are in molarity o k is the rate constant for the reaction at a given temp o and the exponents can only be found experimentally The exponential values determine the order of the reaction. They are usually whole numbers (but not always). o 2 = Second order o 1 = First order o 0 = Zero order (rate of the reaction is independent of the concentration of that reagent) Remember, the exponents are experimentally measured and DO NOT correlate to the coefficients in the reaction equation. 15.3 How do we experimentally determine the exponents? We usually use a technique that allows us to monitor the changes in a reactant or product. Suppose we have a reaction where a gas is evolved. We can measure the volume of gas produced as a function of time. The initial rate of gas evolution will be the slope at time zero. Usually, when we measure rates, we measure initial rates , which are the rates observed immediately after the reactants are mixed. 15.4 Suppose we have the following reaction 2 H 2 + 2 NO 2 H 2 O + N 2 Rate = k [H 2 ] x [NO] y The strategy is to: 1. vary one concentration at a time and monitor the rate 2. evaluate the exponents separately 3. combine the exponents into a rate law 4. substitute one data set to find k Collecting experimental data, we have initial rates Expt # [H 2 ] (mol L 1 ) [NO] (mol L 1 ) Rate i 0.1 0.2 5 ii 0.2 0.2 10 iii 0.1 0.4 20 The exponents can be evaluated by logically thinking about the data (the preferred way), or by doing some math....
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 Fall '07
 Griffith
 Kinetics

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