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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 Æ 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 ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Δ + = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Δ − = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Δ − = t t t ] Δ [NH 2 1 ] Δ [H 3 1 ] Δ [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 Æ 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 Æ 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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This note was uploaded on 04/17/2008 for the course CHEM 020 taught by Professor Griffith during the Fall '07 term at UWO.
 Fall '07
 Griffith
 Kinetics

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