CHAPTER 16 CHEMICAL KINETICS (Part 2)

CHAPTER 16 CHEMICAL KINETICS (Part 2) - Experimental...

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Experimental Determination of the Rate Law Expression The orders (exponents) in the rate law expression may or may not correspond to the coefficients in the balanced equation. There is no way to predict the order of the reaction from the balanced equation. This must be calculated using experimentally determined data. Problems on page 653-655 illustrate the general method for determining the order of reaction. Note: 1. If doubling the concentration of reactant 'A' (while the concentration of the other reactant is held constant) doubles the rate of reaction, then the reaction is first order with respect to 'A'. 2. If doubling the concentration of reactant 'A' (while the concentration of the other reactant is held constant) quadruples the rate of reaction, then the reaction is second order with respect to 'A'. 3. Once the exponents in the rate law have been determined, the rate constant can be evaluated from any set of data. 4. The unit for rate of reaction is always M/time (also written as mol.L -1 .time -1 5. The unit of k for a reaction depends on the order of the reaction. Relation between Concentration of Reactants and Time. (a) For First Order Reactions For a first order reaction of the type: A Products, Rate = k[A]. Integrating this equation gives: kt A A t = ] [ ] [ ln 0 where [A] 0 is the concentration of A at time t = 0, [A] t is the concentration of A at time 't' from the start of the reaction and k is the rate constant. The above equation can be rewritten as ln[A] t = -kt + ln[A] 0 Plotting the natural logarithm of the concentration of A versus time gives a straight line for first order reactions. The slope of this graph = -k, and the intercept on the Y-axis = ln[A] 0. (You might be able to derive the equations in this section from what you learned in Calculus. If you do not know how this equation is obtained, read on. You will not be asked to recall this equation.)
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Half-Life of a First Order Reaction: The time required for a reactant to reach half its original concentration is called the half- life or a reaction and it is designated by the symbol t ½ . When t = t ½ , [A] t = ½[A] 0 Therefore, kt A A t = ] [ ] [ ln 0 now can be written as ln(2) = kt ½ or     t ½     =   k 693 . 0
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