117 notes-2010 - Chapter 13 Chemical Kinetics Rates of...

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Chapter 13 Chemical Kinetics: Rates of Reactions I. Introduction A. To completely describe a chemical reaction it is necessary to understand two factors 1. Whether the reaction will occur (thermodynamics) 2. How fast the reaction will occur (kinetics) 3. Example: Hydrogen peroxide is unstable but does not decompose rapidly at room temperature unless a catalyst (potassium iodide) is added. Then the decomposition becomes rapid. a. Decomposition reaction 2 H 2 O 2 (aq) → 2 H 2 O(l) + O 2 (g) a. Does the potassium iodide affect the kinetics or thermodynamics of the reaction? B. Chemical kinetics is a study of the rates of chemical reactions and by what mechanism they occur. 1. Reaction rate: Increase in the concentration of a product or the decrease in the concentration of a reactant per unit time, i.e. the amount of a product formed or a reactant used up per unit time. 2. Reaction mechanism: The pathway (series of molecular interactions) by which a reaction occurs. 1
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II. Reaction Rate A. A chemical reaction occurs when atoms, ions and/or molecules collide and form new atoms, ions and/or molecules. If these species are all in the same phase, the reactions are referred to as homogeneous. If the species are in different phases, the reactions are referred to as heterogeneous. B. The rate at which a chemical reaction occurs is a function of 1. The properties of the reactants, in particular their molecular structure and bonding. 2. The concentrations of the reactants. 3. The temperature at which the reaction occurs. 4. The presence of a catalyst. C. The reaction rate is measured by a change of concentration over a given time. This concentration may be for a reactant or for a product and the concentrations are related by the stoichiometry of the reaction. 2
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1. Example: Consider the following reaction a. H 2 (g) + 2 ICl(g) → I 2 (g) + 2HCl(g) b. Concentration data at 230 ° C time (t) [H 2 ] [ICl] [I 2 ] [HCl] rate = - [H 2 ]/ t (seconds) (mol/L) (mol/L) (mol/L) (mol/L) 0 1.000 2.000 0.000 0.000 1 0.674 1.348 0.326 0.652 2 0.526 1.052 0.474 0.948 3 0.436 0.872 0.564 1.128 4 0.374 0.748 0.626 1.252 5 0.328 0.656 0.672 1.344 6 0.293 0.586 0.707 1.414 7 0.265 0.530 0.735 1.470 8 0.242 0.484 0.758 1.516 c. The reaction rate is determined by taking the difference in the concentrations of the reactant or product and dividing by the difference in the time. Because rates are always positive, the rates calculated from reactants are multiplied by –1. 3
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d. The reactions rates are consistent with the stoichiometry of the reaction. rate of decrease in [H 2 ] = 0.5 rate of decrease in [ICl] = rate of increase in [I 2 ] = 0.5 rate of increase in [HCl] or e. The reaction rate changes during the course of the reaction because the reactants are being used up as shown in the below figure. 4
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D. Effect of concentration on the reaction rate. The rate law
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