C401Ch12LN1 - Chapter 12: Chemical Kinetics Part 1 Chemical...

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Chapter 12: Chemical Kinetics Part 1 Chemical kinetics is the study of how fast chemical reactions occur and how they occur. Four important factors affect rates of reactions: Concentration of reactants. Temperature of reactions. Presence or absence of a catalyst. Surface area of solid or liquid reactants or catalysts. Goal: to understand chemical reactions at the molecular level. Reaction Rates The speed of a reaction is defined as the change that occurs per unit time. It is often determined by measuring the change in concentration of a reactant or product with time. The speed of the chemical reaction is its reaction rate . For a reaction A ± B Here the change in the number of moles of B is defined as: ² (moles of B) = (moles of B at final time) ³ (moles of B at initial time) Illustrate this with an example: Suppose A reacts to form B. Let us begin with 1.00 mol A. • At t = 0 (time zero) there is 1.00 mol A and no B present. • At t = 10 min, there is 0.74 mol A and 0.26 mol B. • At t = 20 min, there is 0.54 mol A and 0.46 mol B. Eventually, there will be no more A left, and only B will be present. We can use this data (and others determined at other times) to find the average rate: For the reaction A ± B there are two ways of measuring rate: The rate of appearance of product B (i.e., change in moles of B per unit time) as in the preceding example. The rate of disappearance of reactant A (i.e., the change in moles of A per unit time). Note the minus sign! This reminds us that rate is being expressed in terms of the disappearance of a reactant. Thus, rates are always positive numbers. in time change B of moles of number in the change rate Average = min mol 026 . 0 0 10 0 0.26 rate Average = ± ± = t ± ± ² = A) of (moles rate Average Average rate = ± (moles B) ± t = (moles of B at t = 10 min) ² (moles of B at t = 0 min) 10 min ² 0 min
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Rates in Terms of Concentrations In most chemical reactions we will determine the reaction rate by monitoring a change in concentration (of a reactant or product).
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This note was uploaded on 06/25/2008 for the course CHEM 400-401 taught by Professor Dr.samples during the Fall '06 term at American River.

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C401Ch12LN1 - Chapter 12: Chemical Kinetics Part 1 Chemical...

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