chapter 13 notes

chapter 13 notes - Chapter 13 Chemical Kinetics Rates and...

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©2010 Donald L. Siegel, All Rights Reserved Chapter 13 Chapter 13 Chemical Kinetics: Rates and Mechanisms of Chemical Reactions
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©2010 Donald L. Siegel, All Rights Reserved Rates of reaction Rates of reaction W Up to now, we’ve only considered whether a reaction not how fast. W The study of the rates of reactions is called kinetics. W We define the reaction rate as the change in concentration of a species per unit time [ ] t A t t t time at A of ion concentrat t time at A of ion concentrat A Rate Δ Δ = = 1 2 1 2 ) (
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©2010 Donald L. Siegel, All Rights Reserved Stoichiometry and rates Stoichiometry and rates W Consider the reaction 3A + B 2P W What do we mean by the rate of reaction? W Answer will depend on what we are measuring. A will disappear three times faster than B P will be produced twice as fast as B disappears 1/3(- [A]/ t) = 1/1(- [B]/ t) = ½( P/ t) x x 1 rate reaction of rate ν = ν x is the stoichiometric coeffecient of x
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©2010 Donald L. Siegel, All Rights Reserved Average and Instantaneous Rates Average and Instantaneous Rates W Rates are usually given as positive numbers For a reactant, Rate = - [A]/ t W Problem with the equation we’ve used is that it’s an average. Rates can change quickly. W Use instantaneous rate The instantaneous rate is the tangent of the [ ] vs. time curve. In calculus terms, d[A]/dt In practical terms, find average over a short time span
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©2010 Donald L. Siegel, All Rights Reserved
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©2010 Donald L. Siegel, All Rights Reserved Rate Laws Rate Laws W Rates of reactions are not random. W They can be quantified. Any reaction behaves the same way under the same conditions The mathematical description of the kinetic behavior is called a rate law Rate = k[A] a [B] b [C] c a, b, c need not have anything to do with the stoichiometry. W Misinformation in book! Products can appear in the rate law Most often, they don’t.
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©2010 Donald L. Siegel, All Rights Reserved More about rate laws More about rate laws W Rate = k[A] a [B] b [C] c Exponents a, b, c, … are the order of the reaction with respect to A, B, C, etc. The order can be anything (integer, fraction). The sum of the exponents a + b + c + … is the overall order of the reaction k is called the rate constant. Don’t confuse it with the rate. W This form of the rate law is called the differential rate law Calculus term. It’s a differential equation There is also an integrated form [] [] [] [] K c b a C B A k dt A d =
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©2010 Donald L. Siegel, All Rights Reserved Units of k Units of k W The rate of reaction has units of amount/time M/s atm/s mol/s W The units of k can be found from the rate and the total order 1 st order: M/s = k M, k is s -1 2 nd order: k is M -1 s -1
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©2010 Donald L. Siegel, All Rights Reserved Determining the rate law Determining the rate law W To avoid the complication of reverse reactions, frequently data is taken early in the reaction. Called method of initial rates
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chapter 13 notes - Chapter 13 Chemical Kinetics Rates and...

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