Chem 1120 1st Edition Lecture 6 Outline of Last Lecture I Reaction Rates II Concentration and Rate III Average Rate vs Instantaneous Rate A Definitions Examples B Reaction Order and Units of Rate Constants Outline of Current Lecture I Method of Initial Rates A Deriving the Rate Law II Concentration and Time A Quick Logarithm Review B Reaction Orders III Half Life Current Lecture I Rate laws must be determined experimentally and what will usually happen is that your professor will give you a bunch of data from an experiment and you will have to use the data given to figure out the rate law etc The method of initial rates determines reaction orders from the effect of changing a reactant s concentration on the initial rate of the reaction Example Given the reaction 2N2O5 4NO2 O2 g These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute EXPERIMENT N2O5 O mol L INITIAL RATE mol L s 1 0 45 M 2 7 X 10 4 2 0 90 M 5 4 X 10 4 Rate k N2O5 A The easiest way to explain how to derive the rate law is to show an example I will debrief it first and then once you read over the example it will make more sense To find the rate law for a reaction you must use the experimental data and use the reactants and products to put the law into the correct form Let s say the equation is A B C The rate law would look something like this r k reactant A n reactant B m product C p r rate k constant n 1 m 1 p 2 order we will learn how to find these exponents below Example Deduce the rate law for the reaction from the following experimental data BrO3 aq 5Br aq 6H aq 3Br2 l 3H2O l EXPERIMENT BrO3 O Br O H O INITIAL RATE mol L s 1 0 10 M 0 10 M 0 10 M 8 X 10 4 2 0 20 M 0 10 M 0 10 M 1 6 X 10 3 3 0 20 M 0 20 M 0 10 M 3 2 X 10 3 4 0 10 M 0 10 M 0 20 M 3 2 X 10 3 Solution Assume a rate law of the form rate k BrO3 n Br m H p How can n m and p be found use two experiments data EXPERIMENT BrO3 O Br O H O INITIAL RATE mol L s 1 0 1 M 0 1 M 0 1 M 8 X 10 4 2 0 2 M 0 1 M 0 1 M 1 6 X 10 3 II A lot of times we are interested in the rate or speed of an equation but sometimes instead we want to know how the concentration of a reactant or product change with time not the rate Calculus can help us derive these re lationships between the reaction rates and concentrations as a function of time A In case you do not remember logarithms B Equations expressing the relationship between concentration and time can be derived by calculus The resulting equations depend on the rate law used Zero Order Reactions A bB Products First Order Reactions A bB Products Second Order Reactions A bB Products Example At 300 oC is the reaction NO2 g NO g 1 2 O2 g 1st or 2nd Order III Half life t1 2 the time required for the concentration of a reactant to decrease to one half of its initial value After one half life A 1 2 A o m Zero Order Reactions First Order Reactions Second Order Reactions REVIEW OF ZERO FIRST AND SECOND ORDERS ON NEXT PAGE
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