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UT CH 302 - Kinetics I

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CH 302 1st Edition Lecture 18Outline of Last Lecture I. Beta DecayII. Band of StabilityIII. WorksheetIV. Interpreting the GraphV. Half-Life QuestionsOutline of Current Lecture I. Clicker Questions II. KineticsIII. Reaction RatesIV. Method of Initial ratesCurrent LectureClicker Question:1. Hydrogen-3 (tritium, H-3) is sometimes formed in the primary coolant water of a nuclearreactor. Tritium is a beta emitter with a t1/2 = 12. 3 years. For a given sample containing tritium, after how many years will only about 12% of the sample remain?A) 12.3 years B) 24.6 years C) 36.9 years D) 49.2 years E) 61.5 yearsKinetics:- How fast chemical reactions occur- Measure the rates of reactions (macroscopic) gives us insight into the way reactions are actually happening (microscopic)Reaction Rates:- There are average, instantaneous, and initial reaction rates- Instantaneous rate: slope of the tangent line- Initial rate: the beginning of the graph- Average rate: Initial rate at t = 0, best to define and compare, you can define concentrations with confidence Method of Initial Rates/Determining Rate Laws:- You can't simply look at an overall reaction and know the associated rate law. Instead you must measure it in the lab.- The easiest way to do this is to run a series of experiments with different initialconditions. - Since the rate of a reaction can vary with time, we compare the very initial rate of the reaction. - This also avoids any complications with changes in rate due to backwards reactions.- By changing the initial concentrations, we can see what concentrations affect the rate and in what way. For example for the reaction- The concentration dependence of the rate can be determined by comparing the different experiments. - For example the difference between experiments #1 and #2 is that the CO concentrationhas been doubled in the 2nd experiment. o Looking at the initial rate measured, it is clear that it is the same in both experiments. o Thus the rate is independent of CO concentration. o We would say the rate is zeroth order in CO. o The NO2 dependence can be determined by comparing experiments #2 and #3. Here the NO2 concentration has been increased by a factor of 3 in the third experiment compared to the second. o This increases the initial rate. o If we compare the initial rate from experiment 3 to experiment 2, we see that therate has increased by a factor of nine. o Since we increase the concentration by 3 and the rate increase by 9 we know the rate is dependent on the NO2 concentration squared. o Or we would say the reaction is second order in NO2. Finally, we could also say this rate is second order overall.- We could also determine a value for the rate constant since we now know the rate


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UT CH 302 - Kinetics I

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