CHEM 1120 1nd Edition Lecture 7 Outline of Last Lecture I. Half-Lifea. Equations for each orderII. How can we control the rate of a chemical reaction?III. Temperature and Ratea. Collision Modelb. Activation Energyc. Arrhenius EquationOutline of Current Lecture I. Arrhenius Equationa. How do we know it is true?II. Reaction Mechanismsa. Elementary stepb. Molecularityc.Current LectureI. Arrhenius Equationa. How do we know it is true?i. Measure k at different temperaturesii. Plot k vs tempiii. Must be exponentialiv. Plot ln k = -E/R(1/T) + ln (A) (should be linear)1. Slope: -E/RII. Reaction Mechanismsa. Reaction mechanism: the sequence of elementary steps resulting in an overall chemical reactionb. Elementary step: a simple reaction having no intermediates and having only one transition state (also elementary reaction or elementary process)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.c. If you are going to write a rate law, only want to write it in terms of things you know (things that go in and go out), don’t measure intermediatesi. Intermediates? NOT an elementary stepii. A B, BC, B would be an intermediate stepd. If a reaction equation represents an elementary step then the reaction coefficients are related to the orders of the rate lawe. How is a mechanism judged as acceptable?i. Sum of elementary steps must result in overall reactionii. Mechanism must be consistent with the experimentally determined rate lawf. Since elementary steps are simple collision processes, their rate laws are determined by their molecularityi. Unimolecular: A products, Rate=k[A]ii. Bimolecular: A+A or A+Bproducts Rate= k[A]^2 or k[A][B]iii. Termolecular: A+A+A or A+A+B or A+B+Cproductsg. Rate-determining step: a mechanism’s slowest stepi. Determines the overall reaction rateii. Larger activation energy = slower
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