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UI CHEM 1120 - Reaction Mechanisms and Catalysis

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CHEM 1120 1st Edition Lecture 8Outline of Last Lecture 1. Half lives2. Collision model theory3. The Arrhenius EquationOutline of Current Lecture 1. Reaction mechanisms2. Reaction mechanisms and rate laws3. Catalysts introductionCurrent Lecture14.6 Reaction Mechanisms- Reaction mechanismo The sequence of elementary results in an overall chemical reaction- Elementary stepo A simple reaction with a well-defined rate law that depends on the number of molecules (also, elementary reaction or elementary process- If a reaction equation represents an elementary step, then the reaction coefficients are related to the orders of the rate law- Through mechanisms, rate laws gain chemical significance… we do not just have a mathematical rule, but more importantly molecular level insight- Elementary steps and Their Rate Lawso Since elementary steps are simple collision processes, their rate laws are determined by their molecularityo Molecularity How many molecules are involved in the process- A mechanism must satisfy two requirementso The sum of the elementary steps must result in the overall reaction 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.o The mechanism must be consistent with the experimentally determined rate law (experiments only provide evidence, not proof)- Determining the rateo All elementary steps do not have the same rateo Usually one of the elementary steps is slower than the others and thus limits the overall rate of the reaction; called rate-determining step (or rate-limiting step)o In a multistep process, one of the steps will be slower than all otherso The overall reaction cannot occur faster than this slowest, rate-determining step- Reaction intermediate o A substance that is formed and uses up during the overall reactiono Intermediate concentration should not appear in the rate law- From the Ch. 14 powerpoint, slides 68 through 72, reaction mechanism exercises with detailed pictures and explanations how to do each exercise.14.7 Catalysis - Catalysto Substances that increases the rate of a chemical reaction without undergoing permanent chemical changeo Catalysts are extremely important in industry producing NH3, gasoline, plastics, etc. numerous catalyst-related Nobel Prizes- Homogenous catalysis o Catalyst and reactants in the same phase- Heterogeneous catalysiso Catalyst and reactants in different phaseso Catalytic converter found in the exhaust of an automobile Converts hydrocarbons, CO and nitrogen oxides into CO2, H2O and N2 Used is Pt and Rh deposited on Al2O3- Enzymeso Biological catalystso Most large proteins with molar masses from 10^4 to 10^6 g/molo Common mechanism: 1) substrate + enzyme enzyme-substrate complex 2) enzyme-substrate


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