CHEM 1120 1st Edition Lecture 8 Outline of Last Lecture 1 Half lives 2 Collision model theory 3 The Arrhenius Equation Outline of Current Lecture 1 Reaction mechanisms 2 Reaction mechanisms and rate laws 3 Catalysts introduction Current Lecture 14 6 Reaction Mechanisms Reaction mechanism o The sequence of elementary results in an overall chemical reaction Elementary step o 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 Laws o Since elementary steps are simple collision processes their rate laws are determined by their molecularity o Molecularity How many molecules are involved in the process A mechanism must satisfy two requirements o 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 rate o All elementary steps do not have the same rate o 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 others o 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 reaction o 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 Catalyst o Substances that increases the rate of a chemical reaction without undergoing permanent chemical change o 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 catalysis o Catalyst and reactants in different phases o 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 Enzymes o Biological catalysts o Most large proteins with molar masses from 10 4 to 10 6 g mol o Common mechanism 1 substrate enzyme enzyme substrate complex 2 enzyme substrate complex enzyme products