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1 • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Experiment 10 Chemical Kinetics Discovery Lab CH 204 Fall 2008 Dr. Brian Anderson • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Last week in lab Titration curves Changing composition of a weak acid solution as strong base is added pH meters, pH standards Witnessed the awesome power of a buffer solution to resist changes in pH • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Thermo vs Kinetics Thermodynamics is concerned only with where you start and where you finish. Kinetics is all about how you get there: reaction rates and reaction mechanisms (the individual steps of a reaction).2 • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Reaction rate How quickly a reactant disappears, or how quickly a product forms. Δ[A] d[A] Δt dt How fast the reaction goes depends on the concentrations of the reactants. This is expressed in an equation we call a rate law. = Rate = - - • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Rate Laws Rate laws tell you how the rate of a reaction (M/sec) depends on the concentrations. For the reaction A + B → C, a typical rate law is Rate = k[A][B] Most rate laws are 1 st or 2 nd order. (Sum of the exponents equals 1 or 2.) This example is 1 st order in A and B, and 2 nd order overall. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Reaction Mechanisms The actual step-by-step chemical pathways that reactions take. These often include intermediate products and catalysts that don‛t appear in the overall equation – but they might appear in the rate law.3 • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Catalysts and Intermediates catalysts – consumed early in a reaction and regenerated in the same amount later. No net change in concentration. reaction intermediates – produced early in a reaction and then consumed later. Even though they are essential to the reaction pathway, neither of these show up anywhere in the overall reaction. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level In the reaction A + B → C A might not actually react with B at all, because the mechanism could be something like A + catalyst → Intermediate Intermediate + B → C + catalyst In this case the rate law might include a catalyst that isn‛t in the overall reaction. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Elementary Reactions Each individual step in a reaction mechanism is called an elementary reaction. There are no “hidden participants” in an elementary reaction, and the rate law is always exactly what you would expect: For aA + bB → cC, the rate law is Rate = k[A] a [B] b4 • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Rate Laws for overall reactions Generally speaking they are often exactly what you would expect, but there are enough exceptions out there that that‛s not a safe bet. Rate laws can‛t be predicted from overall reaction equations, and are determined empirically. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level “All right, men... ...here‛s what we‛re gonna take and do.” 125 mL of unknown solution 2.5 g of unknown solid 2 drops of methylene blue indicator Mix ‛em up and watch the reaction go. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Your mission, if you decide to accept it... Determine the mechanism of the reaction. Break the overall reaction into individual elementary reactions. Identify the role of the unknown solid and any other reactants, catalysts, and intermediates in the system. (The unknown liquid does NOT play a role.)5 • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Things to watch for Rates of reactions – what happens quickly? What happens slowly? How does doubling the concentration of a reactant affect the rate of the reaction? Which changes in the reactants result in the formation of more product? How do you know when you‛ve formed more product? What is the limiting reagent in the reaction? • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level “My brain hurts!” The answers aren‛t in the lab manual, and your TA won‛t just hand them to you either. You‛ve got to noodle your way through to the answer on this one. Make sure you understand the mechanism before you leave the lab. Your TA will not answer questions about the mechanism once lab is over. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level In your notebook Record all your data and observations. Part 15 – simple graphs. Two or three points is enough. No need for Excel, just draw a simple graph in your lab notebook. Record all observations directly into your lab notebook.6 • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Before you leave the lab Look over the discussion questions and make sure you know the answers. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Well, that lecture wasn‛t very helpful, was it? No, it wasn‛t. • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level Special Announcements NO LECTURE NEXT WEEK NO QUIZ NEXT WEEK Check out of your drawer at the end of lab today. Experiment 10 reports are due by the beginning of lab time next week. If you are doing a make-up lab today, bring your TA evaluation to the stockroom so they can get it in the right envelope.7 • Click to edit Master text styles – Second level • Third level – Fourth level » Fifth level More Special Announcements IF YOU HAVE A MAKE-UP LAB Go directly to lab at your normal lab time next week. I will post make-up lab room assignments on the course web page and on signs posted outside the labs.


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