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CORNELL CHEM 2080 - Chemistry 2080: General Chemistry II Quiz

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© Melissa A. Hines, 2020 Group Quiz 2 Chemistry 2080: General Chemistry II Spring 2020 Due: Thursday, February 13 Deposit in box outside Baker 131 between 8:00 am – 2:00pm Before submitting your group quiz, please staple a completed cover sheet, which lists your group number, the chair and all participating members of your group, to the top of your problem set. Blank cover sheets will be available next to the assignment box outside Baker 131 and from the course website. Please show all work and state all assumptions. 1. NO2 Chemistry One of the principal components of air pollution are the nitrous oxides, which are often abbreviated as NOx. At room temperature, one of these gases, nitrogen dioxide or NO2, is stable indefinitely. But when the gas is raised to a temperature of 313°C (at time t = 0), the data pictured at right were obtained. (a) Assuming that only NO2, NO and O2 are in the bulb, what (balanced) chemical reaction is occurring? (Show your work and explain your reasoning.) (b) What is the initial rate of NO2 reaction? The rate 80 seconds after heating begins? The average rate over the 0–100 second time range? (c) How are the rates of NO2 disappearance and the rates of NO and O2 appearance related? Back up your answer with explicit calculations. Explain this relationship. (d) Graphically determine the order of the NO2 reaction rate and the rate constant. 2. Experimental Determination of a Rate Law You have been asked to experimentally determine the rate law for the chemical reaction 2 HgCl2 (aq) + C2O42– (aq) → 2 Cl– (aq) + 2 CO2 (g) + Hg2Cl2 (s). You have stock solutions of HgCl2 and C2O42–, each with a concentration of exactly 0.200 M. You can add arbitrary volumes of each solution to your reaction flask as well as an arbitrary volume of H2O. There is one catch, though. Your experimental technique is not perfect, so your volumes will have a few percent random error (similar to what will actually happen in lab). For example, two identical experiments will not give you exactly the same initial rate. Design your experiment carefully to minimize the effects of your errors! (a) In a few sentences, describe your strategies (plural!) for choosing volumes and minimizing the effects of experimental errors. Hint: You will be designing a similar set of experiments in the lab, albeit with a different chemical reaction. Now is the time to think about a strategy for your volume choices (including total volumes), as working a problem on paper is much faster than working one in the lab! 20151050Concentration/M x 10–3100806040200Time/s NO2 NO O2Chem 2080 — Group Quiz 2 page 2 of 3 © Melissa A. Hines, 2020 (b) Use the rate calculator web page (linked on Group Quizzes and Exams page) to “measure” the initial reaction rate as a function of experimental parameters. Use these rates to calculate the full rate law for the reaction. Your answer should include a table of your experimental parameters and results. 3. A New Integrated Rate Law You have come up with a new kinetic model for the reaction of molecule A which predicts an integrated rate law of the form where [A]0 is the concentration of A at time t = 0, and k is a constant. (a) Derive an expression for the (initial) half-life of A in terms of [A]0 and any other constants. (b) You would like to test your data graphically to see if they obey this rate law. To do this, you need to put the integrated rate law in linear form, then plot your data. What should you plot on the x- and y-axes? (Back your answer up with the appropriate equation.) If the data are indeed linear when plotted in this fashion, how can you extract the rate constant from a linear fit to your data? 4. The Pharmacokinetics of Celexa Nerve cells are not physically connected. Instead, there is a small gap between one nerve cell and the next. To transmit a signal, a nerve cell releases a chemical, known as a neurotransmitter, that then diffuses to the next nerve cell and binds to it. After a certain number of neurotransmitters bind to the nerve cell, the nerve triggers. When the neurotransmitter is finished, it is released from the second cell and reabsorbed by the first. Celexa (citalopram) inhibits the readsorption of one specific neurotransmitter — serotonin. Some think depression leads to a short supply of this neurotransmitter and that Celexa allows each serotonin molecule to remain active for a longer period of time. Celexa is decomposed in the liver (80%) and kidneys (20%). Because of this decomposition, Celexa has a measured half-life in the body of 35.0 hours. In truth, the mechanism of Celexa decomposition is complicated, but for our purposes, we will assume that Celexa has a constant half-life that is independent of dosage. For this problem, please express your concentrations in units of ng/ml and time in units of hours. (a) What is the rate law for the decomposition of Celexa in the body (including the rate constant)? What is the integrated rate law? Write your integrated rate law in the form “[Celexa]t = …” (b) Suppose you have a patient who has never taken Celexa before. You instruct them to take one 20 mg Celexa capsule every day at 7 AM. You may assume that each capsule is immediately absorbed into the blood (a bad A[ ]= A[ ]02− 2ktDose(ng/ml)1 6:59 AM1 7:01 AM 251 7:00 PM2 6:59 AM2 7:01 AM 252 7:00 PM3 6:59 AM3 7:01 AM 254 6:59 AM4 7:01 AM 255 6:59 AM5 7:01 AM 256 6:59 AM6 7:01 AM 257 6:59 AMTimeDayChem 2080 — Group Quiz 2 page 3 of 3 © Melissa A. Hines, 2020 assumption) where it increases the concentration of Celexa in their plasma by 25.0 ng/ml. [In other words, if a patient with a Celexa concentration of 31.0 ng/ml takes one capsule, their concentration will immediately rise to 56.0 ng/ml.] Calculate your patient’s Celexa level at the times listed in the table. (c) Graph your patient’s Celexa concentration for the first 6 full days (i.e., up to 8:01 am on Day 7). Note: If you complete the extra credit successfully, you can use that graph to satisfy part (c). (d) Drugs like Celexa often take a week or more to take full effect. Based on your calculations, give one explanation for this. Would there be any advantages to a new antidepressant with the same efficacy as Celexa but a longer half-life? Explain your reasoning. (Extra credit)


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