Chemistry Week 15

Chemistry Week 15 - Unit #4 1 (6) Chemical Kinetics...

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Unit #4 1 -- Chemical Processes -- (6) Chemical Kinetics TEXTBOOK READING : BLB-10 , Chapter 14, 21.4 Practice PROBLEMS: (Ch 14) 16, 20, 24, 28, 43, 50, 66, 72; (Ch 21) 34, 40, 42 Chemists have developed ideas about predicting and understanding what chemical changes occur. To characterize or optimize a chemical process, we often need to know: (1) how fast a reaction occurs (= rate ); and (2) what molecular steps occur during the process (= mechanism ). The study of reaction rates aids in determining mechanisms. Knowing these details of a chemical reaction allows us to make changes in reaction conditions (concentrations, temperature, etc.) which will speed up or slow down a process (without guessing). For example, we need to make useful compounds (nylon, ethylene glycol, etc.) in a reasonable time (speed up these reactions); or we might want to prevent unwanted reactions like corrosion. The behavior of a chemical system may be determined by thermodynamic (equilibrium effects) or by reaction rates. Consider the following two different systems: (a) To an equimolar aqueous solution of Cl (aq) and I (aq), a small amount of Ag + (aq) is added. The following equilibria are in effect: Ag + (aq) + Cl (aq) ZZX YZZ AgCl(s) K 1 = 5.6 × 10 9 Ag + (aq) + I (aq) AgI(s) K 2 = 1.2 × 10 16 Here, since K 2 > K 1 , AgI(s) will precipitate from solution but AgCl(s) will not. The reason for this observation is thermodynamics – chemical equilibrium greatly favors the formation of AgI(s) over AgCl(s). (b) Dehydration of ethanol by sulfuric acid depends on temperature and acid strength: CH 3 CH 2 OH 24 0 conc. H SO 170 C ⎯⎯⎯⎯→ CH 2 CH 2 + H 2 O H +40 kJ (Ethylene) 2 CH 3 CH 2 OH 0 dil. H SO 140 C ⎯⎯⎯⎯→ CH 3 CH 2 OCH 2 CH 3 + H 2 O H 0 kJ (Diethyl ether) Here, the production of ethylene proceeds faster at 170 ° C in concentrated acid than the production of diethyl ether (it also depends on the lower pH – there is a greater concentration of H + ). NOTE: there is also a thermodynamic affect – the more endothermic process occurs at higher temperature. What are the important factors that determine how quickly a chemical reaction occurs? Two vital parameters are amounts of reactants involved, as measured by their concentrations, and temperature. In the Iodine Clock demonstration, two colorless solutions are mixed until the mixed solution becomes dark, due to the formation of a starch-iodide (tri-iodide) complex. The net reaction carried out in class is
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Unit #4 2 -- Chemical Processes -- IO 3 (aq) + 3 HSO 3 (aq) ⎯→ I (aq) + 3 HSO 4 (aq). In the mechanism for this reaction, I 2 (aq) is generated, which is also reduced to iodide ions. The demonstration is run, first, at three different levels of concentration: (i) “full strength;” (ii) 90% “full strength;” and (iii) 70% “full strength.” For this case, the most concentrated solutions completed the reaction soonest. The second run used identical concentration levels but at three different temperatures: (i) from an ice bath (cold); (ii) room temperature (intermediate); and (iii) from a hot water bath (hot). Here, the highest temperature solution completed the reaction first. In
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Chemistry Week 15 - Unit #4 1 (6) Chemical Kinetics...

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