JHU_GL14C_Lab.docx - 27 3 KINETICS CONCENTRATION DEPENDENCE...

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1O2 Pressure as a Function of Time706050403020100100 200 300 400 500 600 700Time (s)al rate region40Initi302010150200me (s)273. KINETICS – CONCENTRATION DEPENDENCEJHU INTRO. CHEM. LABSUMMER 2017You will have one lab period for this experiment.There is a video prelab lecture on Blackboard and aprelab assignment. You will complete a Chem21labspostlab assignment on this experiment.This experiment and the next experiment willgive you experience in determining reaction rates,reaction orders, activation energy, and the effect ofcatalysts on reaction rates. You will also observe howa catalyst reacts to form a reaction intermediate andthen the reaction intermediate disappears and thecatalyst reappears.The decomposition of hydrogen peroxide is aspontaneous reaction:2H2O2(aq) 2H2O(l) + O2(g)(1)However, hydrogen peroxide has a long shelf-life atroom temperature, indicating that the rate of reactionis very slow. The reaction rate can be increased usinga catalyst. Many species catalyze this reaction,including halides such as I- and Br-, transition metalscations such as Fe3+, Cu2+, and MnO2 and biocatalystssuch as yeast and catalase, which contain Fe3+-hemegroups, where heme is a large, cyclic molecule thattraps the iron ion in the center. Hemoglobin is anexample.You will observe the formation of oxygen as thereaction progresses using a pressure sensor in aconstant volume apparatus. Since pressure is yable todetermine the dependence of oxygen formation as afunction of time. You will then use the method ofinitial rates to determine the reaction order. To dothis, you will determine the tangent to the rate curveat the time the reaction starts (after a brief inductionperiod) as shown below:Figure 1: Sample rate of oxygen formationAn enlargement of the initial 200 seconds looks asfollows:Figure 2: The first 200 seconds from figure 1. Theinduction period in this example is the first 25seconds. Then, the initial rate region where the dataforms a straight line tangent to the curve is from 25– 125 seconds. After that, the rate begins to level offas reactants are consumed and competing reactionsmay occur.The initial rate method is easier to analyzecompared to analyzing the entire curve. It limits theanalysis to a region where concentrations of thereactants are close to their initial values. This avoidsany complications of products or intermediates thatmight affect the rate through competing reactions orequilibrium formation.The rate expression for the forward reactionfor the reaction of aA + bB cC + dD can bewrittenrate = k[A]m[B]n(2)where k is the rate constant and m and n are integersthat give the order of the reaction with respect to Aand B, respectively. The overall reaction order is m +n. Rate expressions and reaction orders depend onthe reaction mechanism. The stoichiometry of theoverall chemical equation does NOT determine thereaction orders. However, once the reaction ordersare known, information about the mechanism can bepostulated.0

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