# Exam 2 Review - 1 Practiceproblems theprelabhomework...

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5/2/20111Review for test 2Experiments 471How to study for the test1. Practice problems – the prelab homeworkproblems.2. Lecture notes3Lab reportcalculations and theory3. Lab reportcalculations and theory4. Worksheets – calculations and theory2Experiment 4KineticsS2O32–(aq) + 2H3O+(aq)SO2(g) + S(s) + 3H2O(l)The solid sulfur product forms acolloidal suspension, producing thewhiteout condition.3By measuring the time it took to achieve whiteoutconditions, you experimentally determined thereaction orders and the activation energy Ea.S2O32–(aq) + 2H3O+(aq)SO2(g) + S(s) + 3H2O(l)[S][S]dTime at whichXdisappearsinstantaneous rate = dS/dt (tangent)average rate =S/t (chord)initial rate = dSi/dt (tangent at start) (no reverse rxn)average initial rateS/t = Sd/t (chord near start)4timeInitial rateS2O32–(aq) + 2H3O+(aq)SO2(g) + S(s) + 3H2O(l)rate = k[S2O32–]m[H3O+]nThe instantaneous initial rateis the rate of the forwardreaction.There are no products initially, so no reverse reaction ispossiblepossible.The average initial rate is primarily the rate of theforward reaction.The rate is proportional to the inverse of the time ittakes to reach whiteout conditions.5[S]d/t = k[S2O32–]m[H3O+]n1/t =k*[S2O32–]m[H3O+]nwherek*= k/[S]dandk=Aexp(Ea/RT)andmandnare experimental valuesThere are 3 things that could change the rate of reaction:[S2O32–][H3O+]kkYou determine the amount that they change the rate bychanging only one at a time.Changing the concentrations one at a time lets you determine thereaction orders.Changing the temperature lets you determine the activation energy.NOTE: Look at the prelab hw and your worksheetassignment to be sure you can do the calculations.6
5/2/20112Temperature dependence andactivation energy, Eak=Aexp(Ea/RT)(Arrhenius equation)With [S2O32–] and [H3O+] constant, the rate is proportional tok1/t = C’exp(Ea/RT)aln(1/t) = – Ea/RT + lnC’y = mx + bA plot of ln(1/t) as a function of 1/T gives a straight linewith a slope of –Ea/R(t = time, T = temperature)Eais positive for all elementaryreactions.NOTE: Look at the prelab hw and your worksheetassignment to be sure you can do the calculations.7When Eais positive, the reaction rate constantincreases with increasing temperature.As the temperature increases, more energy isavailable for the reactants.This energy can be used toovercome an activationReactionintermediateovercome an activationbarrier.
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