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Prepared by NGUYEN Dinh Lam DUT - UD1NG HA HC VHA L H PHN TN- Ti liu tham kho chnh:1. Ha l v Ha Keo, Nguyn Hu Ph, Nh Xut bn Khoa hcv K thut, 20032. Bi tp Ha l c s, Lm Ngc Thim, Trn Hip Hi, NguynTh Thu, Nh Xut bn Khoa hc v K thut, 20033. Physical Chemistry, Third Edition, Robert G. Mortimer, Elsevier Inc., 20084. Physical Chemistry Understanding our Chemical Word, Paul Monk, John Wiley & Sons, Ltd, 2004Prepared by NGUYEN Dinh Lam DUT - UD2Phn 1: ng ha hcPrepared by NGUYEN Dinh Lam DUT - UD3Mt s khi nim c bn- ng ha hc l mn hc nghin cu v tc v c ch ca qu trnh ha hc.- Nghin cu ng hc: Nng sut v Cng ngh camt qu trnh.Prepared by NGUYEN Dinh Lam DUT - UD4Mt s khi nim c bn-Tc phn ng (Reaction rates)- nh lut tc dng khi lng (Rate Law).- Bc phn ng (Reaction Order).- Phn ng nguyn t (Elementary step).- Phn t s (Molecularity).- C ch phn ng (Reaction Mechanism).- L thuyt va chm (The Collision Theory).- Nng lng hot ha (Activation Energy).-Xc tc (Catalyst).Prepared by NGUYEN Dinh Lam DUT - UD5Part 1: Chemical KineticsPrepared by NGUYEN Dinh Lam DUT - UD6Kinetics Studies the rate at which a chemical process occurs. Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).At 298K, reaction: H2(g)+ 1/2O2(g)= H2O(l) Go298(r)= -254,8 kJ.mol-1 Thermodynamics does a reaction take place?Kinetics how fast does a reaction proceed?Prepared by NGUYEN Dinh Lam DUT - UD7Outline: KineticsReaction Rates How we measure rates.Rate LawsHow the rate depends on amounts of reactants.Integrated Rate LawsHow to calc amount left or time to reach a given amount.Half-lifeHow long it takes to react 50% of reactants.Arrhenius Equation How rate constant changes with T.MechanismsLink between rate and molecular scale processes.Prepared by NGUYEN Dinh Lam DUT - UD8Chemical Kinetics(Simple Homogenous reaction)Reaction rate is the change in the concentration of a reactant or a product with time (M/s).A Brate = - [A]trate = [B]t[A] = change in concentration of A overtime period t[B] = change in concentration of B overtime period tBecause [A] decreases with time, [A] is negative.Prepared by NGUYEN Dinh Lam DUT - UD9A Brate = - [A]trate = [B]ttimePrepared by NGUYEN Dinh Lam DUT - UD10Exercise 1: Br2(aq) + HCOOH (aq) 2Br-(aq) + 2H+(aq) + CO2(g)time393 nmlightDetector[Br2] Absorption393 nmBr2(aq)Prepared by NGUYEN Dinh Lam DUT - UD11average rate = - [Br2]t= -[Br2]final [Br2]initialtfinal- tinitialslope oftangentslope oftangentslope oftangentinstantaneous rate = rate for specific instance in timeExercise 1: Br2(aq) + HCOOH (aq) 2Br-(aq) + 2H+(aq) + CO2(g)Prepared by NGUYEN Dinh Lam DUT - UD12rate [Br2]rate = k [Br2]k = rate[Br2]= rate constant= 3.50 x 10-3s-1Exercise 1: Br2(aq) + HCOOH (aq) 2Br-(aq) + 2H+(aq) + CO2(g)Prepared by NGUYEN Dinh Lam DUT - UD13Exercise 2: 2H2O2(aq) 2H2O (l) + O2(g)PV = nRTP = RT = [O2]RTnV[O2] = PRT1rate = [O2]t RT1 Pt=measure P over timePrepared by NGUYEN Dinh Lam DUT - UD14Exercise 2: 2H2O2(aq) 2H2O (l) + O2(g)Prepared by NGUYEN Dinh Lam DUT - UD151. The absorbance of radiation at some wavelength at which a given product or reactant absorbs.2. The intensity of the emission spectrum of the system at a wavelength at which a given product or reactant emits.3. The volume of a solution required to titrate an aliquot removed from the system.4. The pressure of the system (for a reaction at constant volume).5. The volume of the system (for a reaction at constant pressure).6. The electrical conductance of the system.7. The mass spectrum of the system.8. The ESR or NMR spectrum of the system.9. The dielectric constant or index of refraction of the system.10. The mass loss if a gas is evolved.The classical methods for determining the reaction ratePrepared by NGUYEN Dinh Lam DUT - UD16Reaction Rates and StoichiometryFactors That Affect Reaction Rates Concentration of Reactants As the concentration of reactants increases, so does the probability that reactant molecules will collide. Temperature At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. Catalysts Speed reaction by changing mechanism.Prepared by NGUYEN Dinh Lam DUT - UD17Reaction Rates The average rate of the reaction over each interval is the change in concentration divided by the change in time:C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)Average Rate, M/s[ ][ ]sMtCl H Crate averagetCl H Crate average0 . 0 0 . 500905 . 0 1000 . 09 49 4= = =Prepared by NGUYEN Dinh Lam DUT - UD18Reaction Rates Note that the average rate decreases as the reaction proceeds. This is because as the reaction goes forward, there are fewer collisions between reactant molecules.C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)Prepared by NGUYEN Dinh Lam DUT - UD19Reaction Rates A plot of concentration vs. time for this reaction yields a curve like this. The slope of a line tangent to the curve at any point is the instantaneous rate at that time.C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)Prepared by NGUYEN Dinh Lam DUT - UD20Reaction Rates The reaction slows down with time because the concentration of the reactants decreases.C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)Prepared by NGUYEN Dinh Lam DUT - UD21Reaction Rates and Stoichiometry In this reaction, the ratio of C4H9Cl to C4H9OH is 1:1. Thus, the rate of disappearance of C4H9Clis the same as the rate of appearance of C4H9OH.C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq)Rate =-[C4H9Cl]t= [C4H9OH]tPrepared by NGUYEN Dinh Lam DUT - UD22Reaction Rates and Stoichiometry What if the ratio is not 1:1?H2(g) + I2(g) 2 HI(g) 2 mol of HI are made for each mol of H2used.Prepared by NGUYEN Dinh Lam DUT - UD23Reaction Rates and Stoichiometry To generalize, for the reactionaA + bB cC + dDReactants (decrease) Products (increase)Prepared by NGUYEN Dinh Lam DUT - UD24Write the rate expression for the following reaction:CH4(g) + 2O2(g) CO2(g) + 2H2O (g)rate = - [CH4]t= - [O2]t12= [H2O]t12= [CO2]tPrepared by NGUYEN Dinh Lam DUT - UD25The Rate LawThe rate law expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers.aA + bB cC + dDRate = k [A]x[B]yreaction is xth order in Areaction is yth order in Breaction is (x +y)th order overallPrepared by NGUYEN Dinh Lam DUT - UD26Concentration and RateEach reaction has its own equation that gives its rate as a function of reactant concentrations. this is called its Rate LawTo determine the rate law we measure the rate at different starting concentrations.The Rate LawPrepared by NGUYEN Dinh Lam DUT - UD27F2(g) + 2ClO2(g) 2FClO2(g)rate = k [F2]x[ClO2]yDouble [F2] with [ClO2] constantRate doublesx = 1Quadruple [ClO2] with [F2] constantRate quadruplesy = 1rate = k [F2][ClO2]Example:This equation is called the rate law, and k is the rate constant.Prepared by NGUYEN Dinh Lam DUT - UD28Rate Laws A rate law shows the relationship between the reaction rate and the concentrations of reactants. For gas-phase, reactants use PAinstead of [A]. k is a constant that has a specific value for each reaction. The value of k is determined experimentally.Constant is relative here - k is unique for each reaction, k changes with Trate = k [F2][ClO2]Prepared by NGUYEN Dinh Lam DUT - UD29F2(g) + 2ClO2(g) 2FClO2(g)rate = k [F2][ClO2]Rate Laws Rate laws are always determined experimentally. Reaction order is always defined in terms of reactant (not product) concentrations. The order of a reactant is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation.1Prepared by NGUYEN Dinh Lam DUT - UD30Exercise 3: Determine the rate law and calculate the rate constant for the following reaction from the following data:S2O82-(aq) + 3I-(aq) 2SO42-(aq) + I3-(aq)Experiment [S2O82-] [I-]Initial Rate (M/s)1 0.08 0.034 2.2 x 10-42 0.08 0.017 1.1 x 10-43 0.16 0.017 2.2 x 10-4rate = k [S2O82-]x[I-]yDouble [I-], rate doubles (experiment 1 & 2)y = 1Double [S2O82-], rate doubles (experiment 2 & 3)x = 1k = rate[S2O82-][I-]=2.2 x 10-4 M/s(0.08 M)(0.034 M)= 0.08/Msrate = k [S2O82-][I-]Prepared by NGUYEN Dinh Lam DUT - UD31Experiment [S2O82-] [I-]Initial Rate (M/s)1 0.08 0.034 2.2 x 10-42 0.08 0.017 1.1 x 10-43 0.16 0.017 2.2 x 10-4rate = k [S2O82-]x[I-]yy = 1x = 1rate = k [S2O82-][I-] Exponents tell the order of the reaction with respect to each reactant. This reaction isFirst-order in [S2O82-]First-order in [I-] The overall reaction order can be found by adding the exponents on the reactants in the rate law. This reaction is second-order overall.Exercise 3: Determine the rate law and calculate the rate constant for the following reaction from the following data:S2O82-(aq) + 3I-(aq) 2SO42-(aq) + I3-(aq)Prepared by NGUYEN Dinh Lam DUT - UD32Integrated Rate LawsFirst order reactionConsider a simple first order reaction: A BHow much A is left after time t? Integrate:Differential form:Prepared by NGUYEN Dinh Lam DUT - UD33The integrated form of first order rate law:Can be rearranged to give:[A]0is the initial concentration of A (t=0).[A]tis the concentration of A at some time, t, during the course of the reaction.Integrated Rate LawsFirst order reactionPrepared by NGUYEN Dinh Lam DUT - UD34First-Order ReactionsManipulating this equation produceswhich is in the form y = mx + bPrepared by NGUYEN Dinh Lam DUT - UD35First-Order ReactionsIf a reaction is first-order, a plot of ln [A]tvs. t will yield a straight line with a slope of -k.So, use graphs to determine reaction order.Prepared by NGUYEN Dinh Lam DUT - UD36Exercise 4: The reaction 2A B is first order in A with a rate constant of 2.8 x 10-2s-1at 800C. How long will it take for A to decrease from 0.88 M to