Presentation on theme: "Nanochemistry NAN 601 Dr. Marinella Sandros Lecture 5: Kinetics"— Presentation transcript:
1 Nanochemistry NAN 601 Dr. Marinella Sandros Lecture 5: Kinetics Instructor:Dr. Marinella SandrosLecture 5: Kinetics
2 Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed?Reaction rate is the change in the concentration of a reactant or a product with time (M/s).A Brate = -[A]t[A] = change in concentration of A overtime period trate =[B]t[B] = change in concentration of B overtime period tBecause [A] decreases with time, [A] is negative.
3 Chemical Reaction Rates Why are we interested in reaction rates?• If we wish to synthesize a new compound, or design a process for the industrial preparation of a valuable chemical substance, we would need to ensure that the reaction/s concerned take place at a reasonable rate.• Studies of the rates of chemical reactions can tell us an enormous amount about exactly how the reactions take place at the molecular level, i.e., such studies provide information about the reaction mechanism.
4 Factors that Affect Reaction Rate TemperatureCollision Theory: When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place.Kinetic Theory: Increasing temperature means the molecules move faster.Concentrations of reactantsMore reactants mean more collisions if enough energy is presentCatalystsSpeed up reactions by lowering activation energySurface area of a solid reactantBread and Butter theory: more area for reactants to be in contactPressure of gaseous reactants or productsIncreased number of collisions
5 Reaction Rates•The rate of a reaction could be monitored by measuring the increase in the amount of a reaction product with time.• Suppose that a substance A reacts to form B• For a reaction A→B
14 Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) slope oftangentslope oftangentslope oftangentaverage rate = -[Br2]t= -[Br2]final – [Br2]initialtfinal - tinitialinstantaneous rate = rate for specific instance in time as opposed over an interval of time
15 rate = k [Br2]k =rate[Br2]= rate constant= 3.50 x 10-3 s-1
16 The 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 overall
17 Concentration and Rate Determination of order – Initial rate MethodA reaction is zeroth order in a reactant if the change in concentration of that reactant produces no effect.• A reaction is first order in a reactant if doubling the concentration of that reactant causes the rate to double.• A reaction is nth order in a reactant if doubling the concentration of that reactant causes an 2n increase in rate.• Note that the rate constant does not depend on concentration.
18 F2 (g) + 2ClO2 (g) FClO2 (g)rate = k [F2]x[ClO2]yDouble [F2] with [ClO2] constantRate doublesx = 1rate = k [F2][ClO2]Quadruple [ClO2] with [F2] constantRate quadruplesy = 1
19 Run # Initial [A] ([A]0) Initial [B] ([B]0) Initial Rate (v0) 1 1.00 M 1.25 x 10-2 M/s22.00 M2.5 x 10-2 M/s3What is the order with respect to A?What is the order with respect to B?What is the overall order of the reaction?11
20 Initial Rate (mol dm-3 s-1) [NO(g)] (mol dm-3)[Cl2(g)] (mol dm-3)Initial Rate (mol dm-3 s-1) 0.250 1.43 x 10-60.500 2.86 x 10-61.14 x 10-5What is the order with respect to Cl2?What is the order with respect to NO?What is the overall order of the reaction?123
21 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.F2 (g) + 2ClO2 (g) FClO2 (g)1rate = k [F2][ClO2]
24 First-Order Reactions Rate depends on the concentration of a single reactant raised to the 1st power.rate = -[A]t[A] = [A]0e-ktrate = k [A][A] is the concentration of A at any time t[A]0 is the concentration of A at time t=0ln[A] - ln[A]0 = - ktln[A] = - k t + ln[A]0Y = m X + b
25 First-Order Reactions A graph ln [A] vs T gives a straightLine with a slope –k and a y-interceptof ln [A]0
29 Second-Order Reactions Rate depends on the reactant concentration raise the second power or on the concentrations of two different reactants, each raised to the first power.rate = -[A] t[A] is the concentration of A at any time trate = k [A]2[A]0 is the concentration of A at time t=0Half life for second order1[A]-[A]0= ktt½ = t when [A] = [A]0/2t½ =1k[A]0
30 [A] - [A]0 = kt Zero-Order Reactions [A] rate = - rate = k [A]0 = k [A] is the concentration of A at any time t[A] - [A]0 = kt[A]0 is the concentration of A at time t=0Half life for zero ordert½ = t when [A] = [A]0/2t½ =[A]02k
31 Summary of the Kinetics of Zero-Order, First-Order and Second-Order ReactionsOrderRate LawConcentration-Time EquationHalf-Lifet½ =[A]02krate = k[A] - [A]0 = - ktt½Ln 2k=1rate = k [A]ln[A] - ln[A]0 = - kt1[A]-[A]0= ktt½ =1k[A]02rate = k [A]2
32 Quiz # 3A+B CExperimentNumber[A] (M)[B] (M)Initial Rate(M/s)10.1004.0 x 10-520.200316.0 x 10-5Using these data, determine (a) rate law for the reaction, (b) the magnitude of the rate constant; (c) the rate of the reaction when [A]= M and [B]= M.
33 Answer Rate= k [A]2 [B]0 = k [A]2 K= Rate/ [A]2 = 4.0 x 10 -3 M-1 s-1 Rate= k [A]2 = (4.0 x M-1 s-1)(0.05 M)2=1.0 x 10-5 M/s