Kinetics BLAHH – Flashcards
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| What is the order of a plot of reactant concentration vs. time that has a straight line? |
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| Zero |
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| What does not increase the value of the rate constant for a reaction? |
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| Increasing the [] of reactants |
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| A certain reaction has an enthalpy of -75kJ and an Ea of 40kJ. A catalyst lowers the activation energy of the forward reaction by 15kJ. What is the Ea of the reverse reaction with the same catalyst? |
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| Ea=40kJ Catalyst(-15)= 25kJ Reverse reaction(+75)= 100kJ |
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| What does a catalyst do? |
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| It increases the rate of the reaction and lowers the activation energy. (Makes the curve less steep.) |
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| What is Ea? |
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| The minimum energy required to turn reactants into products. |
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| At a certain temp., the first order decomposition of H202 exhibits these data: time (seconds, s)- 0 15 [H202] (mol L^-1) 2.0 1.0 At what time will the [H2O2]=0.50 mol L^-1? |
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| (This is first order, so you can't find the equation of the line). Concentration decrease by 1/2 every 15 seconds, so the concentration will be 0.50 after 30 SECONDS. |
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| The rate of a reaction with just two reactants is observed to double when the concentration of one reactant is doubled and the second reactant is held constant. The rate is also observed to increase by a factor of nine when the concentration of the second reactant is tripled, holding the concentration of the first reactant constant. What is the overall order for this reaction? |
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| [A]^2[B]= order of 3 |
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| Tritium decays by a first-order process that has half-life of 12.5 years. How many years will it take to reduce the radioactivity of a tritium sample to 15% of it original value? |
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| t 1/2= 0.693/k k=0.05544 ln(0.15)=-0.005544t t=34 yr |
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| What step limits reaction rate? |
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| slowest |
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| What is the overall order of a reaction with a rate constant having the units L•mol-1•s-1? |
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| Second |
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| When reacted with water, the insecticides DDT decomposes with a half-life of 10 years. Approximately how many years will it take for 99% of a given sample to decompose once exposed to water in the environment? |
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| t 1/2= 0693/k 10= 0.693/k k=0.0693 ln(0.01)=-0.0693t t=70 yr |
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| What, if decreased, will increase the rate of a reaction of a solid? |
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| Particle size |
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| What reaction characteristic is changed by adding a catalyst to a reaction at constant temp? |
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| activation energy |
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| Which statement is true for the general rate law: Rate = k[A]m[B]n? A)The exponents in the rate law must be positive integers. B)The values for the exponents must be determined by experiment. C)The overall order of the reaction is equal to m times n. D)It can be written from the stoichiometry of the overall reaction. |
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| C) |
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| Does a catalyst lower the Ea for the forward reaction, reverse reaction, or both? |
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| Both |
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| In the following reaction, what is the relationship between the rate of production of NH3 and rate of consumption of H2? N2(g)+3H2(g)=2NH3(g) |
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| -?[N2]/?t= (-1/3)?[H2]/?t = (1/2)?[NH3]/?t |
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| For each of the following rate equations, describe what would happen to the rate if the concentration of reactant A was tripled and the concentration of reactant B is halved. a. Rate = k[A][B] b. Rate = k[A]2[B] c. Rate = k[A]2[B]2 d. Rate = k[A][B]3 |
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| a. increases by 3/2 b. increases by 9/2 (3^2/2) c. increases by 9/4 (3^2/2^2) d. increases by 3/8 (3/2^3) |
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| At 150?C the decomposition of acetaldehyde CH3CHO to methane is a first order reaction. If the rate constant for the reaction at 150?C is 0.029 min-1, how long does it take a concentration of 0.050 mol L-1 of acetaldehyde to reduce to a concentration of 0.040 mol L-1? |
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| ln[A]=-kt+ln(A0) ln(0.040)=-(0.029)t+ln(0.050) t=7.7 minutes |
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| The decomposition of hydrogen iodide into hydrogen and iodine is a second order reaction. The rate constant k = 0.080 L mol-1s-1. How long does it take an initial concentration of 0.050 M to decrease to half this concentration? |
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| t 1/2= 1/k[A0] 1/(0.080)(0.050)= 250 seconds |
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| The gas-phase decomposition of nitrous oxide has the following two-step mechanism. 1) N2O = N2+O2 2) O+N2O = N2+O2 Write the overall equation. Is O a catalyst or intermediate? Identify the lower step is the rate law = k[N2O]. |
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| 2N20 = 2N2+O2 It is an intermediate because it was formed during the middle step of the reaction. Step 1 (less reactants) |
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| As concentration increases, rate ________. |
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| Increases |
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| What is the differential rate law for a zero order, first order, and second order reaction? |
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| Zero: r = k (unit= m/s) 1st: r = k[A] (unit= s^-1) 2nd: r = k [A]2 (unit= L/(mol•s) |
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| What is the integrated rate law for zero, first and second order reaction? |
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| Zero- [A]=-kt+[A]0 1st- ln[A]=-kt=ln[A]0 2nd- 1/[A]= kt+ 1/[A]0 |
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| What are the rules of collision theory? |
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| •particles must collide •sufficient energy •correct orientation of particles |
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| Find the overall reaction: Cl2(g)?2Cl(g) (fast) Cl(g)+ChCl3(g)=HCl+CCl3 (slow) Cl(g)+CCl3(g)=CCl4 (fast) |
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| Cl2(g)+CHCl3(g)=HCl+CCl4 |
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| Find Ea if: 1) At 701k, rate constant= 2.57 L/mol•s 2) At 895k, rc= 567 L/mol•s |
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| ln(k2/k1)=(Ea/r)(1/T1-1/T2) ln(567/2.57)=(Ea/8.314)(1/701-1/895) Ea= 14,500 kJ/mol |
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| Write overall reaction: 2NO(g)?N2O2(g) (fast) H2(g)+N2O2(g)=H2O(g)+N2O(g) (slow) N20(g)+H2(g)=N2(g)+H20(g) (fast) |
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| 2NO+2H2=2H20+N2 r= k3[H2][N2O2] k1[NO2]=k2[N2O2] 2NO+2H2=2H2O+N2 r= k3[H2][N2O2]= k3[H2](k1[NO]/k2)= (k3k1/k3)[H2][NO] r= k[H2][NO]^2 |
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| A certain reaction has the following general form: aA ==> bB At a particular temperature and [A]0 = 2.00 x 10^-2 M, concentration versus time data were collected for this reaction, and a plot of ln[A] versus time resulted in a straight line with a slope value of -2.97 x 10^-2 min^-1. a) Determine the rate law, the integrated rate law, and the value of the rate constant for this reaction. b) Calculate the half-life for this reaction. c) How much time is required for the concentration of A to decrease to 2.50 x 10^-3 M? |
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| a) k= [A] ln[A]= -0.0297t+ln[0.0200] k= -0.0297 m/s b) t 1/2= 0.693/0.0297= 23.3 minutes c) ln(0.00250)= -0.0297t+0.0200 t= 202.4 minutes |
