lecture 11 - Turn on Camptasia Recording! Exams grades will...

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Unformatted text preview: Turn on Camptasia Recording! Exams grades will be recorded in OWL soon. Hopefully over the weekend. Exam keys will be posted & exams will be returned Wednesday Feb. 27th. No Office Hours today due to school closure closure. SI sessions Sunday Library 1349 Wednesday Library 1085 ... 1 How do you feel you did on the exam. A.Very Well B.Well C.OK D.Kinda Poorly y E.Very Poorly ... 2 Interpreting Rate Laws Rate = k [A]m[B]n If m = 1, rxn. is 1st order in A If [A] doubles, then rate goes up by factor of __ If m = 2, rxn. is 2nd order in A. Doubling [A] increases rate by ________ g y If m = 0, rxn. is zero order. If [A] doubles, rate ________ ... 3 Below is a zero order reaction for the decomposition of Ammonia. What is the rate law? 2NH3(g) --> N2(g) +3 H2 (g) A. B. C. D. Rate = k [NH3]2 Rate = k [NH3]2 [H2]3 Rate = k [N2] [H2]3 Rate = k ... 4 Based on this data, what order is this reaction in NO? 2NO(g) + Cl2(g) --> 2NOCl(g) A. B. C. D. Zero First Second Third ... 5 Based on this data, what order is this reaction in Cl2? 2NO(g) + Cl2(g) --> 2NOCl(g) A. B. C. D. Zero First Second Third ... 6 What is the rate law for this reaction? 2NO(g) + Cl2(g) --> 2NOCl(g) A. B. C. D. Rate = k [NO]2 Rate = k [NO]2 [H2]1 Rate = k [NO]2 [Cl2] Rate = k [NOCl]2 ... 7 Reaction Rate Constants Zero order reaction: First order reaction: Second order reaction: ... 8 Derivation of the Second Order Integrated Rate Law ... 9 Concentration / Time Relations What is the concentration of reactant as function of time? Consider FIRST ORDER REACTIONS The rate law is ... 10 Concentration / Time Relations Integrating -(∆ [R] / ∆ time) = k [R], we get [R] / [R]0 = fraction remaining after time t has elapsed. This is the integrated first-order rate law. ... 11 Integrated Rate Laws For the general reaction R → P, if [R]0 is the starting concentration of R and [R]t is the concentration at time t, Reaction order in R: First Order Rate Equation Integrated Rate Law −Δ[R] = k[R] Δt ⎛ [R] ⎞ ln⎜ t ⎟ = −kt ⎝ [R]0 ⎠ Second Order Δ[R] −Δ[R] = k[R]2 Δt −Δ[R] = k[R]0 = k Δt 1 1 − = kt [R]t [R]0 [R]0 − [R]t = kt ... 12 Zero Order Concentration / Time Relations Sucrose decomposes to simpler sugars like glucose Rate of disappearance of sucrose = k [sucrose] If k = 0.21 hr-1 and [sucrose] = 0.010 M How long to drop 90% (to 0 0010 M)? 0.0010 Glucose ... 13 Rate of disappearance of sucrose = k [sucrose]. k = 0.21 hr-1. If initial [sucrose] = 0.010 M, how long to drop 90% or to 0.0010 M? Use the first order integrated rate law ... 14 The reaction below is second order. k=30 L/mol min. k 30 L/mol•min. If Initial [HI] =0.3 mol/L, 0.3 what will the [HI] be in 5 minutes? 2HI(g) ---> H2(g) + I2(g) A. 0.29 mol/L B. 0 17 B 0.17 mol/L l/L C. 0.032 mol/L D. 0.0065 mol/L ... 15 Using the Integrated Rate Law The integrated rate law suggests a way to tell the order based on experiment. 2 N2O5(g) ---> 4 NO2(g) + O2(g) ---> Time (min) 0 1 2 5 [N2O5]0 (M) 1.00 0.705 0.497 0.173 ln [N2O5]0 0 -0.35 -0.70 -1.75 Rate = k [N2O5] ... 16 Using the Integrated Rate Law 2 N2O5(g) ---> 4 NO2(g) + O2(g) Rate = k [N2O5] 1 [N 2 O5 ] v s . t i m e 0 l n [N 2 O5 ] v s . t i m e 0 0 t i me 5 -2 0 t i me 5 Data of conc. vs. time plot do not fit straight line. Plot of ln [N2O5] vs. time is a straight line! ... 17 Using the Integrated Rate Law 0 l n [N2 O5 ] v s . t i m e Plot of ln [N2O5] vs. time is a straight line! Eqn. for straight line: y = mx + b 5 -2 0 t i me All 1st order reactions have straight line on a plot of ln [R] vs. time (All 2nd order reactions give straight line on a plot of 1/[R] vs. time) ... 18 Summary of Rate Laws ... 19 Half-Life of Reaction HALF-LIFE is the time it takes for 1/2 a sample is disappear. For 1st order reactions, the concept of HALF-LIFE is especially useful. ... 20 Half-Life Reaction is 1st order decomposition of H2O2. ... 21 Half-Life Reaction after 1 half-life. 1/2 of the reactant has been consumed and 1/2 remains. ... 22 Half-Life After 2 half-lives half lives 1/4 of the reactant remains. ... 23 Half-Life A 3 half-lives half lives 1/8 of the reactant remains. ... 24 Half-Life After 4 half-lives half lives 1/16 of the reactant remains. ... 25 Half-Life Sugar is fermented in a 1st order process (using an enzyme as a catalyst) catalyst). sugar + enzyme --> products Rate of disappearance of sugar = k[sugar] k = 3.3 x 10-4 sec-1 What is the half-life of this reaction? ... 26 Half-Life Rate = k[sugar] and k = 3.3 x 10-4 sec-1. What is the halflife of this reaction? Solution S l ti [R] / [R]0 = fraction remaining when t = t1/2 then fraction remaining = _________ ... 27 Half-Life If rate = k[sugar] and k = 3.3 x 10-4 sec-1 and the half-life is 35 min. Starting with 5.00 g sugar, how much i l ft after 2 h and 20 min (140 min)? h is left ft hr d i i )? Solution 2 hr and 20 min = 4 half-lives Half-life 1st 2nd 3rd 4th Time Elapsed 35 min 70 105 140 Mass Left 2.50 g 1.25 g 0.625 g 0.313 g ... 28 Half-Life Radioactive decay is a first order process. Tritium ---> beta + helium 3H 3He ---> beta + Carbon-14 decay: 14C ---> beta ... + 14N 29 Half-Lives of Radioactive Elements Rate of decay of radioactive isotopes given in terms of half-life. 238U 14C 131I --> --> --> 234Th 14N + He + beta 4.5 x 109 yr 5730 y 8.05 d + beta 131Xe ... 30 The decomposition of phosphine, PH3, proceeds according to the equation below. It is a first-order process. The half life is 37 9s first order process 37.9s at 120°C. How much time is required for 3/4 of the material to decompose? 4PH3 (g) --> P4 (g) + 6 H2 (g) A. B. C. D. 37.9 s 79.8 s 120 s 113.7 s ... 31 The decomposition of phosphine, PH3, proceeds according to the equation below. It is a first order process. The half life is 37 9s first-order process 37.9s at 120°C. What percentage of the sample remains after 1 minute? 4PH3 (g) --> P4 (g) + 6 H2 (g) A. B. C. D. 0.421 0.382 0.335 0.212 ... 32 ...
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This note was uploaded on 04/15/2008 for the course CHEM 112 taught by Professor Hardy during the Spring '08 term at UMass (Amherst).

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