ch14houts - —_ l 1 time conc - time 1n [AL = -alct + 1n...

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Unformatted text preview: —_ l 1 time conc - time 1n [AL = -alct + 1n [A]o [A]: = ‘akt + [A10 ' or expression ak log [A]t — —2.303t + log [A]0 In [A] plot as y [A] or __1._ [A] 103 [A] ——alc —ak ak slope (neg- slope) (neg. slope) (pos. slope) (- “" ) 2.303 111 [A10 intercept [A]0 or ‘ L We 105 [A10 tm [_A]_0 0.693 1 2a]: alc ak[A]o Chapter 14 — Kinetics Hdditional Practice Problems 14.26 A hypothetical reaction A —9 products is second order in A. The half-life of a reaction that was initially 1.66 M in A is 310 min. What is the value of the rate constant? 14.27 A reaction shows the same half-life regardless of the starting concentration of the reactant. Is it a first-order or a second—order reaction? $0. In three different experiments the following results were obtained for the reaction A-—> products: [A]() = 1.00 M, rm =‘ 50 min; [Alo = 2.00 M, (1,2 = 25 min; [Alo = 0.50 M, 1,,2 = 100 min. Write the rate equation for this reaction and indicate the value of k. 51. Ammonia decomposes on the surface of a hot tung- sten wire. Following are the half-lives that were obtained at 1100 °C for different initial concentrations of NH3: [NH310 = 0.0031 M, [1,2 = 7.6 min; 0.0015 M, 3.7 min; 0.00068 M, 1.7 min. For this decomposition reaction, what is (a) the order of the reaction; (b) the rate constant k? [14.641 The gas-phase reaction of chlorine with carbon monoxide to form phosgene, Clz(g)+CO(g) —t COCl2(g), obeys the following rate law: A[COC|2] Al Rate = = k[Clz]"‘[C0] A mechanism involving the following series of steps is con- sistent with the rate law. C12 = 2C1 c1+c0 = coct c0c1+c1, .= COC11+C1 ._ Assuming that this mechanism is correct, which of the steps above is the slow, or rate-detennining, step? Explain. Hint: try making each step the slowest step [rate—det. step] and see what rate law you get and if it agrees w. the Hip. rate law giuen. Chemistry 12fl125 — Experiment 16 Lab Report Tips 1.For your graphs you can use a computer to do the graphs. If your graphing program does linear regression (least squares fitting) you can use this to plot your lines as well. Some programs can also draw nice smooth nonlinear curves. If you do use the best fit produced by the computer you need to include, both on the graph and in your report, the equation the computer gives you for the straight line. You should also report the correlation coefficient (usually signified as R) which gives you some idea about how good the fit is. The closer R is to i1 the better your fit is to a straight line, i.e. the closer your points are to falling on the straight line (R=il means all your points fall exactly on a straight line). Some programs report a X2 or variance of the fit, which tell you how good the fit is. Report these if given. If your program can not do a linear regression than just have it plot the points and you draw the “best-fit” line. This means you have to compute the slope yourself. To do this accurately and correctly you need to have the plotting program draw grid lines so it looks something like the graph paper provided in your lab manuals. Having a few tick marks at 0.5 inch intervals will not be acceptable. For all graphs, whether by hand or computer generated, they should occupy essentially the whole page. In other words, spread out your axes so your data occupies as much of the graph and page as possible. 2.For the Beer’s Law Graph you will have 4 experimentally determined data points and the 0,0 point. You want to draw the best straight line through your 4 exp. det. data points while forcing the line to go through the 0,0 point. You probably won’t be able to do this using a linear regression (least-squares) program as they don’t usually allow you to force the line through a data point. 3.If you have access to a spreadsheet program use it rather than the computer program in 160 CE. If you use the computer program you have to adjust your concentrations by a factor of 103 and you have to copy all the data from the screen onto you report forms. If you use a spreadsheet you can program the columns to do the calculations for you and then print out the spreadsheet (with the appropriate titles and column headings) as your report form. In your spreadsheet you will enter your time and absorbance columns and then have the spreadsheet do the appropriate calculations for you. You should have a separate spreadsheet for each solution, just like the report forms. I’ve listed below what should go in the other columns of your spreadsheet. You also need to show an example of each of these calculations in your report. Column 1: time Column 2: Absorbance (A) Column 3: [Cr(EDTA)‘], = A/m where, A: absorbance m: slope of Beer’s Law Graph. Column 4: [Cr3“‘]t = [Cr3+]0 — [Cr(EDTA)‘]t where, [Cr(EDTA)‘]t is Column 3 and [Cr3+]0 is the original Cr3+ conc. at time 0 (a constant for each solution) Column 5: 1n [Cr3+][ Just take the natural log of each cell in Column 4. Column 6: l/[Cr3+][ Just take the reciprocal of Column 4. Some spreadsheets (Excel 7.0) can now do linear regression. You just tell it whcih columns you want to plot. I wouldn’t suggest spending hours trying to figure out how unless you want to learn so you can use it in the future. Just have it plot the points and draw your lines by hand if you can’t figure out how to do the linear regression or the program won’t do it. What ever you do don't just tell it to connect the points so you get a jagged line. ...
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ch14houts - —_ l 1 time conc - time 1n [AL = -alct + 1n...

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