Chemistry 141 Laboratory Lab Lecture Notes for Kinetics Lab Dr Abrash

Transcription

1 Q: What is Kinetics? Chemistr 141 Laborator Lab Lecture Notes for Kinetics Lab Dr Abrash Kinetics is the stud of rates of reaction, i.e., the stud of the factors that control how quickl a reaction occurs. Therefore the primar data that are collected in kinetics eperiments are these rates of reaction, and how the depend on concentrations of reactants, concentrations of catalsts, and the temperature. Q: What reaction are we going to stud? We re going to stud the iodine clock reaction, which is described in detail in our manual. The ke reaction is S O 2I 2SO I In the presence of starch, the iodine reacts to form a blue-black comple. Wh doesn t the solution immediatel turn black when the S 2 O 8 2- reacts? Because we also include some S 2 O 3 2-, which reacts to reconvert the I 2 back to I -. 2S O I 2I S O When we use up our S 2 O 3 2-, the solution turns black. At that point, we will have used up an amount of S 2 O 8 2- equal to ½ of the S 2 O We will measure the time it takes to consume a fied amount of the S 2 O 8 2-, which will be M for all runs. Q: How are rates of reaction defined? When following the concentration of a reactant, these are defined as the negative of the change in concentration divided b the time elapsed, i.e. rate. t To simplif our calculations in this eperiment, the change in concentration of S 2 O 8 2- for all runs is the same, M. The elapsed time in this eperiment will be the amount of time that elapses between the miing of our reagents, and the first sign of a color change from clear to black in our reaction miture. Q: How does a reaction rate depend on concentration?

2 The dependence of a rate on concentration is given b a so called rate law. A rate law simpl sas that the rate can be epressed as a function of the concentrations of reactants. These equations can sometimes be complicated, but man reactions have a rate equation with a simple form: rate k[ A] B C. In this equation, A, B, and C are either reactants or catalsts;,, and are eponents, which are tpicall not related to the stoichiometric coefficients. The eponents are called either partial orders. k is called the rate constant. The rate constant is independent of concentration but depends on temperature. For parts A, B and C of the eperiment, the rate equation ou ll be tring to determine will be rate k I S O. 2 Your job will be to determine the eponents and and the constant k from our data. In part D, the rate equation will be slightl different: 2 2 rate k I S O Cu. Note that in this equation, the eponents and are not necessaril the same as the ones for runs A, B, and C, because a catalst speeds up a reaction b creating a new reaction mechanism. However, in this section we onl wish ou to determine the new eponent for the concentration of Cu 2+. Q: How do we determine the eponents? In order to do this ou need to compare runs in which all variables but one are held constant. So in this eperiment, to determine the order with respect to I -, ou would need to do eperiments in which both temperature and the concentration of S 2 O 8 2- were held constant. The best wa to determine the order once these data are collected is with ecel. Note that if we take the ln of our rate equation for parts a, b, and c, we get ln rate ln k ln I ln S O 2. For eperiments in which the concentration of S 2 O 2-8 is constant, this becomes ln rate constant ln I

3 Thus if ou make a graph with the ln of the rate on the ais, and the ln of the concentration of the iodide on the -ais, the slope of the trendline will be our order,. A similar graph for eperiments in which everthing but the concentration of S 2 O 8 2- is held constant will ield the order. For this eperiment ou should probabl round our slopes to either the nearest integer or the nearest half integer, i.e. for this eperiment orders will most tpicall be 0.5, 1, 1.5, 2, 2.5, and so on. Determination of the order with respect to the catalst Cu 2+ is carried out in a similar manner. This is VERY important. NOTE that in these graphs, the intercept of the trendline is NOT the ln of k. That can onl be determined once all of the orders are known. Q: How do we determine the rate constant k? Once we determine the value of the eponents, (and for the runs in Part D, ), we rearrange our rate law to solve for k: k rate 2 I S2O8 for parts A, B, and C or rate k I S O Cu 2 2 for part D. Q: How does a reaction rate depend on temperature? All of the temperature dependence of the reaction rate is contained in the rate constant k. The dependence of k on temperature is given b the Arrhenius Law, E a k Ae RT Note that in this equation, the temperature must be in Kelvin units. If we take the natural log of both sides, we get: Ea ln k ln A. RT

4 Thus a plot with ln k on the ais and 1/T on the ais will have a slope equal to E a /R and an intercept equal to ln A. Therefore the best wa to determine E a and A is to do the following: Determine our values of k for all of the runs in Part C. Make a plot using ecel of ln k (-ais) vs 1/T (-ais) Create a trendline for our plot of ln k vs 1/T. The slope will be equal to - E a /R and the intercept will equal ln A. Determine our Ea b multipling our slope b R. Note that our R value should be J K-1 mo l-1, and that our units for R will be Joules. Determine our preeponential factor A, b taking the antilog of our intercept intercept, e. Q: Do ou have an tips for us? Sure! You ll be working in groups of 4 (or 5) toda. Each group of 2 will be doing half of the runs in the eperiment. One half of the group will be doing runs 1, 2, 3, and 7 and the other half runs 4, 5, 6, 8, 9. Both halves of the group will come together to do runs 10 and 11. You ll pool the data so that everone has all 14 runs to use in our data analsis and lab report. We want to make sure that everone has good data to work with. To ensure this, we ll set up a big table on the whiteboard with all the runs listed. As ou complete a run, write down the time it took for the black color to appear on the whiteboard. If our result doesn t agree with the others, then ou should redo that run. Whenever ou reuse one of our flasks or beakers, make sure that ou rinse it carefull with deionied (RA) water. Contamination between runs seems to be the primar source of poor data in this eperiment. The temperatures in part B are approimate. It is more important that ou eactl record the temperature than that ou have increments of eactl 10 degrees. Make sure that ou heat or cool the contents of the two flasks before ou combine them. In Part C, make sure that ou don t use an EDTA Remember that ou should dispense our solutions directl from the burets into the beakers ou ll be using for each run. Q: What do we need to do for the lab report? The lab report is described in detail on page of the manual. Q: What are we allowed to collaborate on and what must we do independentl?

5 You can collaborate onl on the eperimental procedure and the table of data. All other work must be done individuall.