Question

will work with up to 3 partners (similar to a lab group) to prepare a written report which analyzes kinetic data that has been provided to you. All student groups will receive data for the hypothetical reaction aAlE) products where a is a numeric variable and A is a chemical variable. The reaction therefore has the form of a decomposition reaction, in which a single substance forms one or more new substances. For consistency, all concentrations start out at 1.000 M and decrease over time as the reactant is consumed in the chemical reaction. The data you will receive shows the remaining concentration at 10 second intervals, with the total timescale being 200 seconds. Data is provided for the following temperatures: 25°C 35"с 45°C 55 oc, 65°C, 75 ec, 85 °C, and 95 oC. Your paper MUST address the following questions / tasks listed below. You can't just give final answers. Your paper must fully describe, explain, and illustrate how the answers have been obtained. What is the order of the reaction? Note that it will be either zero order, first order, or second order. These are the only reaction orders represented in the data that has been provided to students. Apply what you have learned in class to determine the order, and clearly present your methods and results in your report. 1. Determine the reaction rate constant (k) for at least 2 different temperatures. Clearly present the methods used, and the results obtained. 2. 3. Determine the activation energy (E.) for the reaction. Clearly present the methods used and the results obtained. Optional Extra Credit: Determine the value of the frequency factor, sometimes called the "pre-exponential factor" Some things to keep in mind In all data that has been distributed to students, the reactant concentration is listed at 10-second intervals over a 200 second timescale. Data is presented for temperatures ranging from 25 °C at 95 c, in increments of 10 °C. Depending on the reaction, not all of the data will be usable. Don't feel compelled to analyze everything that is there. The minimum standard to is to analyze 2 different temperatures, but there are probably more than 2 temperatures for which the data is usable So how do you decide what is "usable"? The concentration needs to drop significantly during the observation period, but not so fast that all you get is a couple of data points before it decays to zero concentration. If you still have, for example, 95% of t With he original reactant concentration remaining at 200 seconds, that's not likely to yield good results. so little of a change in concentration, practically every graphical plot will look linear, so it will be difficult to decide on the reaction order. For example, consider this plot, taken from the one of the Kinetics PowerPoint slides: On the above graph, you can see that the concentration profile is curved if you look at the entire concentration vs. time plot. However, if you look at only the part of the curve near the beginning of the reaction, the change of concentration with time appears to be linear. It's only when the concentration falls substantially see that the profile is curved. The above plot is for a first order reaction. You must plot the natural log (In) of the concentration to get a straight line across the entire graph (the straight line shown below the curve). But even the concentration itself seems to plot as a straight line if the concentration has decreased by only a small amount from its original value. observation period. from its starting value that you can Therefore, l advise against analyzing data where too little of the reactant is consumed during the At the other extreme, if the reaction is too fast, the reactant may decay to zero concentration in such a short time that very few data points are available for analysis. I would likewise, advise against attempting to analyze such data. You are looking for that "happy medium" where there is substantial loss of reactant during the observation period, but in which enough data is available to give a meaningful analysis. Another problem you may encounter with some reactions at certain temperatures is when the concentration is near zero. The concentrations are measured to the nearest 0.001 M, so for slow decays, you may have multiple time points with the same value when reported to 3 decimal places. For following set of concentrations, taken at consecutive 10-second time points a very slow decay of concentration example, consider the 0.007 M 0.007 M 0.006 M 0.006 M 0.006 M 0.005 M 0.005 M 140 s 150 s 160 s 170s 180s 190 s 200 s Although the concentration drops continuously over the time interval from 140 s to 200 s, the decay rate is so slow that the changes from one time point to the next occur beyond the third decimal place, and therefore, are not shown in the data. The data give the impression that the concentration hangs at one value for an extended period of time, then suddenly drops to a slightly lower value, hangs there for a while, and so on. But this is an artifact of the rounding. If more decimal places were available, we could see that the concentration drops continuously, albeit slowly. If you are plotting this data (whether the data itself, or some function of it, such as the natural log) you will end up with some points having the same Y value for two or more different x values. Data points of this nature seem to suggest that the linear fit is a horizontal line - that is, a line having a zero slope. But that can't be right, because it would mean that the reactant concentration is not changing with time. Even when such "problematic" data points are in your set, you will probably still get a fairly good fit because the data points from earlier in the reaction- when the concentration was changing faster- will help offset the "bad" data points. Even so, since you have a lot of data to choose from, it would probably be a good idea to avoid data that has plateaus, unless the use of such data is unavoidable. The data you have been provided is noise-free and error-free. It's what you would measure in a perfect world using an instrument that measures concentration to the nearest 0.001 M. So if you choose your data carefully (that is, which temperatures to use), you should be able to obtain a perfect linear fit (R 1) when plotting data that that should be linear. Data Set # 2 Data for aA(g) → products All reactions start with (AJo 1.000 mol L4, The reaction of substance A is followed for 200 seconds at the temperatures indicated in the table. Depending on the nature of the reaction, some data may not be usable. If the reaction is very slow, the experiments at the lower temperatures may not show enough loss of reactant to generate reliable data. If the reaction is very fast, the experiments at the higher temperatures may generate so little data that it can't be analyzed. In this experiment, any concentrations below 0.001 mol LJ can not be detected, and might as well be zero. Therefore, concentrations are no longer reported when they fall below 0.001 mol LJ TIME ↓ 1.0001.0001.0001.000 1.000 1.000 1.000 1.000 0.960 0.9330.893 0.835 0.757 0.6580.540 0.412 0.9210.8700.797 0.697 0.573 0.433 0.292 0.170 0.884 0.812 0.711 0.582 0.434 0.2850.158 0.070 0.848 0.757 0.635 0.486 0.329 0.188 0.085 0.029 0.814 0.70.566 0.4060.249 0.123 0.046 0.012 0.781 0.659 0.506 0.339 0.189 0.081 0.025 0.005 0.749 0.6140.451 0.2830.143 0.053 0.013 0.002 0.719 0.573 0.403 0.236 0.108 0.035 0.007 0.690 0.535 0.359 0.197 0.082 0.023 0.004 0.662 0.499 0.321 0.165 0.062 0.015 0.002 0.635 0.465 0.286 0.138 0.047 0.010 0.001 0.610 0.434 0.256 0.115 0.036 0.007 0.585 0.405 0.228 0.096 0.027 0.004 0.561 0.377 0.204 0.080 0.020 0.003 0.539 0.352 0.182 0.067 0.5 0.002 0.517 0.328 0.162 0.056 0.012 0.001 0.496 0.306 0.145 0.047 0.009 0.476 0.286 0.129 0.039 0.007 0.457 0.267 0.115 0.033 0.005 0.438 0.249 0.1030.027 0.004 TIME ph, you can see that the concentration profile is curved if you look at the entire concentration vs. the above gra t. However, if you look at only the part of the curve near the beginning of the reaction, the change of concent h time appears to be linear. It's only when the concentration falls substantially from its starting value that you that the profile is curved. The above plot is for a first order reaction. You must plot the natural log (In) of the centration to get a straight line across the entire graph (the straight line shown below the curve). But entration itself seems to plot as a straight line if the concentration has decreased by only a small amount fr nal value. Therefore, I advise against analyzing data where too little of the reactant is consumed during the rvation period. eother extreme, if the reaction is too fast, the reactant may decay to zero concentration in such a short t ew data points are available for analysis. I would likewise, advise against attempting to analyze such dat g for that "happy medium" where there is substantial loss of reactant during the observation period, bu gh data is available to give a meaningful analysis. encounter with some reactions at certain temperatures is a very slow decay of co measured to the nearest 0.001 M, so for slow d ar erample, conside

Answer 1

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