PROCEDURES

Your instructor will give you a set of kinetic data to use as an unknown. You will use the data to create sets of zero, 1st and 2nd order graphs using Graphical Analysis software. The software will do the actual graphing. You are to determine the order of the reaction, the rate constant at each temperature, and the activation energy of the reaction.

1. ORDER OF REACTION AND RATE CONSTANTS

Open Graphical Analysis. Click on the graph window to make it active, then select “Graph Options from the “GRAPH” pull-down menu. Be sure that point protectors, connecting lines, background grid, and legend are all checked.

Click on the text window and enter your name, section number, and the date on line 1. Enter y our unknown number on line 2, and the type of graph (zero, first, or second order or Arrhenius) on line 3.

Click on the data table window and click on the “x”. This will pop up the “Column Options” window. Enter the name “Time” and the unit “Seconds” then click OK. Now click on the data set title and enter the title “20 degrees”. Click on the top cell in the time column and enter time data for your unknown.

Click on the “y” in the other column. The “Column Options window will pop up again. Enter the name “Concentration” and the unit “mol/L”. Next click “Dec. Places” and enter the number 4 and then press OK. Click on the top cell in the concentration column and enter the 20.0 C data for your unknown. You should see data appear in the graph window. Click on the graph title just under the words “Graph Window.” Enter the title “Concentration vs. Time”. Note that time is your x-axis and concentration is the y-axis.

Select "New Data Set" from the "DATA" pull-down menu. Repeat the process of adding titles to the data set and columns, except that the data set title should be “30 degrees”. Enter the data for 30 degrees. To save time, you can copy the column of time data from the 20-degree data set and paste it into the 30-degree column. You will not be able to see your new data appear on the screen until you click on the y-axis label on the left side of the graph. Click on the 30-degree concentration box and press OK. You should see a second line on the screen and a second indicator in the legend.

Repeat this process until data for all four temperatures have been entered. The software will plot graphs of each data set using different point indicators to differentiate the data. You should see four curves on the screen. If the data produce straight lines, your reaction is zero order. Print this screen in landscape mode (FileÆPrinterSetupÆSetupÆLayoutÆLandscape) for your report.

Now you are ready to make a first order graph. Click once on the column title “Concentration” to highlight the column of data for one of the temperatures. It doesn’t matter which temperature you choose, because the software will make appropriate changes to all four datasets. Select "New Column" and “Calculated” from the "DATA" pull-down menu. Enter the title “ln of Concentration”. There are no units on a logarithmic function. Click “ln” on the keypad and then type “Concentration” as the variable within the parenthesis. Remember to use double quotes on the variable. Note that new columns of data are added for all four data sets.

Click on the y-axis label on the left of the graph and check each of the temperatures to display ln of Concentration vs. Time in the graph window. If the new data produce straight lines, your reaction is first order. Be sure to uncheck the “Concentration” boxes so that only ln of concentration is displayed. Print this graph in landscape mode after changing the title of the graph (click on the title) and the information in the text window.

You are now ready to make a second order graph of your data. Select "New Column" and “Calculated” from the "DATA" pull-down menu. The new column title will be “1/Concentration” and will have the units “L/mol”. In the “New Column Formula” box type in “Concentration”^-1. Be sure to use double quotes around the variable. Repeat the process of clicking on the y-axis label and “checking” to display the new data. Again, all four data sets will be displayed. If the data produce straight lines, your reaction is second order. Remember to change the text window and the graph title before printing in landscape mode.

Now adjust your graph to display the set of data that produced straight lines. Choose "Automatic Curve Fit" from the "ANALYZE" pull-down menu, and finally click “linear”. Note that there is a pull-down list of your data sets. The system will calculate the linear fit of the data set selected from the list. Do the fit for each of the four data sets and carefully record the slope values, including the sign, on your report data sheet in the appropriate temperature column. You can now calculate the specific rate constant for each temperature from the respective slopes. The rate constants will be used in the next section for the Arrhenius graph.

2. ARRHENIUS GRAPH AND ACTIVATION ENERGY

Choose "New" from the "FILE" pull-down menu. You will see a dialog box asking whether to save data. If you have printed all of your graphs (zero, first and second order) and recorded the slopes of your straight lines, you may discard the data safely.

You are to enter the values of the rate constants and their corresponding Kelvin temperatures as the "y" and "x" variables respectively. The system will plot the rate constant on the y-axis and Kelvin temperature on the x-axis. Be sure to enter the correct column title and units, graph title and information in the text window. Now look at equation (7) on Page 11-5. Note that we cannot get a straight line unless we graph ln(k) versus 1/Temperature. You will now convert the data to the appropriate functions to achieve a straight-line relationship.

Highlight the "Temperature" column of data in the data window. Select "New Column" and “Calculated” from the "DATA" pull-down menu. Enter the column title “1/Temperature” and the units “1/Kelvin”. In the “New Column Formula” box type in “Temperature”^-1. Remember to use double quotes around the variable. Now highlight the “Rate Constant” column and make a new column of data titled “ln of k”. The new column will have no units since it is a logarithmic function. Click on “ln” in the key pad and then in the “New Column Formula” box type in “rate constant” as the variable. Remember the double quotes. Now click on the y-axis label and check “ln of k” and uncheck “rate constant”. Click on the x-axis label and check “1/Temperature”. Your graph should display a straight line with negative slope. Print this graph in landscape mode after making sure the graph title and information in the text box are correct. Select “Automatic Curve Fit” from the “ANALYZE” pull-down menu and then select “Linear” and press OK. This will give you the value of the slope, which should be recorded on your data sheet with the proper sign and units. Remember slope = -E_a/R. Use the slope to calculate the activation energy, E_a, which should also be entered on the data page, with its units. See equation (8) for calculation information.

Your report should include: Objective, Procedure, Data Page, Observations, Results/Conclusions, 4 graphs, and the Post-lab questions (Page 11-2). The Post-lab questions count 15 point of your report grade and make use of the graphs prepared in this experiment. You will obtain values by interpolation from the graphs. Remember to label your graphs fully: each axis (with units), graph title, your name, section number, date, and unknown number.