PROCEDURES

1. Your instructor will distribute problem sets. Each set will contain four problems, the first three of which will be purely mathematical relationships. The fourth contains typical data from a kinetics experiment wherein the concentration of a reactant is changing with time.

2. Open Graphical Analysis by double-clicking on the Graphical Analysis icon. Page 19-8 is a screen print of the screen you will see when the software is opened. Before you begin, there are some things you need to know about the Graphical Analysis program. On the left side of the screen (Page 19-8) you will see a data window, with the title “Data Set 1”. On the right side of the screen you will see a graph window with the title “Y vs. X”. The third window in the bottom left corner is a text window where you can enter your name, date, problem # from your data set, the “k” value you calculate from slope, and the resultant formula for the relationship between x and y. Only one of the three windows is active at a time. You can make a window active simply by clicking on it.

The data table in Data Set 1 will initially have x and y data columns. The location of the x or y value on any line is referred to as a cell. If you click on a cell, a box appears. You can type values of x or y into a cell box. There are several ways to move to another cell. The <tab> and <enter> keys move to the next cell in the table. The arrow keys can be used to move up or down in a given column of data, but not from one column of data to another. The <return> key moves to the first cell in the next row.

3. Enter the ordered pairs of the first problem into Data Set 1. Type the x value into the cell box for the first line number and press <enter> or <tab>. Type the y value into the first cell box in the second column. After you press <enter>, the cell box will appear on the next data line. Type the next ordered pair in the same way, pressing the <enter> or <tab> key after each entry. Continue in this manner until you have entered all five ordered pairs. As soon as an ordered pair has been entered, a point will automatically appear on the graph. If you make a mistake, simply click the mouse on the value you want to change and type a new number in the box. If you see multiple lines connecting the data, turn off the connecting line feature by deselecting “Connecting Lines” under the “GRAPH” menu.

4. After you have entered the last value, examine the shape of the curve in the Graph 1 window. If the graph is curved (either inverse or exponential), proceed as described in the introduction of this experiment. To do this, select the x column in the data table by clicking the cursor on the “x” at the top of the first column. The selected column should appear highlighted in black. Click and hold on the “DATA” menu at the top of the screen, select “New Column” and then “Calculated”. A new window titled “NEW COLUMN” will appear. It will look like a calculator keypad. Just click on the key you want. The first data entry bar has the title “New Column Name” and will have the name “Column 3” automatically entered by the system. You may now create a new column of data by entering a formula for the column in the second long data entry box. First, decide what exponent you want to use for x. When you modify the formula for the column, you must specify the variable in double quotes. For example, raising x to the second power (x²) would appear as “x”^2; an inverse relationship would appear as 1/”x”, while an inverse cubic relationship would be 1/”x”^3. The system will put the values of x into the new column definition (formula) and calculate a corresponding set of values in the new column.

5. To see if you made the correct choice of exponents, click anywhere on the graph to make the graph window active. When you press and hold the mouse on the x-axis label, a pop-up menu will appear listing all three columns in the data table. From this list, choose the new column you just made, probably “Column 3”. The new formula will replace x on the horizontal axis, and a new curve will appear on the graph. If a straight line results, you have made the correct choice. If it is still curved, click on Data Set 1 to make it the active window. Double click on the modified column. Choose “Definition” from the pop-up menu that appears. Type in a new exponent for x in the New Column Formula” edit box and click <OK>. A new set of values for this power of x will appear in the modified column; these values will automatically be plotted on the graph. Proceed in this manner until a straight line is obtained. After you have a straight line, double click on column 3 in the data window and change the name of the column to be the formula of the column. The example screen shows “1/x^2.

6. After you have obtained a straight-line graph, again click on the graph to activate the window. Click “ANALYZE” from the menu bar and then select “Automatic Curve Fit”. Click on “Linear” and then click “OK”. A new screen will appear and you should choose to “Keep the Fit”. The linear curve fit calculates a regression line, which is a best-fit, straight line passing through or near the data points. The line should pass through each data point exactly for the data supplied in this experiment. The system will also give you a pop-up bar containing the formula of the line in the form Y = mX + b. Click on the pop-up bar and tow it off of the regression line.