Chemical Kinetics: Integrated Rate Laws. ** updated Procedure for Spec 200 use **

Transcription

1 Chemical Kinetics: Integrated Rate Laws ** updated Procedure for Spec 200 use ** *DISCLAIMER: It is highly recommended that students bring in their own computers to lab this week to use excel. There may be places, like UGL, where students may be able to borrow a laptop for a certain amount of time. Procedure: Note: For each stage of the experiment, use the WASH, RINSE, FILL technique to clean the cuvette and minimize sample contamination. Part 1: Determination of the order of the reaction with respect to [P 2 ] In this part of the experiment, the rate of disappearance of P 2 in the presence of an excess of [OH ] will be recorded as a function of reaction time. One method used to simplify the determination of the order of a reaction with respect to one of the species is to set up conditions so that the other reactant is present in large excess. Therefore, during the reaction, the concentration of that species remains essentially constant. In our experiment we will add an excess of OH. Under these conditions, the rate equation can be rewritten as: rate = k [P 2 ] m The apparent rate constant k is related to the true rate constant, k, by the equation k = k[oh ]. Measuring the change in concentration of phenolphthalein with time under these conditions will allow you to determine the order of the reaction with respect to phenolphthalein, m. The experiment can then be repeated at a different concentration of OH and a value determined for n, the order of the reaction with respect to [OH ]. 1. Turn on the spec 200. Once it has loaded select the Spec 200modern interface. 2. On the first window keep the measurement mode on Abs for absorbance but change the wavelength to 550 nm. Note: To change the wavelength by units of 10 rotate the knob. To change by units of 1 press the knob down and rotate at the same time.

2 3. Pour about 10mL of the 0.3M sodium hydroxide solution into a 50mL beaker and cut a few small squares of parafilm for later use. The parafilm should be the correct size to cover the top of a cuvette. 4. Using a mohr pipet and bulb, pipet 4 ml of the sodium hydroxide solution into a square cuvette. (NOTE: Make sure you are following the graduations on the mohr pipet; the solution should not run below the last line) 5. Wipe the outside of the cuvette that contains the sodium hydroxide solution with a kimwipe then insert the cuvette into the sample compartment on the Spec Press the auto zero button (0.00) to blank the spec 200 with the sodium hydroxide solution. 7. Remove the cuvette from the compartment and add one drop of the kinetics phenolphthalein solution. Cover the top of the cuvette with the parafilm and carefully invert the cuvette to mix. 8. Remove the parafilm, wipe the outside of the cuvette with a kimwipe and insert it into the sample compartment. 9. Allow the absorbance to drop to 0.8 before you begin your timer and start recording data readings. If the absorbance is lower than 0.8 when you first insert the cuvette, start recording data readings immediately. If the absorbance is lower than 0.55, remake the solution in the cuvette as in step 4, add a new drop of phenolphthalein, and begin again. 10. Keep the cuvette in the compartment and continue to write down the absorbance once every 15 seconds in your lab notebook until the absorbance drops below 0.2. Record this data in the second column of table If you have a computer, follow the excel directions to finish the analysis for this part. If you do not have access to a computer, use the Hand drawn graph instructions. EXCEL Directions: 1. Open Microsoft excel on your computer 2. In column A and box A1 type time. In the boxes below, enter your time values starting from zero and ending when your absorbance consistently read 0.2 in 15 second increments. 3. Title column B, in box B1 Absorbance and enter in the values that you recorded. 4. Copy and paste the Times into a new column, such as Column D, with the titles time in box D1 and title column E ln (A). 5. In box E2 type in =LN(B2). A blue box should form around your initial absorbance reading and the E2 box will be outlined darker in black. 6. Hover over the bottom right of box E2 until that corner shows a darkened black square and your cursor is a + sign. Pull down the box until it is outlined in black through the entire E column where you have times in the D column. You should get a column filled with numbers as excel calculated the values based off of your absorbance. 7. Copy and paste your time values again into column G and title column H 1/A.

3 8. In box H2 type in =1/(B2). And repeat the rest as you did in the later part of step 5 and all of step 6. Use the values calculated to fill out the rest of table To create a graph hold down the left mouse button and drag your cursor over the data table you created. Then go to the top left insert tab, then the charts area in the top rectangle, and select scatter with straight lines and markers. 10. Right click on the data points in the graph and select Add trend line. In the Format trend line box that shows up select the boxes next to Display Equation on chart and Display R-squared value on chart. 11. Repeat steps 9 and 10 for the other two data tables. 12. Use the values from the equations to fill in table 9-2 and determine from the data which graph is the most linear HAND DRAWN graph instruction 1. Using a calculator convert your absorbance values to ln[a] and 1/[A] to fill out the rest of table Graph your values with time, the independent value, on the x- axis and the Absorbance, natural log A, or reciprocal A, respectively on the y- axis. Each graph should take up about a page so you can see all the data clearly. These data points should be connected with a smooth line. 3. Once you have graphed your points use a ruler to draw a best fit line. The best fit line will have the same number of data points above and below it. Some points may be quite far from the best fit while others may be directly over it as well. 4. Calculate the slope of the line by picking two places on the line that are NOT data points and locating the x, y values for each point. The values can be obtained by dropping a line straight down from one of the points to hit the x-axis as well as straight over to the y- axis, (x1,y1) and then repeating this process for the second point (x2,y2). Use the equation m = (y2 y1) / (x2 x1) to solve for m. 5. Calculate the y-intercept using y = mx + b. Solve for b by using the calculated slope value and any point on the trend line. Again, the point should NOT be an actual data point. 6. To calculate the RMSE value find the corresponding y- value on the best-fit line at the times you took the absorbance. Subtract the actual absorbance value from the best fit line value, then square that number. Repeat this for each data point and then add them all together. Divide the sum of those by the total number of data points that you took then take the square root of that value. 7. Use these values to complete table 9-2 and determine from the data which graph is most linear.

4 Part 1 continued analysis instructions 1. After deciding which graph is the most linear, draw the graph in your lab notebook, noting the axis labels and the slope, intercept, and RMSE. Enter your data into table Have your TA look over y o u r d a t a i n t a b l e 9-2 and initial your results in the space provided. 3. Analysis of this data will allow you to determine the order of the reaction with respect to [P 2 ] PART 2: Determination of the order of the reaction with respect to [OH - ] 1. Recalibrate the spec 200 by following steps 4-6 in part Pipet 4 ml of 0.3 M NaOH and 4 ml of 0.3 M NaCI into a clean, dry test tube and mix thoroughly. Rinse your cuvette with a small amount of this soluti on, then pipet 4 ml of the mixed solution into the cuvette. Check that the spectrophotometer is st ill calibrated. NOTE: The [OH] is half as large as in the first experiment, but the ionic strength of the solutions is the same due to the addition of the NaCl solution. 3. Add 1 drop of phenolphthalein, P 2-. Wipe the outside of the cuvette with a Kimwipe. Invert the stoppered cuvette several times to mix, and then place it i n the spectrophotometer (Remember to be quick!). Close the sample compartment. 4. Begin collecting data by following the same procedure as used in steps 9 and 10 of Part 1. Record your data in the second column of Table If from part 1 you choose a zero order reaction you will only need to fill out the absorbance column of table 9-3. If you believed it to be a first order reaction you will only fill out the column for both absorbance and ln (A). If you believed it to be a second order reaction you will only fill out the columns for absorbance and 1/(A). 6. Follow the same analysis procedure (excel or hand drawn) to find the slope and fill out table Again, sketch the results in your notebook, including the slope, intercept, and correlation. Have your TA initial your results. Part 3: Effect of temperature 1. Pipet 4mL of the 0.3M Sodium hydroxide solution into a clean cuvette. Place the cuvette in an ice and water bath and let it cool for about 10 minutes. 2. With the chilled solution recalibrate the spec 200 following the steps in part Replace the cuvette in the ice bath for another minute or two. Be careful that it does not spill over. This might be a good time to take note of the temperature of the ice bath and the temperature of the room.

5 LAB 9 4. Remove the cuvette from the ice bath, add one drop of phenolphthalein, and cover with parafilm and invert to mix. Then take off the parafilm, wipe the outside with a Kimwipe and place the cuvette in the sample compartment of the spec Take note of the absorbance value. 6. Immediately after getting an absorbance remove the cuvette from the spec 200 and return it to the ice bath. 7. Take readings in this manner every minute for 5 minutes. And fill out the absorbance column in table Analyze your data the same way as you did in parts 1 and Be sure to record the temperatures of the ice bath and room as you will use these to estimate the activation energy of the reaction.

6 Table 9-1 Time (in seconds) Absorbance ln(a) 1/A Table 9-2 Data to determine order with respect to phenolphthalein [A] vs. T Ln[A] vs. T 1/[A] vs. T TA initials Equation in form of y=mx+b Slope of best fit line Intercept of best fit line RMSE of best fit line

7 Table 9-3 Time (in seconds) Absorbance ln(a) 1/A

8 Table 9-4 Data to determine order with respect to hydroxide Slope of best fit line Table 9-5 Time (in second) absorbance Ln(A) 1/A Table 9-6 Data to determine the activation energy Room temperature ( C) Temperature of ice bath ( C) Slope of best fit line Intercept of best fit line