Determination of the Rate of a Reaction, Its Order, and Its Activation Energy

Transcription

1 Determination of the Rate of a Reaction, Its Order, and Its Activation Energy Reaction kinetics is defined as the study of the rates of chemical reactions and their mechanisms. Reaction rate is simply defined as a change in a measurable quantity divided by the change in time. In chemistry, the measurable quantity is usually molar concentration or absorbance. Consider the generalized chemical reaction equation A + B C + D. Symbolically it can be represented in multiple ways: Δ A Rate = = k[a] m Δ time Note that the units on rate are always M/time which can also be expressed as M time 1. The negative sign on the first expression indicates that the molar concentration of reactant A will decrease as time goes by. The second expression is simply the differential rate law expression where the rate constant k, and the order of reactant A (the exponent m) must be experimentally determined. Never, ever forget that the value of k is temperature dependent. Since two reactants are present in our example reaction we can write comparable expressions for reactant B, but beware that the order of B will not necessarily be the same as the order for A, so we often use a different variable, such as n, for the exponent on B. The differential rate law can be integrated to link changes in concentration with time as opposed to rate. This sounds way more complicated that it really is! ( Integrated is a Calculus term you need not worry about in this course we will linearlize the data to avoid Calculus since it is not a prerequisite for AP Chemistry.) In this experiment you will investigate the reaction of crystal violet with sodium hydroxide. Crystal violet, in aqueous solution, is often used as an indicator in biochemical testing. The reaction of this organic molecule with sodium hydroxide can be simplified by abbreviating the chemical formula for crystal violet as CV. CV + (aq) + OH (aq) CVOH(aq) As the reaction proceeds, the violet-colored CV + reactant will slowly change to a colorless product, following the typical behavior of an indicator. The color change will be precisely measured by a colorimeter (see Figure 1) or spectrophotometer set at 565 nm (green) wavelength. You can assume that absorbance is directly proportional to the molar concentration of crystal violet according to Beer s law. [ ] Figure 1 The rate law for this reaction is in the form: rate = k[cv + ] m [OH ] n, where k is the rate constant for the reaction, m is the order with respect to crystal violet (CV + ), and n is the order with respect to the hydroxide ion. Since the hydroxide ion concentration is much more than the concentration of crystal violet, [OH ] will not change appreciably during this experiment. This technique is often referred to as swamping. Thus, you will find the order with respect to crystal violet (m), but not the order with respect to hydroxide (n). Therefore, the rate constant you will determine is a pseudo rate constant.

2 You will use integrated rate law methods to determine the order m and the value of the rate constant by graphing the absorbance and time data that you collect. Set up your axes so that time is always on the x-axis. Plot the absorbance of CV + on the y-axis of the first graph. Plot the natural log of the absorbance of CV + (ln [CV + ], NOT log[cv + ]) on the y-axis of the second graph and the reciprocal of the absorbance of CV + on the y-axis of the third graph. You are in search of the best linear fit. Here comes the elegant part If you do the set of graphs in this order with the y-axes being concentration, natural log of concentration and reciprocal concentration, the alphabetical order of the y-axis variables leads to orders of 0, 1 and 2 respectively for CV +. You can then quickly derive the integrated rate law equations using y = mx + b. Zero order k = negative slope First order k = negative slope Second order k = the slope OBJECTIVES In this experiment, you will React solutions of crystal violet and sodium hydroxide at different temperatures. Graph the concentration-time data and use integrated rate law methods to determine the order of CV and the value of a pseudo rate constant, k, for the reaction. MATERIALS Data collection device computer or handheld colorimeter or spectrophotometer thermometer cups or beakers 0.10 M NaOH solution M crystal violet solution plastic cuvettes syringes PROCEDURE 1. Obtain and wear goggles. 2. Set up the data collection system. Prepare a blank by filling an empty cuvette 3/4 full with distilled water. Seal the cuvette with a lid. To correctly use a Colorimeter cuvette, remember: All cuvettes should be wiped clean and dry on the outside with a tissue. Handle cuvettes only by the top edge of the ribbed sides. All solutions should be free of bubbles. Always position the cuvette with its reference mark facing toward the white reference mark at the top of the cuvette slot on the Colorimeter.

3 3. Calibrate the Colorimeter. a. Place the blank in the cuvette slot of the Colorimeter and close the lid. b. Press the < or > button on the Colorimeter to set the wavelength to 565 nm (Green). Then calibrate by pressing the CAL button on the Colorimeter. When the LED stops flashing, the calibration is complete. 4. On the Meter screen, tap Length. Change the data-collection length to 180 seconds. Select OK. Data collection will last 3 minutes. 5. This first trial will be performed at room temperature. You have been 0.10 M NaOH (colorless solution) and M crystal violet (purple solution). Suck up 5 ml of each into one of two syringes. a. Simultaneously squirt both solutions into a beaker or cup then QUICKLY do step 5b. Swirl the cup the mix, rinse the cuvette with the mixture, discard the rinse into the sink, refill the cuvette at least 2/3 full, wipe off finger prints, place a lid on the cuvette and place it correctly in the colorimeter. Close the lid of the colorimeter and start data collection. Once the trial is finished, discard the reaction mixture into the sink. 6. Analyze the data graphically to decide if the reaction is zero, first, or second order with respect to crystal violet using the following methods: Zero Order: Choose Curve Fit from the Analyze menu. Select Linear as the Fit Equation menu. The linear-regression statistics for these two data columns are displayed for the equation in the form y = mx + b where x is time, y is absorbance, m is the slope, and b is the y-intercept. The correlation coefficient, r, indicates how closely the data points match up with (or fit) the regression line. A value of 1.00 indicates a nearly perfect fit. Select OK. Record regression equation and the associated r value into your table. First Order: Follow these directions to create a column of the natural log (ln) of absorbance. a. Tap the Table tab to display the data table. b. Choose New Calculated Column from the Table menu. c. Enter the Name (ln Abs) and leave the Units field blank. Select the equation, Aln(X). Use Absorbance as the Column for X, and 1 as the value for A. d. Select OK. 7. Follow this procedure to plot a best-fit regression line on your graph of ln absorbance vs. time: a. Choose Curve Fit from the Analyze menu. b. Select Linear as the Fit Equation menu. Select OK. Second Order: 8. Follow these directions to create a calculated column, 1/Absorbance. a. Tap Table to display the data table. b. Choose New Calculated Column from the Table menu. c. Enter the Column Name (l/abs) and leave the Units field blank. Select the equation, A/X. Use Absorbance as the column for X, and enter 1 as the value for A. d. Select OK. 9. Repeat Step 7 to plot a best-fit regression line on the graph of 1/Absorbance vs. time. Record the equations of the three graphs and the associated r 2 values. Determine the order of the reaction. Data Table 1: (NEXT PAGE)

4 Graphing Equation Regression (r 2 ) Zero 1 st 2 nd Which order is it? order! LAB WRITE UP INSTRUCTIONS NAME, DATE, TITLE, PURPOSE, PROCEDURE PRE-LAB QUESTIONS 1. Compare the molar concentration of the crystal violet solution to that of the sodium hydroxide solution. Approximately how much more concentrated is the sodium hydroxide? Justify your answer. 2. Why do we set the spectrophotometer or colorimeter to a wavelength of 565 nm or green in order to measure the absorbance of crystal violet? 3. What law allows us to use and graph absorbance in lieu of concentration? Explain. 4. A student mixes 3.00 ml of M crystal violet solution with 3.00 ml 0.1 M sodium hydroxide both at 24.5 C and collects the following data: Time(s) Absorbance (a) Describe the graphical analysis steps the student should perform in order to determine the (i) (ii) Order of the reaction with respect to crystal violet Value of the rate constant, k (b) Use a graphing calculator or computer graphing software to determine the order of the reaction with respect to crystal violet. Justify your answer. (c) Write the law expression for this reaction. Justify your answer. (d) Determine the value of k including its units. Justify your answer. (e) Calculate the half life of the reaction. Include units with your answer. (f) Determine the absorbance of crystal violet at 3.00 minutes. Justify your answer. (g) Determine the time at which the absorbance of crystal violet is equal to DATA TABLE (copy as part of prelab work but start on fresh page)

5 POST-LAB QUESTIONS 1. Write the rate law of the reaction (include units on K) 2. What is the order with respect of the sodium hydroxide? Why? 3. What is the initial concentration of the CV? (see regression equation)