EXPERIMENT 15 TF Notes 1. Have students log onto LoggerPro3 and start heating water as soon as they arrive 2. This reaction is easily contaminated so students must never insert the thermometer directly into the cuvette containing the reaction. Instead they should rather measure the temperature of the water and equate that to the temperature of the solution in the cuvette. 3. We will be using hot gloves in this experiment. 4. Make sure you are familiar with the calculations necessary to fill out the chart. All the necessary equations are given at the top fo the page. Note that the temperature must be converted from °C to K. EXPERIMENT 15 Thermodynamics of Complex-Ion Equilibria Introduction Thermodynamic data for a reaction system provides researchers with information that is important from both theoretical and practical points of view. There are several thermodynamic properties that chemists pay close attention to when designing or carrying out experiments such as thermodynamic stability, the change in free energy of a reaction, and temperature dependence. For example, if a chemist wants to create a new type of solar cell that combines a semiconductor material with a novel conductive oxide and wants to make sure that the two materials will not react with each other, thermodynamics provide the answer. By finding the free energy change associated with the reaction, s/he can determine how stable the layers are in contact with each other and to what temperature. In this experiment, you will learn how to determine those parameters from a controlled experiment by using spectrometry to find concentration data at various temperatures. This will allow us to determine the equilibrium constant, which can then be used to derive ΔG°, ΔH°, and ΔS°. Scientific Background The equilibrium we will study is the complexation of aluminum ions with xylenol orange, symbolized as – H4Q, which results in the formation of the complex ion AlQ . Because both the reactants and products strongly absorb light, but at different wavelengths, this reaction is ideal for spectrophotometric study. 3+ – + H4Q + Al ←→ AlQ + 4H yellow colorless red colorless Recall that molecules are colored because they absorb certain wavelengths of visible light, while allowing other wavelengths to pass through. Xylenol orange is a yellow-colored organic molecule containing four acidic –COOH groups attached to a system of three benzene-like carbon rings (Figure 3.1). The xylenol orange acts as a ligand, which can bind to the aluminum ion through the six atoms shown in boldface type. In the process, the four protons attached to the –COOH groups are released from the molecule. The color of this complex ion AlQ– is red. O O O O O O O OH HO O N Al N N N HO OH Al3+ O O HO O HO O O O H3C H3C O O S S OH OH OH OH -) Xylenol Orange (H4Q) Xylenol Orange bound to Aluminum (AlQ Yellow Red Measuring absorbance at equilibrium using Beer’s law – The complex AlQ absorbs very strongly at a wavelength of 550 nm. The unreacted molecule H4Q, on the other hand, absorbs much more weakly at this wavelength. The total absorbance at 550 nm can be determined by applying Beer’s law to both species and taking the sum: – A = ε1[H4Q]L + ε2[AlQ ]L where L = 1 cm In our experiment, we will start with a certain initial concentration of H4Q and then heat the mixture to form the complex AlQ–. The complex AlQ– does not form at room temperature, but will form as we heat the – mixture. At any moment, the sum of the concentrations of H4Q and AlQ will be constant and equal to the initial concentration of H4Q, which we can represent as [H4Q]i: – [H4Q]i = [H4Q] + [AlQ ] By combining this equation with the expression for the total absorbance, we can express the absorbance in – terms of the initial concentration [H4Q]i and the equilibrium concentration [AlQ ]: – A = ε1[H4Q]i + (ε2–ε1)[AlQ ] Note that the quantity (ε2–ε1) is positive because ε2 is much greater than ε1, and that we have incorporated the knowledge that the path length is 1 cm. To further simplify this expression, we can define Ai as the initial absorbance before the reaction starts, when there is none of the complex AlQ– : Ai = ε1[H4Q]i Finally, by combining this with our absorbance equation and solving for the concentration of the complex, we find that A " A [AlQ–] = i #2 "#1 4 –1 –1 The quantity (ε2–ε1) has been determined as 2.50 × 10 L mol cm . By measuring the initial absorbance Ai and the absorbance at a specific time A, the concentration of the complex AlQ– can be determined. ! Determining ΔG°, ΔH°, and ΔS° from equilibrium measurements The equilibrium constant K for the complexation reaction is: 4 AlQ" H+ H Q + Al3+ → AlQ– + 4H+ [ ][ ] 4 ← K = 3+ [H4Q][Al ] In this experiment, we will use the spectrometer to find the equilibrium concentration of AlQ– at several + – 2– different temperatures. The concentration of H will be controlled by an HSO4 /SO4 buffer, which should maintain a pH of 2.0. The equilibrium concentrations of the! other species can be determined from the their initial concentrations using simple stoichiometric relationships: – [H4Q] = [H4Q]i – [AlQ ] 3+ 3+ – [Al ] = [Al ]i – [AlQ ] Therefore you will be able to calculate the equilibrium constant K at several different temperatures. From this you can calculate ΔG° at each temperature from ΔG° = –RT ln K Finally, we can use the definition of Gibbs free energy to find ΔH° and ΔS° : ΔG° = ΔH° – TΔS° A plot of ΔG° as a function of temperature should give a straight line with a slope of –ΔS° and a y-intercept of ΔH°. Be sure to use the correct units in all your calculations. Procedure Safety Precautions • Safety glasses and gloves must be worn at all times. • The chemicals used in this experiment are toxic. Change gloves if you suspect that you have spilled any chemical on your gloves. Rinse for 15 minutes at an eye wash station if your eyes are accidentally exposed. • If your skin is accidentally exposed to chemicals, rinse the area with water for 15-20 minutes. • You will be working with hot beakers and cuvettes. Use padded gloves to handle the hot beakers and a test tube holder to handle the hot cuvettes. Waste Disposal • Empty the contents of all cuvettes into the “Used Chemicals” beaker at your lab bench. Use a squirt bottle to rinse the cuvettes with water and pour the rinse into the “Used Chemicals” beaker. • Dispose of empty cuvettes and cuvette caps in the solid waste container. • Empty the “Used Chemicals” beaker into the hazardous waste collection bucket in the back of the lab. • Leave everything else at your lab bench. Using the Spectrometer with Logger Pro Open the Logger Pro software (Go → Applications → LoggerPro) and look inside the spectrometer. You should see a light inside the spectrometer that has a violet color. If the spectrometer light is not on, the spectrometer USB cable may be unplugged. Quit Logger Pro, check the cable, and restart Logger Pro. If the light still does not come on, ask your TF for help. Setting up the Reaction At your bench, you will find two nested beakers. Add distilled water to the inner beaker to the 30-ml mark. Then add water to the outer beaker so that its water level is below that of the inner beaker. Set these beakers on the hot plate and heat the water. Keep an eye on the water; it should boil gently. Add distilled water as needed if it boils too much. Go to the center bench in the laboratory room and obtain two clean plastic cuvettes and two square cuvette caps. Note that each cuvette has clear sides and opaque sides. Handle the cuvettes only by the opaque sides. Never touch the clear sides; fingerprints will interfere with your measurements. If the clear sides of the cuvette become marked or scratched in any way, you must repeat the experiment with new cuvettes. Fill one cuvette with distilled water from your wash bottle, and cap it with one of the square caps. This is your reference cuvette. Bring the other cuvette to the center bench in the lab. You will see two bottle-top dispensers that are calibrated to dispense exactly 1.75 ml of solution. One solution contains xylenol orange 3+ (H4Q) and the other contains aluminum ions (Al ). Carefully squirt 1.75 ml of each solution into the cuvette by smoothly pulling up the top of the dispenser and pushing it down slowly. Your cuvette should be filled almost to the top with 3.5 ml of a pale yellow solution. Put a cap on this solution; this is your sample. Again, be extremely careful not to mark or scratch the clear sides of the cuvette. Calibrating the Spectrometer Gently insert the reference cuvette into the spectrometer. Make sure it is inserted all the way into the spectrometer. The opaque edges should be on the sides of the cuvette when it is inserted, and the light should pass through the clear sides. In Logger Pro, go to the “Experiment” menu. Select “Calibrate ▶ Spectrometer.” Once the dialog box appears, click “Finish Calibration.” You will have to wait a few seconds, and then you can click “OK.” Click on the spectrometer icon at the top of the window (this icon has a “rainbow” appearance). A new window will appear. Select the button labeled “Abs vs. Time.” In the list of check-boxes at right, uncheck any wavelengths that are already selected, and check the box for 550 nm.