Differences in Free Energy of Solvation in a Two-Layer System

Transcription

1 Differences in Free Energy of Solvation in a Two-Layer System (Original exercise designed by William Steel and modified by James Foresman) Purpose: By observing the changes in the concentration of a solute in different phases of a liquid-liquid system we can determine the difference in free energies of solvation for the two solvents. Further, we can compare this quantity to one obtained using quantum mechanical calculations which employ a reaction field model for the solvent. Introduction: The process we will examine is the migration of 5-chlorosalicylic acid across the interface between adjacent layers of two solvents: water and cyclohexane. cyclohexane 5-chlorosalicylic acid Cyclohexane is a non-polar solvent while 5-chlorosalicylic acid contains both polar and non-polar components (overall we would consider it polar). Given that water is very polar, the 5-chlorosalicylic acid can, and will be attracted to both solvents. Thus, if we begin with some 5-chlorosalicylic acid in water and introduce some cyclohexane to the system we should observe a decrease in the concentration of the 5-chlorosalicylic acid in the water phase and an increase in its concentration in the cyclohexane phase. If we began with the acid in the cyclohexane, we would expect the opposite to be true. In order to follow the migration of the 5-chlorosalicylic acid between the solvents we need to be able to observe it. To do so we can use UV-vis spectroscopy. The benzene ring in 5-chlorosalicylic acid is known as a chromophore it will absorb light at different wavelengths depending on the solvent in which it is dissolved. In cyclohexane the maximum absorbance occurs at 312 nm, while in water it occurs at 300 nm. If we make a Beer s Law plot of the absorbance of the solute at

2 different concentrations for each solvent we can use it to determine the concentration of the solute in each solvent layer. Knowing this there are several ways in which we could proceed to observe the migration of the solute. In this lab we will conduct the partitioning experiment in a single cuvette this has the benefit of reducing the chemical waste we produce and simplifies the procedure. Since the water layer will always be more dense than the cyclohexane layer it will always occupy the lower portion of the cuvette this presents a small challenge for determining the concentration of 5-chlorosalicylic acid in the cyclohexane layer. Essentially all of the acid that we add to our system initially must be in either the water or the cyclohexane (a very small amount can actually be stuck on the interface). By observing the concentration of 5-chlorosalicylic acid in water we can use subtraction to determine the amount that is in the organic layer. Procedure: 1. Gather supplies for this experiment: 5-chlorosalicyclic acid several volumetric flasks (5 x 25 ml and 2 x 500mL) distilled water and cyclohexane 5 dry and clean plastic cuvettes several Pasteur pipettes (both 1 and 2 ml) 100 ml and 250 ml beakers for waste 2. Label each pipette so that you have one for use with the aqueous solution of 5-chlorosalicylic acid, one for the cyclohexane solution, one for pure water, and one for pure cyclohexane. Turn on the UV-vis spectrometer and adjust the instrument so that it will record absorbance at 300 nm. 3. Prepare stock solutions by dissolving approximately 20 mg of the acid in 500 ml of each solvent (water and cyclohexane). The mass should be known precisely. 4. Prepare 5 serial dilutions of the aqueous solution of 5-chlorosalicyclic acid for use in a Beer s Law analysis (use 25ml volumetric flasks and add 1, 2, 4, 6, and 8 ml portions to each, diluting to the mark). Collect data for a Beer s Law plot by placing 3 ml of each solution into clean, dry cuvettes. Clean the cuvettes and empty into a large beaker that will serve as your waste container. 4. In a clean, dry cuvette, place 3 ml of water. Now add 1 ml of the cyclohexane solution. Cap and shake gently for two minutes. Allow a few minutes for the system to reach equilibrium. Obtain the absorbance of the aqueous layer several minutes apart until there is no change. 5. Clean up all glassware being sure to collect all solutions in a waste container.

3 Data Analysis: Using the absorbance value of your water/cyclohexane system and your Beer s Law plot, find the concentration of solute in each layer. Use this to calculate the equilibrium constant for the migration of 5-chlorosalicyclic acid from the cyclohexane layer to the water layer. Computational Work: Use Gaussian to determine the free energy of solvation (ΔG solv ) of the 5-chlorosalicylic acid in each of the solvents. You should use the B3LYP theory model and G(2d,p) basis set. Look carefully at the structure as presented on the first page of this handout (the orientation of the hydroxyl and carboxy hydrogens). Use this conformation for your calculations. You should divide up the work among the group so that one person performs a geometry optimization and frequency calculation using the SMD model and cyclohexane as the solvent. The other person can perform a geometry optimization and frequency calculation using the SMD model and water as the solvent. Using your two G values, determine the ΔG for the partitioning of the solute. This value can be related to the equilibrium constant for the partitioning: ΔG = -RTlnK eq Using your K eq values determine an experimental value for this ΔG and compare it to your theoretical value. Discuss the sources of any differences that you observe. Use Gaussian to predict what would happen if benzene were used instead of cyclohexane in this experiment. Discuss.

4 Physical Chemistry Lab Name Gibbs Free Energy of Solvent Partitioning 1. What was the Molarity of the stock solutions used in this lab? Cyclohexane Water If you prepared the solutions yourself, what mass of solute was used in how many Liters of solvent? 2. Data for the Beer s Law Plot: volumetric flask used for dilutions ml ml of stock Molarity Absorbance Please attach your Beer s Law Plot to this datasheet. 3. Data from Partitioning Total Moles of solute Absorbance in water Molarity in water Molarity in CHEX 4. Explain how you performed the partitioning part of the lab. Describe how you created a two layer system in the cuvette and how you obtained the absorbance data. 5. Calculate the experimental equilibrium constant for the solvent partitioning. CHEX is the reactant, water is product.

5 6. Record the Gibbs Free Energy calculated for the solute from Gaussian09: Functional and Basis set Temp Solvent G [a.u.] 298 CHEX 298 Water 298 Benzene 7. Calculate the G for partitioning from the Gaussian data in kj/mol. 8. Compare and discuss your experimental and theoretical results. How well does Gaussian predict the preference for solvation in this case? You will need a calculation that relates G and K (watch units!) 9. What if we considered using benzene instead of cyclohexane? Use a Gaussian calculation to estimate the partitioning equilibrium constant between benzene and water. What does your prediction say about the relative solubility of this solute in benzene versus cyclohexane?