Raman Spectroscopy as the Method of Detection for Constructing a Binary Liquid Vapor Phase Diagram

T physical chemistry laboratory course is an essential experience to the student wishing to pursue an advanced degree in chemistry or a career in a chemistry related field. A study in 2004 on physical chemistry curriculum points out that physical chemistry bridges physics, chemistry, and mathematics and develops complex critical-thinking skills required by scientists working on projects with interdisciplinary foci. Physical chemistry now encompasses subareas too numerous to list and contributes to the study of almost all the other disciplines in chemistry. Today in a growing number of universities, however, the physical chemistry lab has been scaled back from the traditional two-semester period to just one semester. For example, in a recent informal survey of chemistry programs in the Southern Universities Group, approximately a quarter of the schools now only require one semester of physical chemistry lab to obtain a B.S. degree in chemistry. The loss of a whole semester has different roots, such as the desire to reduce the number of hours in a major or to make room for other new specialty courses such as environmental or forensic chemistry. The problem, therefore, arises in being able to provide students with enough lab experience to encompass the important areas of physical chemistry. Moore pointed out that it is extremely important to make sure the physical chemistry curriculum reflects what physical chemists really do. With this in mind, in 1999, the University of Maryland integrated a program of physical and analytical techniques to investigate significantly more complex systems than those incorporated with one curriculum. In 2004, Grand Valley State University sought to incorporate both computational and traditional physical chemistry laboratories in their curriculum, stating that the coupling of laboratories allows students to experience an experiment from amacroscopic, empirical point of view as well as an abstract, theoretical level that considers molecular-level events. Combining multiple laboratory procedures into fewer multidimensional lab exercises allows students to be provided with opportunities encompassing multiple facets of physical chemistry in a constrained time frame. Constructing a binary liquid vapor phase diagram is a classic physical chemistry experiment that is still taught in many chemistry departments and serves to illustrate an important physical concept. Here, we show that two other important concepts can be taught concurrently in this laboratory exercise, specifically molecular spectroscopy and quantum chemistry. The classic binary liquid vapor phase diagram experiment utilizes Raoult’s law, which states that vapor pressure of an ideal solution is directly proportional to the mole fraction of each component. This law is a good approximation for a component only when its mole fraction is close to unity. Deviations, positive or negative, from Raoult’s law give insight into the thermodynamic properties of the components involved. There have been many adaptations to the analysis of the components involved, including using refractive index, gas chromatography, an isotensicope, and a mercury manometer. 12 When choosing an analytical method, a physical property must be selected that changes significantly and smoothly over the composition range. Raman spectroscopy is such a technique that allows for the relative concentration of a component in a mixture to be ascertained in a nondestructive manner, as it is quite reasonable to assume that the intensity of the Raman signal is proportional to the mole fraction of the component in question. Raman spectroscopy, like infrared, yields frequencies that stem from the normal vibrational modes of the molecule under study. ABSTRACT: The physical chemistry laboratory is sometimes constrained to one semester, resulting in pedagogical deficiencies for the students taking the course. The use of a multidimensional laboratory exercise offers students the opportunity to encounter multiple experimental techniques and physical chemistry concepts while not sacrificing a significant amount of time. Here, we show that the classic binary liquid vapor phase diagram experiment can be modified to incorporate both molecular spectroscopy and quantum chemistry. Using Raman spectroscopy as a means of detection and quantum chemistry to investigate the resulting vibrational spectra, students can explore three important subject areas in one laboratory exercise.