The topic of phase behavior in physical chemistry has strong appeal for the visual learner. First, a large body of data is consolidated into a multi-dimensional map of equilibrium behavior: the phase diagram. Second, conversations about equilibrium phase behavior are readily supported by live demonstrations or laboratory experiments. These direct observations of phase transitions emphasize the dynamic process by which a chemical system achieves a new state of phase equilibrium. However, the standard phase diagram is constrained to the printed page, and correlating the information contained in this static image with experimental observations of dynamic behavior requires an imaginative and informed mental leap on the part of the student. The many different lines and areas found in two-component phase diagrams are often challenging for a student to decode and analyze, although knowledge of the Gibbs phase rule can aid a student in this task (1).
To help students connect a dynamic change of state with its representation on an equilibrium phase diagram, six interactive two-component phase diagrams have been created using Excel spreadsheets. Spinner controls embedded in each spreadsheet allow the system composition and the temperature (or pressure) to be varied stepwise using mouse clicks (Figure 1). The spreadsheet automatically updates the location of the system on the phase diagram, drawing in the isopleth, generating a tie line if the equilibrium state is two-phase, and indicating the compositions of all phases present. For additional clarity, every single-phase region in the diagram is its own color. All two-dimensional regions in which the system is either metastable or unstable as a single phase are left white. Several spreadsheets also have a cartoon representation of the physical state of the system; these cartoons are likewise updated as the spinner controls are clicked.
Figure 1. Screen capture of an interactive methanol/cyclohexane liquid–liquid phase diagram created in Excel. The system composition and temperature can be varied stepwise by clicking on the spinner controls. Tie lines are automatically generated in the phase diagram when the system separates into two phases. A representation of the state of the system is shown in the cartoon ampule to the left of the phase diagram.
The collection of interactive phase diagrams includes one solid–liquid system, two liquid–vapor systems, and three liquid–liquid systems. These systems are listed in Table 1, along with a brief explanation of the motivation behind their selection. All coexistence curves presented in the interactive phase diagrams are based on experimental literature data combined with theoretical models at various levels of sophistication. The online documentation provides greater detail, including student study questions, theoretical assumptions, coexistence-curve equations, citations, and guidance as to how these spreadsheets may be adapted to other binary systems.
Table 1. The Six Two-Component Systems for Which Interactive Phase Diagrams Have Been Created with Excel Spreadsheets
System
Type
Comments
Durene:Phenanthrene
S-L
A simple two-component eutectic system that does not form compounds. A laboratory experiment based on this system has been reported (2).
Benzene:Toluene
L-V
Often used as a model of liquid–vapor equilibria in physical chemistry textbooks, in part due to the near-ideal interactions of the two components. Laboratory experiments based on this system have been reported (3,4).
Butane:Propane
L-V
The coexistence curves of this system occur over a pressure range of a few bars at room temperature. Vapor-phase molar volumes are relatively small at these pressures, making the cartoon piston representation easier to visualize.
Methanol:Cyclohexane
L-L
A well-studied system with an upper critical point. Several laboratory experiments based on this system have been described (5–8).
Acetone:Glycerol
L-L
Another system with an upper critical point. The molar volumes of the two components are nearly equal, so mole ratios predicted by application of the lever rule to the phase diagram match the volume ratio of the liquid layers in the cartoon.
Butanal:Water
L-L
An analog for an oil/water immiscible system, except with sufficient miscibility such that the coexistence curve boundaries are clearly displaced from the vertical axes of the phase diagram.
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