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Le Chat: Simulations in Chemical Equilibrium is a computer-based graphical illustration of the chemical equilibrium phenomenon, in relation to Le Châtelier's principle. Le Chat illustrates the movement toward equilibrium for reactions in the gas phase, as well as the changes produced in the equilibrium state by a given perturbation of concentration or partial pressure, temperature, or volume (total pressure).
Le Chat is intended for high school and freshman university students, but can be used to investigate problems of interest to more advanced students.
The basic steps in running Le Chat are choosing a chemical system, setting initial conditions for the system, and running a simulation of the system's progress to equilibrium. The main screen is illustrated in Figure 1. For any chemical system, each substance in the system is represented by a specific color. Color-coding is continued in the chemical formula for the reaction, in the bar-graph representations of the initial and final conditions for the reaction, in all of the plots, and in the reaction simulation area. In the reaction simulation area, located in the lower left corner of the window, colored dots represent molecules of the substance of the corresponding color. The number of dots of each color qualitatively reflects the relative concentrations (molar fractions).
Le Chat defines a simulation as a plot of concentration or partial pressure versus time for a specific chemical system with given initial conditions. The free energy value is plotted during a simulation and students can watch it change until it reaches zero (at equilibrium). Values for G°, H°, and S° are also shown.
A sequence of simulations is defined in Le Chat as a group of simulations for the same chemical system in which each simulation has unique initial conditions. In Figure 1 a second simulation at the same initial concentrations of compounds but at a higher temperature is about to reach equilibrium. A record of each simulation that has been performed on the current chemical system is located in the upper-left corner of the screen. Each time a new simulation is performed on the chemical system, a graphical representation of the simulation is placed in a panel in that area. It is possible to review a simulation by clicking its panel, to print the results of any simulation in the sequence, and to delete simulations. The sequence of simulations can also be saved and reloaded later.
The on-screen help window in the upper-left corner provides content sensitive background information concerning Le Châtelier's principle and the main operation instructions. By default, this area uses components from Internet Explorer and shows information in HTML format. There is also an option to view-screen help in rich text format (RTF).
Students can select from 11 pre-programed equations, as shown in Figure 2. You can also add or delete equations (see Fig. 3) or chemical substances (see Fig. 4).
Selected Images from Le Chat, Part of the General Chemistry Collection
Figure 1. Le Chat's main screen.
Figure 2. It is easy to select a pre-programed equation in Le Chat.
Figure 3. New equations are entered in this window.
Figure 4. New substances can be added to Le Chat's data base.
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