WHAT IS IT? ----------- This project shows how a simple chemical system comes to different equilibrium states depending on the concentrations of the initial reactants. It is very similar to its partner model, "Chemical Equilibrium 2," which shows how an identical system's dynamics depend on the kinetics of the forward and backward equilibrium reactions. Equilibrium is the term we use to describe a system in which there are no macroscopic changes. This means that the system "looks" like nothing is happening. In fact, in all chemical systems microscopic processes continue but in a balance that yields no changes at the macroscopic level. This model simulates two simple reactions of four molecules. The reactions can be written A + B yields C + D. At the same time, of course, C + D yields A + B. A classic real-life example that would illustrate such reactions is the reactions of carbon monoxide with nitrous dioxide to produce carbon dioxide and nitrous monoxide. The reverse is also possible. All the reactants are gases. We could watch such an equilibrium system because NO2 is a reddish colored gas which is visible. Upon combining NO2 with carbon monoxide, the two gases forming the colorless products NO and CO2, and the system loses reddish color. However, not all the color is lost. Ultimately the system comes to equilibrium with some of the "reactants" and some of the "products" present. How much "reactant" and "product" a system ends up with depends on a number of factors. The inherent kinesthetics of the reaction are of vital concern... some reactions tend to go in a particular direction because energy is released in that direction, for instance. A system's equilibrium is also affected by the concentrations of the system's reactants and by the temperature of the system. HOW TO USE IT ------------- As stated above, this model simulates a chemical system of four different molecules. They are represented on the graphics screen as squares of four different colors. In this simulation, yellow molecules react with blue molecules to produce green molecules and brown molecules. The model is setup by first adjusting the YELLOWMOLS and BLUEMOLS sliders and pushing the SETUP button. YELLOWMOLS sets how many yellow molecules the simulation starts with, while BLUEMOLS sets how many blue molecules the simulation starts with. The GO button sets the simulation in motion. Molecules move randomly and react [changing color to represent rearrangement of atoms into different molecular structures] with each other. The system soon comes into equilibrium. Four monitors show how many of each kind of molecule are present in the system. There is also a plot window which plots the number of each kind of molecule present versus time. RUNNING THE MODEL ----------------- 1) THINGS TO NOTICE ------------------- Notice that the number of product molecules is limited by the smallest amount of reactant product. Notice that there are always the same number of reactant products since they are formed in a one-to-one correspondence with each other. 2) THINGS TO TRY ---------------- How do different amounts of the two reactants affect the final equilibrium. Are absolute amounts important, or is it a ratio of the two reactants that matters? Try setting the YELLOWMOLS slider to 400 and the BLUEMOLS slider to 20, 40, 100, 200, and 400 in five successive simulations. What sort of equilibrium state do you predict in each case? Are certain ratios predictable? EXTENDING THE MODEL ------------------- One variation of this model already exists. Chemical Equilibrium 2 shows a similar system in which forward and reverse reaction rates are the variables controlled instead of initial concentrations. Look at this other model and consider how the two fit together. You could put the two models together and see if concentration and reaction rates act independently of each other on the final equilibrium state. You could also extend the program by allowing the user to introduce new molecules into the simulation while it is running. How would the addition of fifty blue molecules affect a system that was already at equilibrium? STARLOGO FEATURES ----------------- Notice that the primitive CLEARPLOT is not abbreviated in this program. CP is no longer the abbreviation for CLEARPLOT as it is now used as a primitive to clear all patches. Notice also that the primitive STOP which is used at the end of a series of consequents to IF statements checking for neighboring molecules, stops the turtled from executing not just that single consequent, but the entire procedure. This is vital to this model, or a single blue molecule surrounded by a number of yellow molecules at once could single- handedly change all of them to brown. Instead, the blue molecule changes only one to brown (the first one it "notices") and then stops the entire looking procedure.