Lesson Scenario - Generic Chemical Dynamics (Vensim)

Basic Model:

Description

This is a simple dynamics model of generic chemical reaction relationships. The reactions model consists of three compounds - the reactant, an intermediate compound, and the product. As the model progresses, reactants change into intermediaries, which then change to products. The results of this model simply show how reactants change upon reactions. Users can change the parameter of reactions 1 and 2 to view the rates of transformation.

Background Information

Generic Chemical Dynamics is a process that leads to the transformation of one set of chemicals/substances to another. The substance (or substances) initially involved in a chemical reaction is known as reactants or reagents. Chemical reactions are usually characterized by a change that yields one or more products, which usually have properties different from the reactants. Chemical reactions happen at different rates/times. At a given temperature and chemical concentration rapid reactions, described as spontaneous, requiring no input of extra energy other than thermal energy can take place. Non-spontaneous reactions are considerable slower and require the input of additional energy (such as heat, light or electricity) to complete the reaction.

Science/Math

Chemical reactions are described with chemical equations, which graphically present the starting materials, end products, and sometimes intermediate products and reaction conditions. For each chemical equation there can be a mixture of numbers and letters, usually element symbols, to represent the substances present in the model/reaction. An example chemical equation used in the Vensim model can be derived using (A-Reactant, B- Intermediate Compound, C-Product)

A + B --> C or Reagent + Intermediate Compound --> Product

More elaborate reactions are represented by reaction schemes, which in addition to starting materials and products show important intermediates or transitions.

Teaching Strategies

When introducing this subject, it is important to teach or review different chemistry vocabulary pertaining to chemical reactions and equations. Be sure to explain chemical changes and their reactions with emphasis on cause and effect. This can be demonstrated with simple vinegar and baking soda experiments or using online videos/simulations. For further explanation and examples visit: http://www.ric.edu/faculty/ptiskus/reactions/

Implementation:

How to use the Model

This is a very short and simple Vensim model. There are 3 parameters in the model.

1. Circular Variables- K1 or K2 - Connected to both the rate flow and box variables and serve constant factors to the rate outcome
2. Rate flows (hourglass symbols) RXN1 (reaction 1) or RXN2 (reaction 2) - Help to control the flow of the box variables to represent change over time. The direction of the rate arrow shows the flow from reactants to products.
3. Box Variables- the reactant, an intermediate compound, and the product- Serve as the quantities to the model. Each box variable has its own level that affects the results of the model.
   • Ex. Similar to a real chemical reaction; i) symbolizing the amount of the reactant and ii) symbolizing the amount of intermediate compound will yield iii) symbolizing the amount of the product.

The blue arrows show which variables depend on each other. Reaction1 can be manipulated by changing either the K1 or Reactant amounts and Reaction 2 can be manipulated by changing either K2 or Intermediary amounts. After setting the parameters for rates of transformation between the stages, click run to simulate the reaction. For more information on Vensim, reference the Vensim tutorial at: http://shodor.org/tutorials/VensimIntroduction/Preliminaries.

Learning Objective:

1. Understand the relationship of chemical reactions as system models
2. Understand the effect of parameters on the chemical reactions

Objective 1

To accomplish this objective, first review simple cause and effect concepts using chemical reaction examples. Later have students look at the model to see if they can find any relationship between the values over time, without changing the initial amounts. Suggest trying different amounts for the reagent and intermediary compound and then look at how the product amount changes.

Objective 2

Have students experiment with each of the manipulable parameters to see the effect on the simulation. Ask the following questions to guide their exploration:

1. What happens to the graph if you increase or decrease the amount of reactant? Intermediary compound?
2. Does the k1 or k2 values have any effect on the product values on the graph? If so, what is the effect?
3. How much Product is created after 1 second if the initial reagent is 500, k1 is 0.5, k2 is .003 and the intermediary is 2? What happens to the value after 2 seconds if we change k1 to 0.05? 0.25? What do you think would happen if we made the reagent amount extremely small?
4. What happens to the shape of the graph if you increase or decrease the reagent amount?

Extension:

1. Model how other variables can affect the product result (i.e. added heat, temperature, etc�)

Extension 1

Ask students to project what would happen if there were more variables put into the equation. As a group or individually have students come up with some variables they would like to add to the model for example, heat, light or electricity. Then add the variables to the model and using the blue arrows decide what each variable should affect and be connected too. Ask the following questions:

1. In what ways do we want the added variables to change the model? Increased product? Decreased starting materials?
2. What do the variables have in common, if any?
3. How do you interpret the model flow/behavior with the added variables?

Related Models

Reaction Data (Excel)

This model represents the progress of a chemical reaction from start to finish. In general, the speed of a chemical reaction is related to the concentration of the reactants, so as the reaction progresses it tends to slow down, eventually approaching completion. This model documents several different reactions over the course of several experiments each in order to get the most accurate measure possible of their speed. The model allows the user to set the concentration of the reactant with the �fraction� slider, after which point the results are displayed on the graph to the right. Each reaction includes a graph of reactants and products.

Set the initial fraction of the reactants, and then update the cells and graph. The results will be shown automatically. To view other reactions, switch to any of the other four pages.

Reversible Consecutive First-Order Reactions (Vensim)

This model represents a two-stage reversible reaction. Many organic processes operate by such a reaction, in which the reaction can both transform products to reactants, and reactants back to products. In such a scenario, rather than going to completion the reaction will reach equilibrium. There are three stages in the reaction, represented by groups A, B, and C. As time goes by, A transforms to B, and B to C, but the reverse also occurs. The parameters of interest are k1, k2, k3, and k4, which determine the rate at which each stage of the reaction occurs. Once you have changed the parameters to your liking, simply run the reaction to view the results.

Michaelis-Menton Equations (Vensim)

The Michaelis-Menten model of chemical reactions in essence states that a substrate combines with an enzyme to form an activated complex. This complex then reacts to form an enzyme and a product. The reaction specified by this equation is the most common representation of a chemical reaction, used in fields from biochemistry to neuroscience.