Exploration of chemical kinetics of sulfur trioxide formation.
Purpose: To visualize a graphical representation of the chemical reaction: 2SO2 + O2 
2SO3, as well as calculate the equilibrium constant.
Tool(s) used: Vensim PLE
Approximate time to completion: 15-20 minutes, or longer if you care to alter any
aspects of the setup.
Educational level: High school or beginning undergraduate students
Overview: In addition to being very a very toxic gas to humans in close contact with it,
sulfur dioxide reacts with oxygen in the atmosphere to form sulfur trioxide. Sulfur
trioxide then reacts with water molecules (present in the atmosphere as well) to form
sulfuric acid droplets, one of the important components of acid rain. Acid rain exerts its
harmful and destructive effects on soil, plants, aquatic animals as well as infrastructure.
In recent years, efforts have been put in place to reduce emissions of sulfur (and
nitrogen) compounds. Coal fired power plants are capable of producing the majority of
sulfur dioxide in the Midwest, which can have devastating effects on the environment.
This exercise covers the formation of sulfur trioxide from the reactants sulfur dioxide
and oxygen, in order to investigate the subject of kinetics and limiting reactant.
Exercise:
1. Open Vensim PLE and construct the model seen here:
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Note: SO2i (i: initial), O2i, SO2, O2, k fwd and k rvrs are all variables (auxiliary/constant,
not box variable). SO3 is a box variable.
2. Click on the ‘Equations’ icon, and remember that any icons highlighted in black
need numerical values or formulas added to the model. Use the following values
for the respective icons:
SO2i: = 2 (for initial value)
O2i: = 1 (for initial value)
SO2: = SO2i – SO3
O2: = O2i-SO3
k fwd: = 4.36
Rxn fwd: = k fwd*O2*SO2
SO3: = Rxn fwd-Rxn rvrs, initial value = 0
k rvrs: = 4.36
Rxn rvrs: = k rvrs*SO3
3. Click on ‘Model’ in the menu bar and choose ‘Settings.’ Change the values to
these:
Initial time: 0
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Final time: 0.5
Time step: 0.007812
(Make sure that ‘Save results every TIME STEP’ is checked)
Units for time: Second
Click ‘OK’
4. Click the ‘Set’ icon and make sure you are using the ‘RK4’ integration method.
5. Click the ‘Run’ icon and run the model.
6. Click on the ‘Control Panel’ icon, on the ‘Graphs’ tab, click ‘New,’ and enter in the
values as seen below and then click ‘OK’
7. Click ‘Display’ on the control panel window to see the graph. It should look
something like this:
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8. Now click on the ‘Automatically simulate on change’ icon (next to the ‘Run a
simulation’ icon). Click ‘OK’ to overwrite data set. This will display slider bars for
all of the variables. Play around with the different sliders, paying particular
attention to what happens when you move the ‘k fwd’ or ‘k rvs’ bars.
Questions:
1. How does moving the ‘k fwd’ slider affect the concentrations of the reactants and
products?
2. Regardless of the rate of chemical reaction, which reactant acts as the limiting
reactant? How does consuming all of the limiting reactant affect the product?
3. There is an extremely large supply of oxygen in the atmosphere, and the
formation of sulfur trioxide in the presence of water vapor (also present in the
atmosphere) leads to the subsequent (and rapid) formation of sulfuric acid which
falls to the earth as acid rain. Plants are the main source of oxygen production on
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earth. What might happen if sulfur dioxide emissions from places like coal fired
power plants were to go unchecked forever?
©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright