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Reversible Consecutive First-Order Reactions Learning Scenario

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Lesson Scenario - Reverse Consecutive First Order (Vensim)

Basic Model:


This model represents a two-stage reversible reaction. Many organic processes operate in which the reaction can both transform reactants to products, and products back to reactants. 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, and upon equilibrium the reverse can also occur

Background Information

A reversible reaction is a different type of chemical reaction where reactants form products, and when reacted together give reactants back. Reversible reactions can be distinguished from normal reactions by double arrows, which symbolize the idea that equations can flow in two directions. Reversible reactions will never complete if they are performed within a closed vessel. In a reversible chemical process, a state of equilibrium is obtained when the rate of forward chemical reaction is equal to the rate of reverse chemical reaction.


Le Chatelier's principle is regularly paired with reversible reactions to help explore the connection of equilibrium concentrations and reverse reactions in chemistry. Le Chatelier states, "The position of equilibrium shifts to try to cancel out any changes you make." Ex.

A + B <--> C + D Increasing the concentration of A means more C and D are produced to counteract the change

A + B <--> C + D + Heat Heating the mixture means the equilibrium moves to the left to counteract the change.

Teaching Strategies

Before teaching this semi-complex topic it is best to reference the generic chemical dynamics learning scenario and resources. Next proceed to explain chemical equations and reversible reactions with the following talking points:

  1. The concept of reactants turning into products. Point out that in many cases chemical equations are oversimplified
  2. Equilibrium. Explain that many reactions reach an equilibrium and the reaction mixture contains both reactants and products particles. The percent of reactants converted to products varies considerably.
  3. Le Chatelier's principle. Point out that the principle connects reverse reactions to equilibrium.


How to use the Model

There are three stages in the 2-staged reaction model, 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 quantity of each group is shown in the graph on the right. Each stage has circular variables labeled K1, K2, K3, K4 connected with blue arrows showing their dependency, and serve as constant factors to the rate outcome. In addition to circular variables there are rate flow arrows which control the organizational flow of the model and 2 sets of box variable reactions labeled frxn1 (First order reactions 1), frxn2 (First order reactions 2), rrxn1 (Reverse order reactions 1), and rrxn2 (Reverse order reactions 2). Each box variable has its own level that affects the results of the model and serves as the quantities to the model. 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. For more information on Vensim, reference the Vensim tutorial at:

Learning Objectives:

  1. Understand the concept of reversible reactions, equilibrium, and Le Chatelier's principle
  2. Understand the main elements on the chemical reaction model

Objective 1

To accomplish this objective have students observe the current model without any added changes and be sure they understand the flow, both first order and reversible, as well as the graph results. Have students complete the reversible reactions review worksheet to keep the concept fresh in their minds.

  • Write reversible reactions and ask students to use Le Chatelier's principle to predict the outcome and how they could increase the yield of a reaction product.
  • Have students refer to CSERD for more examples and further detailed explanations.

Objective 2

Have students experiment with each of the manipulable parameters (k1, k2, k3, k4) 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/product A? Reactant/product B? or Reactant/product C?
  2. Do the k1, k2, k3, or k4 values have any effect on the product values on the graph? If so, what is the effect?
  3. What happens to the graph if you change the dependency of any of the K parameters? (Try connecting the blue arrows to different boxes and see how the simulation model results change)


  1. Look for other biological pathways that act as reverse reactions
  2. Look at models of reverse reactions in agent modeling applets

Extension 1

Have students research real life examples of reversible chemical reactions. Once they find one they understand have them sketch a picture of the process and draw a graph predicting what the end results look like. Students can also try to make their own Vensim model to create a graph of the processes end result.

Ex. A real life example of a reverse reaction includes: Glycolysis, a metabolic pathway that has 3 irreversible reactions coincided with 7 reversible reactions to carry out the processes in the cytosol...

Extension 2

Have students review reverse reactions and be sure they have a clear understanding of the system Vensim model. Have students use and review the Netlogo chemistry reversible reactions model:

Related Models:

Reaction Data (Excel)

This model represents the progress of a chemical reaction from start to finish. It documents the speed of a reaction as it relates to the concentration.

Generic Chemical Dynamics (Vensim)

This is a simple dynamics model of generic chemical reaction relationships. As the model progresses, reactants change into intermediaries, which then change into products. Users can change the parameter of reactions to view the rates of transformation and concentration's of compounds

Michaelis-Menton Equations (Vensim)

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

Chemical Kinetics (Vensim) Chemical Kinetics (Excel) Chemical Kinetics (AgentSheets)

Simulates a first-order chemical reaction, in which the rate is proportional to the concentration of the reactant