Teaching focus
Use this simulation to help students connect three views of a reversible reaction: what happens to the particles, how the forward and reverse rates compare, and how the amounts of substances change.
Students should already understand reversible reactions and the meanings of concentration, reaction rate, exothermic and endothermic. Begin with the simple A ⇌ B model before introducing the chemical examples.
Getting started
Select a reaction and press Play. The simulation begins with reactants, allowing students to watch products form and the reverse reaction develop.
Use Pause and the speed control to discuss changes. Switch the graph between Concentrations, Amounts and Rates, and use the amount bars alongside it.
At equilibrium moves the mixture directly to its equilibrium composition at the current conditions and starts the animation. It also clears the graph, so use it when you want to investigate an equilibrium mixture without waiting for the initial reaction.
Suggested teaching sequence
- Establish what dynamic equilibrium means
Start with A ⇌ B and select the Rates graph. Ask students why the reverse rate begins at zero, then increases as product forms.
When the rates become equal, compare the amount bars. Emphasise that equilibrium means constant amounts, which can be unequal. Draw attention to the continuing reaction flashes and both rate displays: chemical reactions are still occurring even though the overall composition is steady.
Ask: “What evidence shows that the reaction has reached equilibrium? What evidence shows that the reactions have not stopped?”
- Disturb an equilibrium by changing concentration
Establish equilibrium, then select Pin reference to mark the starting amounts, concentrations and yield. Add reactant or remove product.
Use the concentration graph to distinguish the immediate change caused by adding or removing a substance from the gradual response caused by the reaction. Identify which reaction is temporarily faster and follow the mixture until the rates become equal again.
Explain Le Chatelier’s principle as the system responding in a direction that opposes the change. Removing product leads to some replacement product forming; it does not necessarily replace everything removed.
Ask: *“Which change did we cause directly, and which change happened because the mixture reacted?”
- Compare pressure changes**
Choose 2NO₂(g) ⇌ N₂O₄(g) and establish equilibrium. Reduce the volume using the pressure control. The piston compresses the gases at constant temperature.
First compare the Amounts and Concentrations graphs: compression immediately increases concentrations without initially changing the amounts present. The subsequent reaction favours the side with fewer gas molecules, increasing the amount of N₂O₄.
Repeat with A(g) ⇌ B(g). Because there is one gas particle on each side of this equation, compression does not change the equilibrium composition.
Ask: “How can the concentrations increase without the equilibrium amounts changing?”
When counting gas molecules, use the coefficients in the equation. Subscripts tell students about the atoms within each molecule.
- Investigate temperature
Use 2NO₂(g) ⇌ N₂O₄(g), where the forward reaction is exothermic. Heat an equilibrium mixture and watch the rate displays immediately. Both rates increase, but the endothermic reverse reaction increases proportionately more, so the equilibrium shifts towards NO₂.
Repeat with N₂O₄(g) ⇌ 2NO₂(g). This is the same equilibrium written in the opposite direction; the forward reaction is now endothermic, so heating favours products.
Ask: “Why does heating favour products in one equation but reactants in the other?”
Encourage students to identify the endothermic direction before predicting the effect of heating.
- Separate a catalyst’s effect on rate from its effect on yield
Add the model catalyst to a mixture already at equilibrium. Both rates increase while the amounts and equilibrium yield remain unchanged.
Then compare runs starting from reactants, with and without a catalyst. Reset restores the original conditions, including removing the catalyst, so add it again for the catalysed run.
Ask: “Does reaching equilibrium sooner mean that more product will be present at equilibrium?”
- Apply the ideas to the Haber process
Select N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Ask students to explain why higher pressure favours ammonia and why cooling favours the exothermic forward reaction.
Use the rate displays to discuss the compromise between equilibrium yield and reaction speed. Finish with the six questions in Check your thinking.
Supporting discussion and assessment
- Enable Predict before applying to ask students which direction will be favoured. This option establishes equilibrium before each prediction.
- Use Hide explanation while students discuss their reasoning, then reveal it.
- Pin reference marks current values for comparison; it does not save an entire previous run.
- Turn Auto scale off when you want the graph scale to remain fixed during a comparison.
- Encourage explanations in this order: change made → direction favoured → effect on the mixture → equal rates restored.
Interpreting the model
The particles, numerical quantities, rates and temperature settings are illustrative. The particle colours are identification aids, and the Haber settings should not be treated as industrial operating values.
The rate displays compare complete forward and reverse reaction changes, taking account of the balanced equation.
Model product yield is the product currently present as a percentage of the maximum possible product from the atoms in the vessel. It includes any product added directly. Adding reactant can increase the amount of product without increasing this percentage, because the maximum possible product also changes. Use the amount bars alongside the yield display.
The vessel is treated as closed while it approaches equilibrium. Adding or removing a substance represents briefly changing the contents and then resealing it.