Collision Theory Simulation
This is a 2D particle simulation of the reaction A + B → P, modelling the four factors that affect reaction rate through collision theory. Cyan circles (Reactant A) and orange circles (Reactant B) move around a reaction vessel, bouncing off walls and each other. Every time an A particle meets a B particle, the simulation checks whether the collision has sufficient energy to overcome the activation energy (Ea):
- Green flash = successful collision — both particles are consumed and a purple hexagon (Product P) appears in their place
- Red flash = unsuccessful collision — particles bounce away unreacted
The energy distribution chart (top right) shows how particle energies are spread at the current temperature, with a dashed Ea line and a shaded area representing the fraction of particles that can react. The rate-time graph below it plots reaction rate live, showing it rise at the start then gradually flatten and reach zero as reactants are used up.
What each control demonstrates:
- Temperature slider — moving it right speeds all particles up, shifts the energy distribution curve, and visibly increases the number of green flashes per second. At very low settings almost no reactions occur even though collisions are still happening — reinforcing that frequency alone is not enough.
- Concentration slider — adds more A and B particles to the same vessel. Pupils can see the vessel become crowded and watch the rate HUD jump immediately.
- Surface Area toggle — adds extra B particles representing newly exposed solid surface sites, modelling the marble chips vs powder comparison.
- Catalyst toggle — moves the Ea line left on the energy chart without changing particle speed, so more of the existing collisions now succeed. Pupils can observe rate increase even at low temperature.
The reaction naturally runs to completion as one reactant depletes — the rate-time graph flatlines and a completion banner appears in the vessel.
Recommended Classroom Activity
“What controls reaction rate?” — guided enquiry (30–40 min)
Learning objective: Pupils can explain, using collision theory, how each factor increases reaction rate in terms of collision frequency and activation energy.
Setup: Open the simulation on the board (whole-class) or one device per pair.
Task sequence:
- Baseline observation (5 min) — start with sliders at default (low temperature, low concentration, no catalyst, no surface area). Ask: “What do you notice about the green and red flashes? What does each one mean?” Establish the two requirements for a successful collision before moving on.
- Temperature investigation (8 min) — pupils drag the temperature slider from cold to hot and record what changes. Prompt: “Does the rate increase because there are more collisions, or because the collisions are more energetic, or both?” Direct them to the energy chart — the shaded reactive area grows as temperature rises.
- Concentration investigation (5 min) — reset, then move only the concentration slider. Ask: “Why does rate increase here? Has the energy of each particle changed?” This isolates collision frequency from collision energy — a key distinction pupils often miss.
- Catalyst investigation (5 min) — add the catalyst with temperature left low. Ask: “Why are there more green flashes now even though the particles aren’t moving faster?” Point to the Ea line shifting left. Key teaching point: a catalyst provides an alternative reaction pathway — it does not give particles more energy.
- Run to completion (5 min) — set temperature to medium, concentration to medium, watch the rate-time graph together. Ask: “Why does the rate slow down and stop even though we haven’t changed anything?” Pupils should link it to reactant depletion reducing collision frequency.
- Exit task (5–8 min, individual) — pupils write two sentences for each factor explaining the change in rate using the words collision frequency, activation energy, and proportion of successful collisions. Sentence starters can be provided via ClassAdapt slide scaffolding for lower-attaining pupils.
Key misconception to address: Many pupils think catalysts make particles move faster (i.e. raise temperature). The catalyst toggle is particularly useful here — temperature slider stays unchanged, yet rate clearly increases, because Ea has dropped not particle speed.