Collision Theory

How concentration, temperature and surface area affect the rate of reaction shown using collision theory

Teaching notes

For a reaction to happen, particles must collide with each other — and even then, not every collision succeeds. This simulation has two tabs, each isolating one factor that raises the rate of reaction: Concentration, which raises how often particles collide, and Temperature, which raises both how often and how successfully they collide.

Both tabs share the same reaction vessel and the same choice of reaction: A → B (a collision converts both particles to product, so the total particle count never changes) or 2A → B (a collision converts one particle and removes the other, so the total falls over time — a deliberate simplification to make the 2:1 stoichiometry visible).

The Concentration tab

Every qualifying collision has the same fixed chance of reacting, set directly by the Collision success chance slider — this tab is entirely about how often collisions happen, not how likely each one is to succeed. There are two independent ways to raise concentration here: add more particles to the fixed-size box, or shrink the box itself with the Vessel volume slider while holding the particle count still. Both routes raise the same underlying quantity — particles per unit volume — and both raise the rate the same way.

Suggested use. Start with a modest particle count and press Run. Reset, then roughly double the particle count and compare — the rate readout should be noticeably higher. Reset again, and this time leave the particle count untouched but drag Vessel volume down instead; use "Show previous run" to overlay the two graphs and confirm they show the same kind of speed-up even though only one of the two controls moved.

The Temperature tab

Here, temperature drives both particle speed and collision success chance at once — the two-part explanation used in GCSE and A-level collision theory. Particle speed scales as √T (kinetic theory: average kinetic energy is proportional to T). Separately, the live Success chance readout follows an Arrhenius-style relationship with temperature and the Activation energy slider: only collisions energetic enough to clear that barrier react, and a hotter sample has a larger share of them.

Both sliders here update live, without resetting the run — dragging Temperature mid-run rescales every particle already in the box, like heating a reaction that's already going. Activation energy behaves the same way, and lowering it without touching temperature is exactly what a catalyst does, which a later module will build on directly.

Suggested use. Start cold and press Run — reactions should be rare. Drag the temperature up while it's running and watch both effects happen together: particles visibly speed up, and the Success chance readout climbs. Reset, then try compensating for a temperature rise by also raising Activation energy, to show a high enough barrier can keep a reaction slow even when it's hot.

The Surface area tab

This tab looks and behaves quite differently from the other two, because the chemistry is different: a solid reactant (blue) sitting in a fluid reactant (teal), like marble chips in acid, rather than two gases mixing freely. The solid is always exactly 100 particles — that never changes. Only how those 100 particles are arranged changes, via the Number of pieces slider.

A fluid particle can only trigger a reaction with a solid particle it can physically touch — one on the exposed surface of a piece. Particles buried inside a block are shielded until the layers above them dissolve away, so a single lump erodes from the outside in rather than dissolving all at once. This is emergent from the geometry, not a shortcut: the simulation tracks which particles are on a boundary and recomputes that after every dissolution.

The Exposed surface stat is the number to watch. Splitting the same 100 particles into more, smaller pieces exposes dramatically more surface for an identical amount of solid — that's the entire mechanism behind surface area's effect on rate, made visible directly rather than asserted.

Suggested use. Start with 1 piece (a single 10×10 block) and press Run — the reaction should be slow, since only the block's outer edge is reachable. Reset with the same fluid count and probability, but set pieces to something like 25 or 100, and compare: Exposed surface jumps enormously even though Solid remaining still starts at 100 either way, and the rate readout follows suit. Use "Show previous run" to overlay the two directly.

Key ideas

Rate of reaction depends on how often particles collide and what fraction of those collisions succeed.
Concentration changes collision frequency only — the same underlying idea whether you get there by adding particles or shrinking the vessel.
Temperature changes both frequency (via speed) and the successful fraction (via the energy barrier) — which is why raising temperature tends to have a much bigger effect on rate than raising concentration by a comparable amount.
Surface area changes how much of a fixed amount of solid reactant is actually available to react at any moment — breaking a lump into pieces doesn't add any solid, it just exposes more of what's already there.
A reaction slows over time as reactants are used up, regardless of which tab you're on, because there are fewer particles (or less exposed surface) left to react with.
Activation energy is the height of the energy barrier a collision must clear. A catalyst works by lowering that barrier, not by heating anything.