Particles react only when they collide with enough energy (the activation energy). Collision theory explains why temperature, concentration, surface area and catalysts change the rate of a reaction.
Milk goes off in days on the bench but lasts a week in the fridge. A glow stick in hot water glows brighter but fades sooner. Same idea: how often, and how hard, particles collide.
Reactions go at different speeds. This lesson gives one model that explains every way to change the speed.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: hotter and faster?
ExploreTry it on the board and compare what happens with your prediction.
Explore: two temperatures
Run it at 300 K first. Then press Reset, move the temperature (try 500 K) and run it again. Watch two numbers in the panel: collisions and reactions.
Try it yourselfNothing is saved: start again as often as you like.
Run the reaction at two different temperatures and compare.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: collisions and energy
In your book: explain why a reaction is faster at a higher temperature. Use "collide", "more often" and "activation energy".
What happens when the temperature goes up?
Particles move faster
Collisions happen more often
A larger share of collisions has enough energyThis is the bigger effect: see the percentage in the panel.
Particles get biggerParticles never change size.
The activation energy gets smallerThe energy needed stays the same; more collisions reach it.
Each change speeds up a reaction. Match it to the reason, in collision terms.
Higher temperature
Higher concentration
Larger surface area (smaller pieces)
A catalyst
Elaborate TypicalUse the same idea in a new context.
Elaborate: concentration
Try it yourselfNothing is saved: start again as often as you like.
Run the low-concentration reaction to the target.
Use the board. This ticks when you get there.
Run the high-concentration reaction to the same target.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: a catalyst
Try it yourselfNothing is saved: start again as often as you like.
Compare the reaction with and without the catalyst.
Use the board. This ticks when you get there.
EvaluateLook back at your first ideas and what changed.
Evaluate: look back
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Particles react only when they collide with at least the activation energy. Most collisions just bounce.
Temperature: faster particles collide more often and, much more importantly, harder.
Concentration and surface area: more collisions each second.
Catalyst: a lower activation energy, so more collisions succeed. It is not used up.
For teachers
Students explain reaction rate with collision theory (collisions with at least the activation energy) and use it to explain temperature, concentration, surface area and catalysts (AC9S10U07, node 5, a hinge node). About 30 minutes.
I can explain that particles react only when they collide with at least the activation energy.
I can explain why more frequent or more energetic collisions make a reaction faster.
I can name and explain at least two ways to increase the rate.
Before class
A concept tutorial for introducing or revising the idea; it does not replace a rate practical. Set the width with ?width=minimum, typical (default) or wider.
The hinge risk is rate factors learned as separate rules. Every factor answers one question: does it change how often particles collide, how hard, or the energy needed?
Model notes: with 15 of each reactant, run times vary from run to run (often by a factor of two), as in real small-scale experiments. Use the class's runs together, or run twice at each temperature. Typical results at 300 K: 10–30 s; at 500 K: 3–5 s. The catalyst halves the activation energy for every collision.
Running the lesson
Engage: record the class's predictions on the board to come back to.
Explore: ask students to read out the percentage of collisions that react at each temperature. That number is the key to the Explain step.
Explain: the third question separates frequency from energy; take it slowly.
Typical: pool the class's two times on the board: the spread is part of the lesson.
The hard part (on purpose)
Explaining a rate change by how often particles collide and how many have enough energy. When a student says heat makes particles want to react, ask what changed in the panel: collisions, or the share that react.
When students get stuck
A student says heat makes the particles want to react. Ask: What changed in the panel: the number of collisions, or the share that react? Fix: Both, but mostly the share: faster particles hit harder, so more collisions have the activation energy.
A student says every collision causes a reaction. Ask: Compare the collisions and reactions numbers in the panel. Fix: Only a few per cent react. The rest don't have enough energy.
A student says a catalyst adds energy. Ask: Did the temperature change when you turned the catalyst on? Fix: No. The catalyst lowers the energy needed instead.
Stretch and support
Stretch: Food in a freezer lasts months. Use collision theory to explain why, and why it still goes off eventually.
Support: Give a two-column frame for each factor: 'More collisions?' and 'More energetic collisions?'.
Where it leads
Next: rate factors investigated in practicals (node 6).
In class: sodium thiosulfate and acid at different temperatures (the disappearing cross), or antacid tablets in water at different temperatures and as powder.
What each step is for
Engage: hotter and faster?: Students' reasons for 'faster' (particles want to react, get bigger) surface here; the board's collision count replaces them with frequency and energy.
Explore: two temperatures: Comparing times and the percentage of collisions that react shows energy, not just frequency, driving the change.
Explain: collisions and energy: Separating 'how often' from 'how hard' is the model; every rate factor is one or both.
Elaborate: concentration: Changing only the number of particles isolates collision frequency: the same share of collisions reacts, but there are more of them.
Elaborate: a catalyst: A catalyst changes the threshold, not the particles' energy; comparing with heating shows two different routes to more successful collisions.
Now you can
Explain that particles react only when they collide with at least the activation energy.
Explain why more frequent or more energetic collisions make a reaction faster.
Name and explain at least two ways to increase the rate.