Particle animations for rates of reaction. Two beakers run side by side, so you can watch how temperature, concentration and surface area change how often successful collisions happen.
Particles must collide, with enough energy
A particle of A meets a particle of B. Whether they react depends on how much energy the collision has.
Not enough energy
No reaction
The particles collide slowly. The energy of the collision is less than the activation energy, so they bounce apart unchanged. No products form.
Enough energy
Reaction
The particles collide fast. The energy of the collision is equal to or greater than the activation energy, so they react and form a product. This is a successful collision.
Collision theory
Chemical reactions can only happen when reacting particles collide with each other, and with enough energy.
Activation energy
The minimum amount of energy that particles must have to react.
Successful collision
A collision with energy equal to or greater than the activation energy. The particles react.
Rate of reaction
Depends on the frequency of successful collisions, which means how many happen each second.
Three ways to get more successful collisions per second
Temperature
What happens to the rate when the temperature goes up? Both beakers hold the same particles. Only the temperature is different.
Explanation hidden. Ask the class to predict and explain what they see first.
What to look for
In the hot beaker the particles move faster and bump into each other more often. On the energy strip their collisions land further to the right, so more of them get past the activation energy line.
Explain it
At a higher temperature the particles have more energy, so they move faster.
They collide more frequently.
The collisions are more energetic, so a greater proportion of collisions have energy equal to or greater than the activation energy.
There are more successful collisions per second, so the rate of reaction increases.
Exam tip
Temperature is the only factor on this page that changes the energy of the collisions. Give both effects: collisions are more frequent, and more of them have enough energy.
The hot beaker finishes first because it runs out of B particles. Both beakers make the same amount of product in the end.
Concentration
What happens when the concentration of A goes up? Same volume, same temperature. The right beaker has more particles of A.
Explanation hidden. Ask the class to predict and explain what they see first.
What to look for
Twice as many A particles in the same volume means A and B meet about twice as often. The particles move at the same speed in both beakers, so the dots on the two energy strips are spread out the same way.
Explain it
A higher concentration means more reactant particles in the same volume.
The particles are closer together, so they collide more frequently.
The collisions are no more energetic, so the same proportion of them are successful.
More collisions per second means more successful collisions per second, so the rate of reaction increases.
Proportionality
Doubling the concentration doubles the number of A particles in the same volume. That roughly doubles the frequency of collisions, so the rate roughly doubles.
Exam tip
Don’t write that the particles have more energy or move faster. Concentration only changes how often they collide.
Pressure works the same way for reacting gases: a higher pressure puts more particles in the same volume. In the required practical you change the concentration and either measure the volume of gas given off, or time how long the mixture takes to turn cloudy.
By the end, both beakers have had a similar number of collisions, because the same 20 particles of B had to react. The higher concentration gets through them faster. That is what a higher rate means.
Surface area
What happens when a solid is broken into smaller pieces? Marble (calcium carbonate) in hydrochloric acid. Both beakers have the same 36 particles of solid and the same acid.
Explanation hidden. Ask the class to predict and explain what they see first.
What to look for
Acid particles can only hit particles on the outside of a piece. The bright edges show the surface facing the acid. Breaking the lump up uncovers particles that were hidden inside, so the acid hits the solid more often.
Explain it
Smaller pieces have a larger surface area to volume ratio.
More particles of the solid are exposed to the acid.
Acid particles collide with the solid more frequently. The collisions have the same energy, because the temperature is the same.
There are more successful collisions per second, so the rate of reaction increases.
Surface area to volume ratio
A 2 cm cube cut into
Surface area
Volume
SA : V
1 cube, 2 cm sides
24 cm²
8 cm³
3 : 1
8 cubes, 1 cm sides
48 cm²
8 cm³
6 : 1
64 cubes, 0.5 cm sides
96 cm²
8 cm³
12 : 1
Halving the side length doubles the surface area to volume ratio. The amount of solid stays the same.
Exam tip
Write “smaller pieces” or “powder”, not “smaller particles”. The particles themselves stay the same size.
The same mass of marble with acid in excess makes the same total amount of carbon dioxide. The powder just makes it faster, so its collision count races ahead early on and the lump slowly catches up.
Exam answers
Every rate explanation follows the same chain, from the change you make to the effect on the rate.
What changesFor example, the temperature goes up.
What happens to the particlesThey have more energy and move faster.
Effect on collisionsMore frequent, and more of them have enough energy.
Effect on rateMore successful collisions per second, so the reaction is faster.
Change
What happens to the particles
Frequency of collisions
Energy of collisions
Rate
Increase the temperature
Move faster, with more energy
Increases
Increases: a greater proportion have at least the activation energy
Increases
Increase the concentration
More particles in the same volume
Increases
No change
Increases
Increase the pressure of a gas
More particles in the same volume
Increases
No change
Increases
Increase the surface area (smaller pieces)
More particles exposed at the surface
Increases
No change
Increases
Model answers
Explain why increasing the temperature increases the rate of reaction.
At a higher temperature the particles have more energy and move faster. They collide more frequently, and a greater proportion of the collisions have energy equal to or greater than the activation energy. So there are more successful collisions per second.
Explain why a higher concentration of acid increases the rate.
There are more acid particles in the same volume, so particles collide more frequently. There are more successful collisions per second.
Explain why powdered marble reacts faster than marble chips.
The powder has a larger surface area to volume ratio, so more particles of calcium carbonate are exposed to the acid. Acid particles collide with them more frequently, so there are more successful collisions per second.
Watch your wording
Instead of
Write
There are more collisions.
Collisions are more frequent, or there are more collisions per second. Rate is about time.
The particles have more energy (for concentration or surface area).
There are more particles in the same volume, or more particles exposed. Only temperature changes the energy.
Smaller particles.
Smaller pieces, or powder.
Heating lowers the activation energy.
More particles have energy equal to or greater than the activation energy.
Limitations of this model
It is a flat, 2D picture of particles that really move in 3D.
A real beaker holds an enormous number of particles. The model shows a few dozen.
Water molecules are not drawn. In the model they nudge the particles now and then, which keeps the temperature steady.
Speeds are exaggerated so the differences are easy to see.
In the marble model each acid particle uses up one calcium carbonate particle. The real reaction needs two acid particles for each one: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.