COLLISION THEORY explains why reactions happen and what affects their rate.
For a reaction to occur:
1. Reactant particles must COLLIDE with each other.
2. The collision must have sufficient ENERGY โ equal to or greater than the ACTIVATION ENERGY.
3. The particles must be in the CORRECT ORIENTATION (for some reactions).
Most collisions are UNSUCCESSFUL โ the particles collide but bounce off without reacting, because they don't have enough energy.
Only EFFECTIVE (successful) collisions lead to products.
This explains all rate factors:
Higher temperature โ faster particles โ more collisions AND more above activation energy โ more effective collisions.
Higher concentration โ more particles per volume โ more collisions โ more effective collisions.
Larger surface area โ more exposed particles โ more collisions โ more effective collisions.
Activation Energy
ACTIVATION ENERGY (Ea) is the MINIMUM energy that colliding particles must have for a reaction to occur.
Even exothermic reactions need activation energy to get started โ you must 'invest' some energy to break existing bonds before the reaction can release more energy.
Example: petrol is stable at room temperature (collisions between Oโ and petrol molecules don't have enough energy). You need a spark (to provide the activation energy) to start the combustion.
EA AND REACTION RATE:
LOW activation energy โ many particles have enough energy โ FAST reaction.
HIGH activation energy โ few particles have enough energy โ SLOW reaction.
BOLTZMANN DISTRIBUTION (energy of particles):
At any temperature, particles have a range of energies.
Most particles have intermediate energy โ very few have very low or very high energy.
Raising temperature shifts this distribution โ more particles move into the high-energy tail (above Ea).
Even a 10ยฐC rise significantly increases the proportion of particles above Ea.
Successful vs Unsuccessful Collisions
Only a small fraction of collisions actually result in reaction:
For a collision to be SUCCESSFUL (effective):
Energy โฅ activation energy โ sufficient energy to break existing bonds.
Correct orientation โ molecules must collide in the right way for bonds to break and reform correctly.
For example, in the reaction A-B + C โ A + B-C:
The C particle must approach the B end of A-B โ not the A end.
A collision at the wrong angle gives an unsuccessful result even if energy is sufficient.
This is why not ALL collisions โ even energetic ones โ lead to products.
INDUSTRIAL RELEVANCE:
Industrial chemists balance:
HIGH TEMPERATURE (faster rate but more energy cost).
CATALYST (lowers activation energy โ faster rate without extra heat).
HIGH PRESSURE (for gas reactions โ more collisions).
CONCENTRATION (higher concentrations mean more collisions).
โ ๏ธ Common Mistake
Increasing concentration or surface area increases the FREQUENCY of collisions but does NOT change the activation energy. Only a CATALYST changes the activation energy. Temperature increases both frequency AND the proportion above activation energy. These are different mechanisms.
๐ Variables
EaActivation energy (Ea) is measured in kJ/mol (kJ/mol)
๐ Key Note
Collision theory: reaction needs collision + sufficient energy (โฅ Ea) + correct orientation. Activation energy = minimum energy for reaction. Low Ea = fast reaction. High Ea = slow reaction. Temperature raises proportion of particles above Ea โ more effective collisions. Catalyst lowers Ea.
๐ฏ Matching Activity โ Collision Theory Concepts
Match each concept to its correct description. โ drag the symbols on the right to match the component names on the left.
Activation energy
Drop here
Effective collision
Drop here
Effect of temperature on Ea
Drop here
Catalyst effect on Ea
Drop here
Why petrol needs a spark
Drop here
Petrol is stable at room temperature โ needs activation energy (spark) to initiate combustion
A collision with sufficient energy AND correct orientation โ results in products
Minimum energy needed for a collision to result in a reaction
Temperature does NOT change Ea โ it increases the proportion of particles that have energy โฅ Ea
Catalyst LOWERS the activation energy โ provides an alternative pathway
โญ Higher Tier Only
Maxwell-Boltzmann distribution: temperature increases the curve height and shifts it right, increasing the proportion above Ea. Explain why a 10ยฐC rise approximately doubles rate โ the fraction of particles above Ea at least doubles. Distinguish between increased collision frequency and increased proportion of successful collisions.
๐ฏ Test Yourself
Question 1 of 2
1. Why do most collisions between reactant particles NOT result in a reaction?
2. A reaction has a very high activation energy. What does this predict about the reaction?
โญ How Well Do You Understand This Topic?
Be honest with yourself โ this helps you know what to revise!
Don't get itGetting thereNailed it!
๐ค Ask Mr Badmus AI
Stuck? Just ask! ๐ฌ
I'll use FIFA for calculations and flag Higher/Triple content clearly.