Collision Theory
Section: 6. Chemical Reactions | Syllabus: Cambridge AS Level Physics 9702
What is Collision Theory? Collision theory explains how chemical reactions occur and why reaction rates vary. It states that for a reaction to occur, reactant particles must collide with sufficient energy and correct orientation.
Collision theory helps us understand the factors affecting rate of reaction at a particle level. The Basic Principles of Collision Theory For a chemical reaction to take place: Particles MUST collide - reactant particles need to come into contact Collisions must have SUFFICIENT ENERGY - energy must be ≥ activation energy (Ea) Particles must have CORRECT ORIENTATION - must collide at the right angle/position Not every collision results in a reaction!
Only collisions that meet ALL three requirements above are called successful collisions . Successful vs Unsuccessful Collisions Type of Collision Characteristics Outcome Successful collision • Particles collide • Energy ≥ activation energy • Correct orientation Reaction occurs - bonds break and new bonds form Unsuccessful collision • Energy • Wrong orientation Particles bounce off each other - no reaction What is Activation Energy (Ea)?
Activation energy is the minimum energy that colliding particles must have for a reaction to occur. Why is activation energy needed? Energy is required to break bonds in reactant molecules Bonds must break before new bonds can form Only particles with energy ≥ Ea can overcome this energy barrier Think of it like this: Activation energy is like a hill you need to climb before you can roll down the other side.
You need enough energy to get to the top! Why Most Collisions Are Unsuccessful In a typical reaction, most collisions do NOT lead to a reaction: Only a small fraction of particles have energy ≥ activation energy at any given time Many collisions occur with wrong orientation (molecules hit from wrong angle) Particles with insufficient energy just bounce off each other This is why reactions don't happen instantly, even though billions of collisions occur per second Rate of Reaction and Collision Theory Rate of reaction depends on: • The FREQUENCY of collisions (how often particles collide) • The ENERGY of collisions (proportion with energy ≥ Ea) • The ORIENTATION of collisions (correct angle) The rate of reaction is proportional to the number of successful collisions per second .
To increase rate, we need to increase the number of successful collisions by: Increasing collision frequency Increasing the proportion of collisions with sufficient energy Maxwell-Boltzmann Distribution The Maxwell-Boltzmann distribution is a graph that shows the distribution of energies among particles in a gas or liquid at a particular temperature.
Key features of the distribution: No particles have zero energy Most particles have energy around the average (peak of curve) Some particles have very high energy (right tail extends far) The area under the curve = total number of particles Only particles to the RIGHT of Ea have enough energy to react Important: The area under the curve to the right of the activation energy line represents the number of particles that can successfully react.
Effect of Temperature on Distribution When temperature increases: The peak of the curve moves to the RIGHT (higher average energy) The peak becomes LOWER and WIDER (energy more spread out) MORE particles now have energy ≥ Ea The area under curve to right of Ea INCREASES significantly Many more successful collisions occur This explains why increasing temperature by just 10°C can approximately DOUBLE the rate!
Comparing Low and High Temperature Aspect Low Temperature High Temperature Average particle energy Lower Higher Particle speed Slower Faster Collision frequency Lower Higher % particles with E ≥ Ea Small percentage Much larger percentage Successful collisions/sec Few Many Rate of reaction SLOW FAST Effect of Catalysts on Collision Theory A catalyst provides an alternative reaction pathway with a LOWER activation energy.
How catalysts work (collision theory explanation): Ea is reduced (lower energy barrier) MORE particles now have energy ≥ the new, lower Ea A greater proportion of collisions are now successful Rate of reaction increases On Maxwell-Boltzmann distribution: Ea line moves to the LEFT, so the area under the curve to the right of Ea increases dramatically - many more particles can now react!
Explaining Reaction Rates Using Collision Theory Factor Changed Effect on Collisions Effect on Rate Increase temperature More frequent collisions + more particles with E ≥ Ea INCREASES Increase concentration More particles → more frequent collisions INCREASES Increase pressure (gas) Particles closer → more frequent collisions INCREASES Increase surface area More surface for collisions → more frequent collisions INCREASES Add catalyst Lowers Ea → more particles have sufficient energy INCREASES Key Equations and Concepts For a reaction to occur: Collision energy ≥ Activation energy AND correct orientation Rate of reaction ∝ Number of success…
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