Knowledge Organiser: Collision Theory & Rate of Reaction
Key Terms
- Rate of reaction — amount ÷ time
- Activation energy (Ea) — minimum energy for reaction
- Successful collision — energy ≥ Ea + correct orientation
- Collision theory — particles must collide with correct energy and orientation
Must-Know Facts
- Rate formula: amount ÷ time (units: g/s, cm³/s)
- Steeper graph = faster rate
- Horizontal line = reaction finished
- Same final height = same amount of product
- CEO rule: Collide + Energy + Orientation
Measuring Rate Methods
- Gas syringe — volume of gas over time
- Mass balance — mass loss over time
- Disappearing cross — turbidity timing
- Colorimeter — colour intensity change
Common Exam Errors
- Forgetting "successful collisions" in explanations
- Confusing rate with total product amount
- Missing units in rate calculations
- Not explaining WHY orientation matters
Key Equations
- Rate = amount of product formed ÷ time (or amount of reactant used ÷ time)
- Rate units: g/s, cm³/s, or mol/s
- Mean rate = gradient of tangent on concentration-time graph
Memory Aid
- C — COLLIDE (particles must physically meet)
- E — ENERGY ≥ Ea (must exceed activation energy)
- O — ORIENTATION (must hit at the right angle)
Common Mistakes
Misconception 1: "All collisions lead to reactions" This is wrong. Only successful collisions produce a reaction. Most collisions do not result in a reaction because either the energy is below the activation energy threshold, or the particles collide at the wrong angle. A faster rate means more successful collisions per second — not more collisions in general. Misconception 2: "Activation energy is the energy of the particles" Activation energy is not a property of individual particles — it is a property of the reaction itself. It represents the energy barrier that must be overcome to break bonds and start the reaction. Different reactions have different activation energies. Catalysts work by lowering this barrier, not by giving particles more energy. Misconception 3: "Rate of reaction and amount of product are the same thing" Rate refers to how fast the reaction proceeds. The final amount of product depends on how much reactant was present (the limiting reagent), not how fast it reacted. Two reactions with different rates can produce the same total amount of product if they start with the same reactants.