Part 1 of 5Conditions for Reaction
Collision
- Particles must collide with each other before a reaction can take place.
Sufficient energy
- The colliding particles must have at least the activation energy, Ea, to break bonds in the reactants.
Orientation
- The particles must meet with a suitable orientation so that the reacting parts of the molecules touch.
Successful collision
- A collision satisfying both conditions is called an effective or successful collision.
Most collisions fail
- Most collisions in a gas or liquid are not successful because particles lack energy or collide in the wrong orientation.
Part 2 of 5Activation Energy
Activation Energy
Definition
- The minimum energy that colliding particles must have in order for a reaction to take place.
Energy barrier
- On a reaction profile it is the height from the reactants to the top of the curve.
Exothermic profile
- Products are at a lower energy than reactants, though the activation energy barrier must still be overcome.
Endothermic profile
- Products are at a higher energy than reactants, so the activation energy is larger than the enthalpy change.
Part 3 of 5Rate of Reaction
1
The rate depends on the frequency of successful collisions between reacting particles.
2
Anything increasing collision frequency, such as higher concentration, increases the chance of a reaction.
3
Anything increasing the proportion of particles with at least Ea, such as higher temperature, increases the rate.
4
A catalyst lowers the activation energy so that more collisions are successful.
5
Rate is usually measured as the change in concentration of a reactant or product per unit time.
Part 4 of 5Collision Outcomes
Compare the different possible outcomes when two particles collide.
Enough energy, right orientation
- The collision is effective, bonds break and new bonds form, so products are made.
Enough energy, wrong orientation
- The reacting atoms do not meet, so the particles simply bounce apart.
Right orientation, too little energy
- The particles cannot break the bonds in the reactants and rebound unchanged.
Neither condition met
- No reaction occurs and the particles continue moving independently.
| Collision | Energy ≥ Ea? | Correct orientation? | Reaction? |
|---|---|---|---|
| Effective | Yes | Yes | Yes |
| Ineffective A | Yes | No | No |
| Ineffective B | No | Yes | No |
Part 5 of 5Exam Checklist & Tips
1.6.1 – Key Points for the Exam
- Collision theory Reactions occur only when particles collide with enough energy and the correct orientation.
- Activation energy The minimum energy needed for a collision to result in reaction.
- Effective collision A collision that results in a chemical reaction.
- Rate of reaction The change in concentration of a reactant or product per unit time.
Common Mistakes
- Saying that all collisions lead to reaction.
- Stating that raising the temperature lowers the activation energy; it does not.
- Saying a catalyst increases the energy of the particles.
- Leaving out orientation when listing the conditions for a successful collision.