R2.2.2 Collision theory

Collision Theory and Factors Affecting Reaction Rates

Key Principles of Collision Theory

1. Collisions Are Necessary for Reactions

For a chemical reaction to occur, reactant particles must collide. However, not all collisions result in a reaction.

Analogy

Think of it like trying to spark a conversation: simply bumping into someone isn’t enough—you need the right timing and approach.

Unsuccessful Collisions: Most collisions do not lead to a reaction. Particles may simply bounce off one another without any changes.
Successful Collisions: These occur only when two critical conditions are met:
- Sufficient Energy: Particles must collide with enough energy to overcome the activation energy ( $E_{a}$ ), the minimum energy required to break bonds and initiate a reaction.
- Correct Orientation: Particles must align in a specific way for the bonds to break and reform into new products.

Common Mistake

Many students incorrectly assume that all collisions result in a reaction. Remember: only collisions with both sufficient energy and proper orientation are successful.

2. Kinetic Energy and Temperature

The kinetic energy of particles increases with temperature (measured in Kelvin).
At higher temperatures, particles move faster, collide more frequently, and with greater energy.
This increases the likelihood of successful collisions.

Analogy

Picture popcorn kernels in a hot pan. As the heat increases, the kernels move more vigorously until they finally pop. Similarly, higher temperatures give reactant particles more energy, increasing the chances of a reaction occurring.

3. Collision Geometry: The Importance of Orientation

Even if particles collide with sufficient energy, they must also align correctly for a reaction to occur.
The spatial arrangement of atoms during a collision determines whether bonds can break and reform.

Example

The Reaction Between AB and CD

For the reaction $A B + C D \to A D + B C$ :

Favorable Orientation: Atom $A$ must approach atom $D$ , while atom $B$ must approach atom $C$ . Only this specific alignment allows the bonds $A - B$ and $C - D$ to break and new bonds $A - D$ and $B - C$ to form.
Unfavorable Orientation: If $A$ approaches $C$ and $B$ approaches $D$ , no reaction will occur because the necessary bonds cannot break.

Illustration of unsuccessful and successful collisions.

Hint

When analyzing reaction rates, always consider both the energy and orientation of collisions.

Reflection

Self review

Why do reactions often occur faster in the gas phase compared to the liquid or solid phase?
How might the shape and size of a molecule influence its collision geometry and reaction rate?
Can you think of a real-world scenario where controlling temperature is essential for controlling reaction rates?

Theory of Knowledge

How might the concept of activation energy in chemistry relate to overcoming barriers in other fields, such as economics or personal development?
For example, are there parallels between the energy needed to start a reaction and the effort required to overcome obstacles in human behavior?

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