Topic 5.5 Notes – Collision Model

1. The Collision Model

At the particle level, a reaction happens when reactant particles collide and rearrange bonds. No collision, no reaction.

In the collision model, particles:

• Move in constant, random motion.
• Collide with one another.
• Only react if the collision is successful.

For an elementary reaction (a single-step reaction), the rate directly reflects that one molecular event. If the step is
$\ce{A + B -> products}$,
then the rate depends on how often A and B collide successfully.

Reaction rate is proportional to:

• Collision frequency (how often particles hit each other)
• Fraction with enough energy
• Fraction with correct orientation

Put simply:

Rate = total collisions × fraction that are successful

Most collisions are not successful. That idea shows up again and again on tests.

2. What Makes a Collision Successful

A collision must meet two conditions at the same time.

a. Sufficient Energy and Activation Energy

Activation energy ($E_a$) is the minimum energy required to begin breaking bonds.

During a collision:

• Some kinetic energy is used to stretch and distort bonds.
• If kinetic energy < $E_a$, particles bounce apart.
• If kinetic energy ≥ $E_a$, bonds can rearrange.

Think of $E_a$ as an energy barrier between reactants and products. Most particles at a given moment do not have enough energy to cross it.

On FRQs, you may need to explain why a reaction is slow at low temperature. The correct reasoning is that few particles have energy ≥ $E_a$, so few effective collisions occur.

b. Correct Orientation

Even with enough energy, molecules must collide in the proper geometry.

• Reactive sites must line up.
• Bonds must be positioned so new bonds can form.
• Complex molecules have stricter orientation requirements.

If orientation is wrong, particles separate unchanged.

This is why the fraction of successful collisions is small. You need:

• Enough energy
• Correct alignment

Both at once.

3. Collision Frequency and Reaction Rate

Now think about how often collisions happen.

Collision frequency increases when:

• Concentration increases
  More particles per volume → more collisions per second.
• Pressure increases (for gases)
  Particles are closer together → collide more often.
• Temperature increases
  Particles move faster → collide more frequently.

For an elementary reaction, if you double the frequency of A-B collisions, you directly increase the rate because that step is the reaction.

But remember: more collisions alone does not guarantee reaction. They must be effective.

4. Maxwell-Boltzmann Distribution

The Maxwell-Boltzmann distribution shows how particle energies are spread out in a sample. The graph below compares two temperatures and highlights the activation energy, $E_a$.

Maxwell-Boltzmann distributions at two temperatures

What this graph tells you:

• The peak represents the most probable energy.
• The total area under each curve is the same (same number of particles).
• Only particles to the right of $E_a$ can react.

When temperature increases:

• The curve shifts right.
• It becomes flatter and broader.
• The area to the right of $E_a$ increases significantly.

Two effects happen at higher temperature:

1. Particles collide more often.
2. A much larger fraction have energy ≥ $E_a$.

That second effect is why temperature has such a dramatic impact on rate. A small temperature increase causes a disproportionately large increase in particles above $E_a$.

On the AP exam, you may be asked to compare shaded areas or explain why rate increases. The correct explanation references the increase in the fraction of particles with energy ≥ $E_a$.

5. Effective vs Ineffective Collisions

Ineffective collisions

• Energy < $E_a$, or
• Incorrect orientation
• No reaction occurs

Effective collisions

• Energy ≥ $E_a$
• Proper alignment
• Bonds break and form → products result

In most reactions, only a small fraction of total collisions are effective. Reaction rate increases when:

• Collision frequency increases
• The fraction exceeding $E_a$ increases
• The fraction with proper orientation increases

Everything in this topic ties back to that single idea.