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The collision model

Molecules have to hit each other to react, but most collisions bounce off harmlessly. A reaction happens only when a collision has enough energy and the molecules are pointed the right way.

§1

Two conditions for a collision to work.

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The collision model says particles must collide to react — but only a fraction of collisions succeed. Two conditions must both be met: enough energy and the correct orientation.

The energy condition is the activation energy, Eₐ: a collision must bring at least this much energy to get over the barrier. The Maxwell-Boltzmann distribution shows what fraction of particles have that much energy.

Raising temperature shifts the distribution to higher energies, sharply increasing the fraction of collisions with energy ≥ Eₐ. That fraction, more than the raw number of collisions, is why heating speeds reactions so strongly.

UNIT 5 TOPIC 5.5 • COLLISION MODEL COLLISION MODEL MAXWELL–BOLTZMANN DISTRIBUTION Eₐ kinetic energy → number of particles lower T higher T Higher T → larger fraction of particles have E ≥ Eₐ (shaded area past Eₐ is bigger for the green curve). Higher-T peak is LOWER and BROADER, shifted right — both curves enclose equal area. COLLISIONS ✓ Effective collision — correct orientation + enough energy products ✗ Ineffective collision — poor orientation or too little energy no products COLLISION MODEL A reaction occurs only when particles collide with: ✓ Correct orientation reactants aligned so bonds can form ✓ Enough energy (E ≥ Eₐ) collision energy reaches the activation energy ✓ Sufficient collision frequency more collisions per second → faster rate Rate depends on collision frequency, orientation, and having energy ≥ Eₐ. AP Chemistry · Unit 5 · Kinetics
Fig. 5.5.1 The collision model: molecules must collide to react, but not every collision works. A successful collision needs enough energy (at or above Eₐ) AND the correct orientation. Raising temperature increases the fraction of collisions above Eₐ.
§2

Reasoning about successful collisions.

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Check both conditions; then think about temperature.

  1. Require enough energy. The collision must supply at least the activation energy Eₐ to react.
  2. Require correct orientation. The molecules must be aligned so the right atoms meet; a well-energized but poorly aimed collision fails.
  3. Read the Maxwell-Boltzmann curve. The area beyond Eₐ is the fraction of particles energetic enough to react.
  4. Raise temperature to speed up. Heating shifts the distribution right, greatly increasing the fraction with energy ≥ Eₐ.
§3

The pieces you'll meet.

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Two gates, one distribution.

collision
Collision model
Particles must collide to react; only some collisions work.
Ea
Activation energy
The minimum energy a collision needs to react.
orientation
Orientation
The molecules must be aligned correctly.
M-B
Maxwell-Boltzmann
Distribution of particle energies; area past Eₐ reacts.
fraction
Energetic fraction
Fraction of collisions with energy ≥ Eₐ.
temperature
Temperature effect
Raises the fraction with sufficient energy, speeding the reaction.
§4

Worked example: why does heating speed a reaction so much?

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Question. A modest temperature rise can double a reaction rate. Why such a big effect?

Fraction over the barrier. Only collisions with energy ≥ Eₐ react. The Maxwell-Boltzmann distribution has few particles far out in the high-energy tail.

Shift the distribution. Heating shifts the whole distribution to higher energy, and the high-energy tail (past Eₐ) grows disproportionately — the fraction able to react jumps.

Conclusion. The rate rises mostly because far more collisions now clear the energy barrier, not merely because collisions are slightly more frequent.

§5

Mistakes that cost real points.

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Pitfall · 01

"A collision with enough energy always reacts."

Energy alone is not enough; the molecules must also collide with the correct orientation. A high-energy collision with the wrong alignment simply bounces apart. Both energy and orientation are required.

Fix. Require both conditions: energy ≥ Eₐ and correct orientation. A poorly aimed collision fails even if energetic.

Pitfall · 02

"Raising temperature helps mainly by making collisions more frequent."

The dominant effect of temperature is increasing the fraction of collisions that have energy ≥ Eₐ, not the collision frequency. That is why a small temperature rise can have a large effect on rate.

Fix. Attribute the strong temperature effect chiefly to the growing high-energy fraction (past Eₐ), not to slightly more frequent collisions.

Pitfall · 03

"The activation energy is the energy released by the reaction."

Activation energy is the energy barrier that must be climbed for a collision to react, not the energy released. Energy released relates to ΔH. Confusing the barrier with the net energy change is a common mix-up.

Fix. Keep Eₐ as the barrier height a collision must overcome; the reaction's energy release is a separate quantity (ΔH).

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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