AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/5/5-5)
Unit 5 · Topic 5.5
5.5 Collision Model
Particles react only if they collide with enough energy to get past the activation energy, and with an orientation that lets the right bonds break and form. Most collisions fail. Raising the temperature increases the fraction of collisions with enough energy, which is the main reason reactions speed up when heated.
Key terms
- collision model
- activation energy (Eₐ)
- orientation
- Maxwell-Boltzmann distribution
The collision model
For an elementary reaction to happen, reactant particles must collide. Bonds can only break and form when particles come into contact. But a collision alone isn't enough. A successful collision needs two things.
- Enough energy: the colliding particles must have at least the activation energy (Eₐ), the minimum energy needed to start breaking bonds and rearranging atoms.
- The right orientation: the particles must hit in a way that puts the reacting atoms next to each other.
Most collisions don't work
In a gas at room temperature, each molecule collides with others billions of times per second. If every collision led to reaction, almost every reaction would be over instantly. In reality, only a small fraction of collisions succeed.
Orientation matters especially for larger molecules. For NO + O₃ → NO₂ + O₂, the N atom of NO has to strike one of the end O atoms of ozone. A collision in which the O end of NO hits ozone doesn't produce products, no matter how energetic it is.
A small, symmetric reactant, like a single atom, has fewer bad orientations, so orientation matters less. A large molecule whose reacting atom is buried on one side has many more ways to collide unproductively.
Using the Maxwell-Boltzmann distribution
Topic 3.5 showed the Maxwell-Boltzmann distribution of particle speeds. Plotted against kinetic energy instead, it shows how many particles have each energy. Draw a vertical line at Eₐ. The area under the curve to the right of that line represents the fraction of particles (and so of collisions) with enough energy to react.
At a higher temperature, the curve shifts right and flattens, and the area beyond Eₐ grows a lot, even though the average energy rises only modestly. That's why heating a reaction by a few degrees can noticeably increase its rate.
Heating also makes collisions a bit more frequent, because particles move faster. But that effect is small. The main reason is the bigger fraction of collisions that have at least Eₐ.
Connecting to concentration and catalysts
Increasing concentration raises the number of collisions per second, so the number of successful collisions per second goes up too, even though the fraction that succeeds stays the same.
A catalyst (topic 5.11) works differently from heating. Instead of giving particles more energy, it provides a pathway with a lower Eₐ. On the Maxwell-Boltzmann graph, the curve stays the same, but the Eₐ line moves left, so more of the area lies beyond it.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Explaining a temperature effect
A reaction's rate roughly doubles when the temperature is raised from 25 °C to 35 °C. The Kelvin temperature went up by only about 3%. Explain the large increase in rate.
Show the solutionHide the solution
- Step 1: 298 K → 308 K is about a 3% rise, so the average kinetic energy rises by about 3% too.
- Step 2: But the rate depends on the fraction of collisions with energy at least equal to Eₐ, which lies in the high-energy tail of the Maxwell-Boltzmann distribution.
- Step 3: When the curve shifts slightly right, the area under the tail beyond Eₐ grows by a much larger proportion.
- Step 4: More collisions per second now have enough energy, so the rate increases sharply.
Answer: Higher temperature greatly increases the fraction of collisions with at least the activation energy (the tail of the distribution), even though the average energy rises only slightly.
- Example 2
Energy alone isn't enough (classic trap)
Two particles collide with more than enough energy to react, but no reaction occurs. A student concludes the activation energy must have been measured wrong. Suggest a better explanation.
Show the solutionHide the solution
- Step 1: A successful collision needs both sufficient energy and the correct orientation.
- Step 2: If the particles struck with the wrong parts facing each other, the bonds that need to break and form weren't in position.
- Step 3: So an energetic collision can still fail.
Answer: The particles probably collided with the wrong orientation; energy above Eₐ is necessary but not sufficient.
Common mistakes
- Explaining the temperature effect only with more frequent collisions. The key reason is a larger fraction of collisions with energy ≥ Eₐ.
- Saying a catalyst gives particles more energy. It lowers Eₐ instead.
- Forgetting orientation as a requirement.
- Drawing the high-temperature Maxwell-Boltzmann curve taller rather than lower and wider.
On the exam
- Free-response questions often ask why a rate increases with temperature. Full credit usually needs you to mention the larger fraction of particles with energy at or above Eₐ, not just faster particles.
- Expect Maxwell-Boltzmann graphs with an Eₐ line. Shade or describe the area beyond it.
Connected topics
Videos
Check yourself
4 questions on 5.5 Collision Model. Pick an answer to see if you got it, and why.
Raising the temperature of a reaction mixture by 10 °C roughly doubles the reaction rate, even though the average kinetic energy of the particles rises by only about 3%. Which of the following best explains this large effect?
In a gas-phase reaction, many collisions between reactant molecules have more than enough energy to react, but most of these collisions still do not form products. Which of the following best explains this?
At constant temperature, increasing the concentration of a reactant in solution usually increases the reaction rate. Which of the following best explains this effect?
Termolecular elementary steps are much less common than unimolecular and bimolecular steps. Which of the following best explains this?
0 of 4 answered