Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Bonds can only break and form when particles collide. No collision, no reaction.
- A collision produces products only if it has enough energy (at least the activation energy, Eₐ) and a suitable orientation (the atoms that must bond actually meet).
- In most reactions only a small fraction of collisions succeed.
- The Maxwell–Boltzmann distribution shows the spread of particle kinetic energies: a peak, then a long tail to the right. The area under the curve beyond Eₐ is the share of particles with enough energy.
- Heating the sample makes the curve lower, wider and shifted right. The area beyond Eₐ grows a lot. The total area stays the same.
Key relationships
| Change | Collision frequency | Fraction with energy ≥ Eₐ | Fraction with good orientation | Rate |
|---|---|---|---|---|
| Increase concentration | increases (in proportion) | no change | no change | increases |
| Increase temperature | increases slightly | increases a lot | no change | increases a lot |
| Larger, less symmetrical particles | — | — | smaller | slower if Eₐ is similar (k smaller) |
For an elementary step A + B → products: collision frequency ∝ [A][B], so rate = k[A][B]. The rate constant k holds the energy and orientation fractions.
Assumptions
- Particles react only during a collision, and only bonds in the colliding particles change.
- Temperature measures the average kinetic energy; individual particles have a wide range of energies.
- The orientation requirement depends on the shapes of the particles, not on temperature or concentration.
Mistakes to avoid
- Saying every collision reacts. Most fail.
- Giving “more collisions” as the main reason for faster rates at higher temperature. The main reason is the larger fraction with energy ≥ Eₐ.
- Saying Eₐ falls when temperature rises. Eₐ stays the same.
- Drawing the higher-temperature peak taller. It is lower and further right, with the same area.
- Linking concentration to collision energy. Concentration changes frequency only.
- Forgetting orientation. Energy alone is not enough.
Quick self-check
- A reaction is first order in A and first order in B. Both concentrations are doubled. By what factor does the rate change? (× 4: collisions between A and B happen four times as often)
- What does the area under a Maxwell–Boltzmann curve to the right of Eₐ represent? (The particles, and so the share of collisions, with at least the activation energy)
- Why does a fast collision between two reactant molecules sometimes fail to react? (The orientation is wrong: the atoms that must bond do not meet)
Next: practice questions.