U5.5 · Collision ModelPractice · 碰撞模型

Most collisions don’t produce reaction: the collision needs enough energy to break the old bonds (at least Ea) and must hit the right part of the molecules (the right orientation). Both are required.

Ratio = $\dfrac{e^{-E_a/RT_2}}{e^{-E_a/RT_1}} = e^{\frac{E_a}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}\right)}$ = $e^{\frac{50000}{8.314}\left(\frac{1}{300}-\frac{1}{310}\right)}$ = $e^{0.647}$ ≈ 1.91. A 10 K rise nearly doubles the energetic collisions, which is why temperature affects rate so strongly.

Heating shifts the speed distribution to higher energies, and the small high-energy tail above Ea grows by a large factor (about 90% in the previous question). Collisions increase by only 1–2%; what matters is the jump in collisions with enough energy. Ea is a property of the reaction and doesn’t change with temperature.

Only particles at the solid’s surface can meet the solution. Grinding exposes far more of them, so collisions happen more often and the reaction speeds up. The energy each collision needs (Ea) and the solution’s concentration are unchanged.

In the reaction, CO takes an O atom from $\ce{NO2}$ and becomes $\ce{CO2}$. The new C–O bond forms between the C of CO and an O of $\ce{NO2}$, so only a C-to-O collision can lead to products.

Only collisions with at least Eₐ can lead to reaction. The T₂ curve is flatter and reaches farther right, so the area under it to the right of the line is much larger: for these values, the fraction of molecules above Eₐ rises from about 9.3% to 23.3%. Eₐ belongs to the reaction, so the line doesn’t move; the total areas are equal, so the number of molecules is the same; and most molecules are still below Eₐ.