Flashcards · 9620 · AS Topic 6

Kinetics (AS Level) flashcards

18 cards on AS Topic 6, subtopics 3.1.6.1 to 3.1.6.5. Pick a subtopic or study them all.

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Activation energy, Ea: The minimum energy that particles must have when they collide for a reaction to occur.
Why do most collisions not lead to a reaction?: Most particles have less energy than the activation energy, so the collision cannot break the necessary bonds and the particles rebound unchanged.
The three links to use in any rate-of-reaction explanation: Do particles collide more or less often? Does a bigger or smaller fraction of collisions have energy ≥ Ea? So what happens to the number of successful collisions per second, and the rate?

Maxwell–Boltzmann distribution: drawing rules: Starts at the origin, one peak (most probable energy), lopsided with a long tail that approaches but never touches the energy axis. Area under the curve = total number of molecules.
What does the area under a Maxwell–Boltzmann curve beyond Ea represent?: The molecules that have enough energy to react if they collide.
Why is the mean energy greater than the most probable energy?: The curve is not symmetrical: the long high-energy tail pulls the mean to the right of the peak.
Maxwell–Boltzmann: how does a higher-temperature curve differ from a lower one?: Lower and broader, with the peak at higher energy. Same area under the curve. The two curves cross once; beyond the crossing the hotter curve is higher.

Rate of reaction: The change in concentration (or amount) of a reactant or product per unit time. Unit for concentration: mol dm⁻³ s⁻¹.
Why does a small temperature rise greatly increase the rate?: Ea lies in the thin tail of the distribution, so a small shift of the curve to higher energy multiplies the fraction of molecules with energy ≥ Ea. Collision frequency rises only slightly.
A product concentration rises from 0 to 0.036 mol dm⁻³ in 120 s. Mean rate?: 0.036 ÷ 120 = 3.0 × 10⁻⁴ mol dm⁻³ s⁻¹
When temperature rises, which effect contributes most to the higher rate?: The larger proportion of molecules with energy at or above Ea, not the slight increase in collision frequency.

Why does increasing concentration increase the rate?: There are more reactant particles per unit volume, so they collide more often, giving more successful collisions per second.
Why does increasing the pressure of a gas increase the rate?: The same molecules are packed into a smaller volume, so they collide more often.
Does increasing concentration or pressure change the Maxwell–Boltzmann curve?: No. The proportion of collisions with energy ≥ Ea is unchanged; only the collision frequency increases.

Catalyst: A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount.
How does a catalyst work?: It provides an alternative reaction route (pathway) with a lower activation energy.
Showing a catalyst on a Maxwell–Boltzmann distribution: Same single curve (temperature unchanged). Mark Ea(cat) at a lower energy than Ea: a larger area lies beyond it, so a larger fraction of molecules can react.
Effect of a catalyst on ΔH and on the enthalpy profile: The hump (activation energy) is lower. Reactant and product levels, and so ΔH, are unchanged.