Kinetics (AS Level) flashcards

Topic 5: Kinetics

Activation energy, Ea

The minimum energy that colliding particles must have for a reaction to occur.

Rate of reaction

The change in amount or concentration of a reactant or product per unit time.

Catalyst, and how it works

A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount. It provides an alternative reaction route of lower activation energy.

Maxwell–Boltzmann distribution: three drawing rules

Starts at the origin; the curve never meets the energy axis at high energy; the area under it equals the total number of molecules.

How does the curve change at a higher temperature?

The peak is lower and further right, with the same area. Many more molecules have E ≥ Ea.

Main reason a small temperature rise greatly increases rate

A much larger proportion of molecules have energy ≥ Ea, so many more collisions are successful. Slightly more frequent collisions is only a minor reason.

Raising concentration or gas pressure: what changes and what does not?

Collision frequency rises (more particles per unit volume). The proportion of particles with E ≥ Ea does not change, and the Maxwell–Boltzmann curve does not move.

Catalyst on a Maxwell–Boltzmann diagram

One curve, unchanged. A lower Ea(cat) marked to the left of Ea, so a greater proportion of molecules have enough energy to react.

Required practical 3 (thiosulfate + acid): what measures the rate?

1/t, where t is the time for the sulfur precipitate to hide a cross. The same amount of sulfur forms each run, so rate ∝ 1/t.