Quick questions on Rate of reaction and collision theory: VCE Chemistry Unit 3

Collision theory says that for a reaction to occur, reactant particles must:

Concentration. Doubling the concentration of a reactant roughly doubles the rate (for a first-order dependence). More particles in a given volume means more collisions per unit time.

The Maxwell-Boltzmann distribution plots the number of particles (y-axis) against kinetic energy (x-axis) for a sample at a given temperature.

An energy profile diagram plots potential energy (y-axis) against reaction progress (x-axis).

Note that concentration and surface area change the collision frequency, which is not visible on either of these diagrams. They are best discussed in words.

Doubling the concentration of a reactant roughly doubles the rate (for a first-order dependence). More particles in a given volume means more collisions per unit time.

Only the particles at the surface of a solid can collide with the other reactant. Grinding a solid into powder increases the surface area dramatically and so increases the rate. This is why a flour mill is more explosive than a flour sack (huge surface area exposed to air).

Raising the temperature has two effects: (1) particles move faster so collide more often (small effect), and (2) more particles have enough energy to overcome Ea (large effect). Effect (2) dominates because the Maxwell-Boltzmann fraction above Ea increases exponentially with temperature. Hence the rule of thumb that a 10°C rise roughly doubles rate.

A catalyst provides an alternative pathway with a lower Ea. The reactants bind to the catalyst surface or active site, the bonds rearrange, and the products leave. The catalyst is regenerated.

rate increases significantly. Powdering increases the surface area of CaCO3, so more carbonate ions are exposed to collisions with H^+ ions per unit time. More frequent collisions per unit time means a faster rate.

rate roughly doubles (assuming first order in HCl, which is the case here). Doubling the concentration of H^+ ions doubles the collision frequency between H^+ and CaCO3 per unit time, so the rate doubles.

Markers want the Maxwell-Boltzmann distribution argument: the fraction of particles above Ea increases.

It must start at the origin (no particles with zero kinetic energy).

The area under the curve is the total number of particles, which is constant. The curve flattens, broadens and shifts right, but the area stays the same.

A catalyst lowers Ea only. ΔH and equilibrium position are unchanged.