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:

<!-- Diagram: Maxwell-Boltzmann at two temperatures with Ea | reviewed 2026-05-21 --> <svg class="fig" viewBox="0 0 540 280" role="img" aria-labelledby="mb-t mb-d"> <title id="mb-t">Maxwell Boltzmann distribution at two temperatures</title> <desc id="mb-d">Number of particles against kinetic energy. Two curves: lower temperature has a higher narrower peak; higher temperature has a lower broader peak shifted to the right. A vertical line marks the activation energy Ea. Particles to the right of Ea can react.

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

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.

Using collision theory, explain why a powdered solid reacts faster than a single chunk with the same mass. [2 marks]

A reaction at $25^{\circ}\text{C}$ takes $400 \, \text{s}$ to complete. At $45^{\circ}\text{C}$ it takes $50 \, \text{s}$. (a) Calculate the rate ratio.

A catalyst is added to the reaction $2 \text{H}_2 \text{O}_2 \to 2 \text{H}_2 \text{O} + \text{O}_2$. (a) State the effect on $E_a$, rate and $\Delta H$. (b) Sketch the energy profile with and without catalyst.