Collision Theory Exam Questions

• Reactant particles must collide with each other [1]
• The collision must have energy equal to or greater than the activation energy [1]
• The particles must collide with the correct orientation / geometry [1]

Examiner note: “Particles must hit each other” earns the first mark, but the other two conditions are equally important. Most collisions are unsuccessful because they lack sufficient energy or have the wrong orientation. All three conditions must be met simultaneously.

• Increasing pressure forces the gas particles closer together / reduces the volume [1]
• This is equivalent to increasing the concentration [1]
• There are more particles per unit volume, so collisions between reactant particles are more frequent [1]
• More successful collisions occur per unit time, so the rate increases [1]

Examiner note: Increasing pressure on a gas has the same effect as increasing concentration of a solution — it puts more particles into a smaller space. The proportion of collisions that are successful does not change, but the total number of collisions per second increases.

• A catalyst provides an alternative reaction pathway [1]
• This alternative pathway has a lower activation energy [1]
• A greater proportion of particles have energy equal to or greater than this lower activation energy [1]
• Therefore more collisions are successful per unit time and the rate increases [1]

Examiner note: A catalyst does NOT increase the number of collisions. It increases the PROPORTION of collisions that are successful by lowering the energy barrier. The total number of collisions stays the same, but more of them lead to a reaction.

• Powdered zinc has a much larger surface area than zinc granules [1]
• More zinc particles are exposed and available to collide with acid particles [1]
• The frequency of collisions between zinc and acid particles increases, so the rate is faster [1]

Examiner note: Surface area affects the number of reactant particles that are available for collisions. Particles inside a large lump cannot collide with the acid — they are shielded by the outer layer. Powdering the solid exposes more particles.

• At temperature T2, the peak of the distribution shifts to the right (higher average energy) [1]
• The peak is lower and the curve is broader [1]
• The area under the curve to the right of Ea (particles with energy >= Ea) is much larger at T2 [1]
• This means many more particles have sufficient energy to react on collision, so the rate increases significantly [1]

Examiner note: This is the Maxwell-Boltzmann distribution. The key point is that the area to the right of Ea increases dramatically with even a small temperature increase. A 10 degC rise typically doubles the rate because the number of particles exceeding Ea increases exponentially with temperature.

(a) The number of collisions per second doubles / increases significantly [1] because there are twice as many HCl particles per unit volume, leading to more frequent encounters with Na2S2O3 particles [1]

(b) The proportion of successful collisions stays the same [1] — the energy distribution of the particles has not changed (temperature is constant), so the same fraction of collisions have sufficient energy to overcome the activation energy

Examiner note: Concentration affects the frequency of collisions, not the proportion that are successful. Temperature and catalysts affect the proportion. This distinction is important for Supplement-level answers.

• Very fine particles have an extremely large surface area relative to their mass [1]
• When dispersed in air, the particles are surrounded by oxygen and can collide with oxygen molecules very frequently [1]
• The reaction with oxygen is very fast and exothermic, releasing a large amount of energy very quickly, which causes an explosion [1]

Examiner note: This is a real industrial hazard. Grain elevators and flour mills have exploded due to airborne dust igniting. The principle is the same as why powdered reactants react faster — extreme surface area leads to extremely rapid combustion.