HSC Exam Practice

Sample response. Activation energy (E_a) is the minimum kinetic energy that colliding particles must possess for a reaction to occur. A collision is only effective if both particles have kinetic energy equal to or greater than E_a and if they have the correct orientation; any collision with energy below E_a does not result in bond breaking and no product is formed.

Marking notes. 1 mark for defining E_a as minimum energy for reaction to occur. 1 mark for linking E_a to whether a collision is effective (must meet or exceed E_a).

Sample response. A homogeneous catalyst is in the same phase as the reactants (e.g. H⁺(aq) catalysing ester hydrolysis in aqueous solution). A heterogeneous catalyst is in a different phase from the reactants (e.g. solid platinum (Pt) catalysing gas-phase reactions in a catalytic converter).

Marking notes. 1 mark for homogeneous: same phase (accept “same state”). 1 mark for heterogeneous: different phase. 1 mark for one valid named example of each type (award 1 if at least one correct pair of name + type is given).

Sample response. For heterogeneous reactions involving a solid reactant, only particles on the surface of the solid are exposed to the other reactant. Grinding the solid into smaller pieces increases the surface area exposed, so more solid particles are available to collide with the other reactant per unit time — collision frequency at the reaction interface increases [1]. The proportion of collisions that are effective (those exceeding E_a) is unchanged because surface area does not affect the kinetic energy of the particles [1]. The activation energy is also unchanged [1].

Marking notes. 1 mark for mechanism (more surface exposed → more solid particles accessible → higher collision frequency at interface). 1 mark for stating the proportion of effective collisions / particle energies is unaffected. 1 mark for stating E_a is unchanged (accept “energy of particles unchanged”).

Sample response. Three features that change: (1) the peak of the curve shifts to a higher energy value (rightward shift); (2) the peak height decreases (the curve is lower/flatter); (3) the high-energy tail extends further to the right, meaning the area to the right of E_a is larger (a greater proportion of particles exceed E_a). One feature that does not change: the total area under the curve remains equal at both temperatures (the total number of particles is the same).

Marking notes. 1 mark each for any three of the four following changes: peak shifts right; peak height decreases; tail extends right; area beyond E_a increases. 1 mark for the unchanged feature: total area under curve (= same number of particles). Award 3 marks for three correct changes + 1 mark for the unchanged feature.

Sample response. The catalytic converter contains platinum (Pt) and palladium (Pd), which are heterogeneous catalysts that operate by adsorbing exhaust gas molecules (CO, NO_x) onto their surface, reacting them, and releasing harmless products (CO₂, N₂) [1]. Heterogeneous catalysis requires the reactant molecules to adsorb onto the Pt surface at active sites; this adsorption step requires the surface to be above approximately 300–400°C [1]. During cold-start the converter surface is at ambient temperature, so CO and O₂ molecules cannot adsorb onto the Pt; the catalytic cycle does not proceed and CO is emitted unreacted. Once the surface reaches operating temperature, adsorption proceeds and CO is oxidised to CO₂ [1].

Marking notes. 1 mark for identifying the catalytic converter as a heterogeneous catalyst (Pt/Pd on a surface). 1 mark for identifying that the surface must reach operating temperature for adsorption to occur. 1 mark for linking cold surface → no adsorption → catalytic cycle inactive → CO emitted.

Part (a) — trend description (2 marks). As temperature increases from 15°C to 45°C, the initial gradient of each curve increases (steeper at higher temperatures), indicating a faster initial rate of reaction [1]. However, all four curves reach approximately the same final volume of H₂ (approximately 100–110 cm³), indicating that the total amount of product is the same regardless of temperature [1].

Part (b) — same final volume (2 marks). The final volume of H₂ is determined by the amount of the limiting reactant — in this case the fixed mass of Mg ribbon — not by temperature [1]. Temperature affects rate (how quickly the reaction reaches completion) but not the total stoichiometric yield. Once all the Mg is consumed, no more H₂ can be produced regardless of temperature [1].

Part (c) — collision theory at 45 vs 15°C (3 marks). At 45°C, the Maxwell-Boltzmann energy distribution shifts to the right compared to 15°C — the peak is lower and broader and the high-energy tail is more extended [1]. This means a significantly greater proportion of H⁺ ions and Mg surface atoms have kinetic energy equal to or greater than the activation energy E_a [1]. The frequency of effective collisions is therefore higher at 45°C, producing H₂ at a faster rate and giving a steeper initial gradient [1].

Part (a) — percentage increase calculation (2 marks). Percentage increase = [(3.8 − 0.4) / 0.4] × 100 = [3.4 / 0.4] × 100 = 850% [1 mark for method; 1 mark for correct answer 850%].

Part (b) — catalyst mechanism vs temperature (3 marks). The zeolite catalyst provides an alternative reaction pathway with a lower activation energy for the cracking of hydrocarbon bonds [1]. On a Maxwell-Boltzmann diagram, the E_a line shifts left (lower threshold) while the distribution curve is unchanged; more hydrocarbon molecules at 400°C already possess energy exceeding the new lower E_a, so more effective collisions occur per second [1]. This is mechanistically different from increasing temperature, which shifts the Maxwell-Boltzmann distribution itself to higher energies (curve moves right), raising the kinetic energy of the particles. The catalyst lowers the barrier; temperature raises the particles — both increase the proportion above E_a but by different mechanisms [1].

Sample response. Four factors increase reaction rate by different collision-theory mechanisms, which can be grouped by whether they affect collision frequency or the proportion of effective collisions.

Concentration and surface area both affect collision frequency only. Increasing the concentration of a dissolved reactant increases the number of particles per unit volume, so the average distance between reactant molecules decreases and collisions occur more frequently. For heterogeneous reactions involving a solid, grinding the solid into a powder increases the surface area exposed to the other reactant, so more solid particles are available for collision per unit time. Neither factor changes the kinetic energy of particles, so the activation energy E_a and the proportion of particles exceeding E_a are unchanged.

Temperature affects both collision frequency and, more importantly, the proportion of effective collisions. On the Maxwell-Boltzmann diagram, the distribution curve shifts right — lower, broader peak, extended tail. A significantly larger proportion of particles now exceed E_a, so effective collision frequency increases substantially. The activation energy itself does not change — it is a fixed property of the reaction.

Catalysts affect the proportion of effective collisions by lowering E_a via an alternative reaction pathway. On the Maxwell-Boltzmann diagram, the E_a line shifts left; the particle energy distribution is unchanged (same temperature). Because the threshold is lower, a greater proportion of particles at the existing temperature already possess sufficient energy. A catalyst does not add energy to particles and does not change ΔH.

In a large-scale industrial context like the Haber process (N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ mol⁻¹), the most effective combination is catalyst + optimised temperature. The iron catalyst lowers E_a, allowing a viable rate at temperatures where the equilibrium yield is still acceptable (~450°C). Using temperature alone to achieve the same rate would require operating above 600°C, where the exothermic equilibrium shifts to reactants and yield collapses below 10%. Increasing concentration (pressure in a gas-phase reaction) and optimising surface area of the catalyst support both contribute but are secondary — the catalyst is the critical enabling factor. This is why all commercial ammonia plants, including Incitec Pivot’s Brisbane facility, operate with an iron catalyst at 400–500°C rather than relying on high temperature alone.

Marking criteria.

• 1 mark — Correctly explains concentration mechanism: more particles per volume → higher collision frequency; no change to E_a or particle energy.
• 1 mark — Correctly explains surface area mechanism: more solid surface exposed → higher collision frequency at interface; no change to E_a.
• 1 mark — Correctly explains temperature mechanism using Maxwell-Boltzmann: curve shifts right, greater proportion exceeds E_a, more effective collisions; E_a unchanged.
• 1 mark — Correctly explains catalyst mechanism: lowers E_a via alternative pathway; curve unchanged; E_a line shifts left; does not change ΔH.
• 1 mark — Correctly distinguishes the two groups: concentration/surface area affect collision frequency only; temperature/catalyst affect proportion of effective collisions.
• 1 mark — Applies to Haber process: identifies catalyst + optimised temperature as most effective combination; explains why temperature alone at high values is counterproductive (equilibrium yield collapse).
• 1 mark — Uses a named industrial example with correct contextual detail (e.g. Incitec Pivot Brisbane: iron catalyst at ~450°C for Haber process; or catalytic converter using Pt/Pd to convert CO and NO_x).
• 1 mark — Reaches a justified, integrated evaluation: ranks catalyst as the critical enabling factor; uses precise terminology (collision frequency, proportion of effective collisions, activation energy, Maxwell-Boltzmann distribution, ΔH, alternative pathway) throughout.