Chemistry • Year 11 • Module 3 • Lesson 12
Factors Affecting Reaction Rate
Build HSC Band 5–6 extended-response technique on rate factors, collision theory, energy diagrams and real industrial contexts.
1. Data + scenario — Haber process temperature trade-off (Band 5–6)
8 marks Band 5–6
Stimulus. Australia’s largest ammonia producer, Incitec Pivot, operates a Haber process plant near Brisbane. The Haber process synthesises ammonia from nitrogen and hydrogen over an iron catalyst:
N2(g) + 3H2(g) ⇌ 2NH3(g) ΔH = −92 kJ mol−1
The table below shows data from a model of the Haber process at different temperatures, using the iron catalyst at constant pressure.
| Temperature (°C) | Rate of NH3 production (relative units) | Equilibrium yield of NH3 (%) | Catalyst activity (relative) |
|---|---|---|---|
| 300 | 0.8 | 52 | Low |
| 400 | 3.2 | 36 | Moderate |
| 450 | 5.0 | 27 | High |
| 500 | 7.1 | 20 | High |
| 600 | 9.4 | 9 | High |
Adapted from Atkins & de Paula (2010). Rate data illustrative only; equilibrium yields approximate.
Q1. Analyse, using the data and your knowledge of collision theory, why Incitec Pivot operates the Haber process at approximately 450°C and why an iron catalyst is essential for commercial viability.
In your answer you must:
- Explain the collision-theory basis for why rate increases with temperature (reference Maxwell-Boltzmann distribution).
- Use the data table to describe the trend in rate from 300°C to 600°C and explain why 300°C gives an unacceptably slow rate.
- Explain the role of the iron catalyst, distinguishing its effect from that of temperature on (a) the Maxwell-Boltzmann diagram and (b) ΔH.
- Use the data table to show that at 450°C the catalyst is at high activity and the rate is substantially faster than at 300°C.
- Reach a justified conclusion about which factor — temperature or catalyst — is more important for achieving a commercially useful rate at 450°C.
2. Source critique — evaluate this textbook claim (Band 5–6)
7 marks Band 5–6
“A catalyst increases reaction rate by giving energy to the reactant particles, raising them above the activation energy threshold. This is why catalysts heat up the reaction mixture as they work. Because catalysts provide extra energy, they also increase the enthalpy change (ΔH) of the reaction — the products have more energy when a catalyst is used. Homogeneous catalysts are always more effective than heterogeneous ones because they mix directly with the reactants.”
Source: adapted from a fictitious Year 11 chemistry revision guide.
Q2. Evaluate this passage. Identify every distinct scientific flaw in the passage, explain the correct chemistry for each, and reformulate the claims about catalysts into a scientifically defensible paragraph. Your evaluation should engage with all four claims made in the passage.
Q1 — Sample Band 6 response (8 marks), annotated
At higher temperatures, the Maxwell-Boltzmann distribution of kinetic energies shifts to the right — the curve becomes lower and broader, and the high-energy tail extends further. The area under the curve to the right of the activation energy Ea (the fraction of particles that can undergo effective collisions) increases significantly. This explains the data: rate increases from 0.8 to 9.4 relative units as temperature rises from 300 to 600°C. [1 — collision-theory rate explanation with M-B reference]
At 300°C the data show a rate of only 0.8 relative units and low catalyst activity. The Maxwell-Boltzmann distribution at 300°C places very few N2 and H2 molecules above the activation energy — even with the catalyst present, effective collision frequency is insufficient for commercial production throughput. [1 — explains why 300°C is undesirable: slow rate (data) + collision-theory explanation]
The iron catalyst lowers the activation energy by providing an alternative reaction pathway with a lower-energy transition state. On a Maxwell-Boltzmann diagram, the Ea line shifts to the left while the particle energy distribution is unchanged. At 450°C a greater proportion of N2 and H2 molecules already exceed the lower catalysed Ea, so the rate at 450°C with catalyst (5.0) is substantially faster than without. Critically, the catalyst does not change ΔH: the reactants and products are chemically identical with or without the catalyst, so the energy difference between them is unchanged. [1 — catalyst mechanism on M-B diagram; 1 — catalyst does not change ΔH]
The 450°C operating point is justified by the data: the catalyst is at high activity (compared to “low” at 300°C), and the rate is 5.0 relative units — more than six times the rate at 300°C. This is consistent with the lesson’s general principle that the 10°C rule applies: each 10°C rise roughly doubles the fraction of particles above Ea, so a 150°C rise from 300 to 450°C produces a very large rate increase. [1 — data-based justification of 450°C using rate and catalyst activity columns]
Of the two factors, the catalyst is more critical to achieving a commercially useful rate at 450°C. At 300°C the catalyst has low activity and the rate is 0.8; raising temperature to 450°C but without catalyst improvement would give a moderate rate, whereas the data show that full catalyst activity at 450°C (rate = 5.0) far exceeds what temperature alone achieves at low catalyst activity. The catalyst enables the 150°C difference between 300°C (unviable) and 450°C (viable) to be exploited fully. [1 — justified conclusion that catalyst is more important for rate at 450°C]
Marking criteria:
- 1 mark — Explains rate increase with temperature using Maxwell-Boltzmann distribution: curve shifts right, greater proportion exceeds Ea, more effective collisions per second.
- 1 mark — Uses data to explain why 300°C is undesirable: very slow rate (0.8), low catalyst activity; collision-theory basis for low rate at low temperature.
- 1 mark — Explains catalyst mechanism on Maxwell-Boltzmann diagram: Ea line shifts left, curve unchanged, more particles exceed lower Ea at same T.
- 1 mark — States catalyst does not change ΔH (reactants and products identical, only barrier height changes).
- 1 mark — Uses specific data from the table to justify 450°C (rate = 5.0, high catalyst activity; contrasts with rate = 0.8 at 300°C).
- 1 mark — Reaches a justified conclusion that the catalyst is more critical for achieving a commercially useful rate at 450°C, supported by comparison of data values.
- 1 mark — Correctly describes the data trend: rate increases steadily from 300 to 600°C; catalyst activity transitions from low (300°C) to high (450–600°C).
- 1 mark — Response uses precise lesson terminology throughout: collision frequency, proportion of particles exceeding Ea, alternative pathway, activation energy, Maxwell-Boltzmann distribution, ΔH.
Q2 — Sample Band 6 evaluation (7 marks)
Flaw 1 — “catalysts give energy to particles.” This is incorrect. Catalysts do not add or transfer energy to reactant particles. They provide an alternative reaction pathway with a lower activation energy. The particle energy distribution (Maxwell-Boltzmann curve) is unchanged by the catalyst — what changes is the activation energy threshold, which shifts to a lower value. More particles already possess energy above the new, lower Ea without needing any additional energy input. [1 — flaw 1 identified + corrected]
Flaw 2 — “catalysts heat up the reaction mixture.” Catalysts do not heat the reaction mixture. They operate isothermally — at the same temperature as the reaction. Any heat observed in a catalysed reaction comes from the exothermic reaction itself, not from the catalyst. This is fundamentally different from raising temperature, which is an external energy input that shifts the Maxwell-Boltzmann distribution. [1 — flaw 2 identified + corrected]
Flaw 3 — “catalysts increase ΔH.” This is incorrect. Because the reactants and products are chemically identical in both the catalysed and uncatalysed pathways, the energy difference between them (ΔH) is fixed by the bond energies of reactants and products. A catalyst only lowers the height of the transition-state energy barrier (the activation energy), not the starting or finishing energy levels. Therefore ΔH is unchanged. [1 — flaw 3 identified + corrected]
Flaw 4 — “homogeneous catalysts are always more effective.” This is an overgeneralisation. Effectiveness depends on the specific reaction and industrial context. Heterogeneous catalysts are often preferred in large-scale industry (e.g. Pt/Pd in catalytic converters, Fe in the Haber process) because they are in a different phase from the reactants, making them easy to separate from the product stream, recover and reuse. Homogeneous catalysts (same phase as reactants) require a separation step after reaction. Neither is universally superior. [1 — flaw 4 identified + corrected]
Defensible reformulation: “A catalyst increases reaction rate by providing an alternative reaction pathway with a lower activation energy. Because the threshold for effective collisions is lowered, a greater proportion of particles at the same temperature can undergo effective collisions — the catalyst does not add energy to any particle. Because the reactants and products are unchanged, the enthalpy change (ΔH) is identical in the catalysed and uncatalysed pathways. Catalysts do not alter the temperature of the reaction mixture; any heat evolved is from the reaction itself. Whether a homogeneous or heterogeneous catalyst is more effective depends on the reaction, required yield, separation requirements, and operating conditions.” [1 — coherent defensible reformulation]
Marking criteria:
- 1 mark — Identifies and correctly refutes Flaw 1 (catalysts lower Ea via alternative pathway; they do not give energy to particles).
- 1 mark — Identifies and correctly refutes Flaw 2 (catalysts do not heat the reaction; they operate isothermally).
- 1 mark — Identifies and correctly refutes Flaw 3 (catalysts do not change ΔH; reactants and products identical, only barrier height changes).
- 1 mark — Identifies and correctly refutes Flaw 4 (homogeneous not always superior; heterogeneous often preferred industrially for separability and reuse).
- 1 mark — Provides a coherent, scientifically defensible reformulation of the passage incorporating all four corrections.
- 1 mark — Uses Maxwell-Boltzmann diagram language correctly: the curve does not shift; the Ea threshold shifts left; more particles already exceed the lower threshold.
- 1 mark — Uses precise lesson terminology throughout: activation energy, alternative reaction pathway, Maxwell-Boltzmann distribution, homogeneous/heterogeneous catalyst, ΔH, effective collisions.