Chemistry • Year 12 • Module 5 • Lesson 6

Le Chatelier's Principle: Pressure, Volume & Catalysts

Build HSC Band 5–6 extended-response technique on pressure, volume and catalyst effects — evaluating industrial trade-offs and diagnosing misconceptions from data.

Master • Extended Response

1. Data + scenario — evaluate industrial pressure and catalyst decisions (Band 5–6)

8 marks Band 5–6

Scenario. Incitec Pivot's Gibson Island plant in Brisbane produces ammonia via the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ/mol. The plant operates at approximately 200 atm and 400–500°C with an iron catalyst. A junior engineer proposes two modifications: (A) increase pressure to 500 atm to improve yield; (B) add twice the amount of iron catalyst to further speed equilibrium attainment.

Pressure (atm)	% NH₃ yield (400°C)	% NH₃ yield (500°C)
100	59	34
200	69	47
300	74	54
500	82	63

Table 1.1. Equilibrium % NH₃ yield at selected pressures. Adapted from Atkins & de Paula (2010), Physical Chemistry, 9th ed.

Q1. Evaluate the two proposals made by the junior engineer. In your response you must:

Use the gas mole count to predict the effect of Proposal A on equilibrium position, citing data from Table 1.1 to quantify the expected gain.
Evaluate whether Proposal A is industrially feasible, considering both the chemical benefit and one named engineering or economic constraint.
Explain, using the mechanism of catalyst action, whether Proposal B would increase the yield of NH₃.
State which factor is the only one that could change K_eq, and whether either proposal does so.
Reach an evidence-based judgement about which, if either, proposal should be adopted.

Plan: mole count → pressure shifts right → cite data (69% → 82% at 400°C) → engineering limit (cost/safety of 500 atm vessels) → catalyst only speeds attainment, not yield → K_eq only changes with T → judgement.

2. Stimulus-based extended response — catalyst misconception in a media article (Band 5–6)

7 marks Band 5–6

Source. The following is an excerpt from an online chemistry study guide: “Adding a catalyst to an industrial reactor always increases the yield of the desired product. This is because the catalyst lowers the activation energy of the forward reaction, meaning more reactant molecules have enough energy to form products. The iron catalyst in the Haber process, for example, raises the equilibrium position toward NH₃, which is why it is an essential part of the process.”

Q2. Identify and explain the scientific flaws in the excerpt above. In your response you must:

Identify all flawed claims in the excerpt (there are at least two distinct errors).
For each error, provide the scientifically correct statement supported by your understanding of activation energy and equilibrium theory.
Explain why a catalyst cannot shift the equilibrium position, using the concept of forward and reverse activation energies.
State the correct role of the iron catalyst in the Haber process.
Describe how an experimental investigation could confirm that a catalyst does not change K_eq.

Three errors to find: (1) catalyst does NOT lower only forward E_a — it lowers both equally; (2) catalyst does NOT increase yield / shift equilibrium position; (3) catalyst does NOT raise the equilibrium position. Correct: catalyst speeds attainment only; yield is set by K_eq which is set by temperature.

Answers — Do not peek before attempting

Q1 — Evaluate industrial proposals (8 marks)

Proposal A — Increase pressure to 500 atm (chemical analysis):

N₂(g) + 3H₂(g) ⇌ 2NH₃(g): left side 1 + 3 = 4 mol gas; right side 2 mol gas. By Le Chatelier's Principle, increasing pressure shifts equilibrium toward fewer gas moles — the right side. From Table 1.1, at 400°C the yield increases from 69% (200 atm) to 82% (500 atm) — a gain of 13 percentage points. This is a real and significant chemical benefit.

Proposal A — Industrial feasibility:

Engineering vessels capable of safely containing 500 atm require substantially thicker walls, higher-grade steel, and more complex safety systems than 200 atm vessels. The capital cost of building and maintaining such infrastructure may exceed the additional revenue from a 13% yield increase. Additionally, the risk of catastrophic vessel failure increases at 500 atm. Incitec Pivot's current 200 atm operation represents the economic and engineering optimum for their plant. Proposal A is chemically valid but may not be industrially feasible without major capital investment.

Proposal B — Double iron catalyst (mechanism analysis):

A catalyst provides an alternative reaction pathway with lower activation energy. Critically, it lowers the activation energy of both the forward and the reverse reactions equally. Doubling the amount of catalyst increases both forward and reverse rates by the same factor — they remain equal — so no net shift in equilibrium position occurs. The yield of NH₃ at equilibrium is unchanged. K_eq is not affected — it depends only on temperature, which has not changed. Proposal B does not increase yield.

K_eq and judgement:

Neither proposal changes K_eq — only a change in temperature would do so. Proposal A does shift equilibrium position (toward products) and increases yield. Proposal B only increases the rate of reaching the same equilibrium. Judgement: Proposal A should be adopted only if the capital cost of 500 atm infrastructure is justified by revenue projections; otherwise, the current 200 atm compromise remains optimal. Proposal B should not be adopted on the basis that it increases yield — it does not. Additional catalyst may be useful if the rate of approach to equilibrium is the bottleneck, but not for yield improvement.

Marking: 1 mark — mole count (4 left, 2 right); 1 mark — LCP + direction of shift (right); 1 mark — cites specific data values for yield gain; 1 mark — evaluates feasibility with one named constraint (cost / safety / vessel engineering); 1 mark — correctly states catalyst lowers E_a equally for both directions; 1 mark — concludes catalyst does not change yield or equilibrium position; 1 mark — states only temperature changes K_eq; 1 mark — reaches an explicit evidence-based judgement on which proposal (if any) to adopt.

Q2 — Catalyst misconception critique (7 marks)

Error 1 — “catalyst lowers the activation energy of the forward reaction”:

This is incorrect. A catalyst lowers the activation energy of both the forward and reverse reactions by the same amount, providing an alternative pathway for both directions. It does not selectively lower the forward E_a. If only the forward E_a were lowered, the forward rate would increase more than the reverse rate — but this does not happen. Both rates increase equally.

Error 2 — “always increases the yield of the desired product”:

This is incorrect. A catalyst does not change the equilibrium yield of any reaction. Yield at equilibrium is determined by K_eq, which depends only on temperature. A catalyst reaches the same equilibrium position faster, but does not shift it. If a system is already at equilibrium and a catalyst is added, both forward and reverse rates increase equally — they remain equal — no shift occurs and no additional product is formed.

Error 3 — “iron catalyst raises the equilibrium position toward NH₃”:

The equilibrium position is not changed by a catalyst. The correct role of iron in the Haber process is to allow the reaction to proceed at a useful rate at the industrially chosen temperature of 400–500°C. Without the catalyst, the rate at this temperature is too slow for commercial production. With the catalyst, equilibrium is reached quickly, but the equilibrium position (and yield of NH₃) is identical to what it would be without the catalyst at the same temperature and pressure.

Experimental confirmation:

To confirm that a catalyst does not change K_eq: set up two identical flasks with the same initial concentrations of N₂, H₂, and NH₃, both at the same temperature. Add iron catalyst to one flask, leave the other without catalyst. After both systems have reached equilibrium, measure the equilibrium concentrations of all species and calculate K_eq for each flask. If the catalyst has no effect on K_eq, both flasks will yield the same value of K_eq (though the catalysed flask will reach this value more quickly). The values should be identical within experimental error.

Marking: 1 mark — identifies Error 1 (catalyst lowers both E_a, not just forward); 1 mark — correct explanation of equal lowering (both rates increase equally); 1 mark — identifies Error 2 (yield / equilibrium position not increased); 1 mark — correct statement that yield is determined by K_eq and temperature only; 1 mark — identifies Error 3 (equilibrium position not raised); 1 mark — correct role of iron catalyst in Haber process (speeds attainment, not yield); 1 mark — valid experimental design to confirm K_eq unchanged (controlled comparison, measuring equilibrium concentrations from both flasks, calculating K_eq).