Chemistry • Year 12 • Module 5 • Lesson 5

Le Chatelier’s Principle: Concentration & Temperature

Build Band 5–6 extended-response technique: synthesise LCP, K_eq, industrial trade-offs, and biological contexts into structured, multi-criteria answers.

Master · Band 5–6

1. Data + scenario + multi-criteria evaluation — Haber process temperature trade-off (Band 5–6)

8 marks Band 5–6

Scenario. Incitec Pivot operates a Haber process facility at Gibson Island, Queensland, producing approximately 150 000 tonnes of ammonia per year primarily for fertiliser manufacture. The table below shows the equilibrium yield (% NH₃ by volume) and the approximate time to reach equilibrium with an iron catalyst at selected operating conditions. The industrial reactor typically operates at 400–450°C and 150–300 atm.

Temperature (°C)	Equilibrium yield NH₃ (%, 300 atm)	Approx. time to equilibrium (iron catalyst)
300	63	very slow (>days)
400	36	moderate (∼hours)
450	26	fast (<1 hour)
500	17	very fast (<minutes)

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹. Yield data adapted from Atkins’ Physical Chemistry (representative values). Time estimates from industrial practice.

Q1. Analyse the data in the table and evaluate why Incitec Pivot operates at 400–450°C rather than at 300°C or 500°C. In your response you must:

State Le Chatelier’s Principle and apply it to explain the effect of temperature on equilibrium yield.
Explain, using the data, why 300°C is commercially unviable despite giving the highest equilibrium yield in the table.
Explain why 500°C also gives a poor industrial outcome, using both K_eq and a named concept from kinetics.
Justify, with reference to the data, why 400–450°C represents the optimal compromise.
State whether K_eq changes when temperature changes and explain the direction of the K_eq change across the table.

Plan first: LCP statement → exothermic + increase T = shift left + K_eq decreases → 300°C too slow → 500°C too low yield + fast but catalyst degrades → 400–450°C compromise. Use data figures throughout.

2. Scenario + multi-criteria analysis — blood pH regulation in emergency medicine (Band 5–6)

7 marks Band 5–6

Scenario. A paramedic arrives at the scene of a panic attack. The patient is hyperventilating (breathing rate approximately 30 breaths per minute, compared with a normal 12–20). The patient reports tingling in the fingers and mild dizziness. Blood pH is measured at 7.56 (normal range 7.35–7.45). The paramedic administers oxygen through a rebreather mask, which limits the loss of CO₂ by causing the patient to re-inhale some exhaled air.

The relevant equilibrium system in blood plasma is:

CO₂(aq) + H₂O(l) ⇌ H₂CO₃(aq) ⇌ H⁺(aq) + HCO₃⁻(aq)

Normal blood [H⁺]: approximately 40 nmol L⁻¹ (pH 7.40). Patient’s blood [H⁺]: approximately 28 nmol L⁻¹ (pH 7.56).

Q2. Using Le Chatelier’s Principle, explain the chain of chemical events that caused the patient’s blood pH to rise during hyperventilation, and evaluate how the rebreather mask intervention corrects this using the same principle. In your response you must:

Define Le Chatelier’s Principle and identify the initial disturbance caused by hyperventilation.
Trace the equilibrium shifts stepwise through the two linked equilibria to explain why [H⁺] fell and blood pH rose.
Use the data (normal vs patient blood [H⁺] values) to quantify the severity of the disturbance.
Explain, using LCP, how the rebreather mask reverses the pH shift and returns the system toward normal.
State whether K_eq changes during any part of this process, and justify your answer.

Plan: LCP definition → disturbance = remove CO₂ → shift left (both equilibria) → [H⁺] falls → pH rises → data: 40 to 28 nmol/L (30% drop) → rebreather restores CO₂ → shift right → [H⁺] rises → pH returns toward 7.40. K_eq unchanged throughout (only concentration disturbances, not temperature).

Answers — do not peek before attempting

Q1 — Haber process temperature trade-off (8 marks)

Marking criteria (in .ws-answers section only):

[1 mark] States Le Chatelier’s Principle correctly: a system at equilibrium shifts to minimise the effect of a disturbance.
[1 mark] Applies LCP to temperature: the forward Haber reaction is exothermic (ΔH = −92 kJ mol⁻¹); increasing temperature shifts the equilibrium left (endothermic direction), decreasing NH₃ yield and decreasing K_eq.
[1 mark] Explains why 300°C is unviable using data: yield is highest at 63% but time to equilibrium is “very slow (>days)” — commercially unusable rate of NH₃ production even with an iron catalyst.
[1 mark] Explains why 500°C gives poor industrial outcome: yield is only 17% at 300 atm (K_eq is very small; equilibrium greatly favours reactants) and catalyst deactivation/sintering occurs at high temperatures, shortening catalyst life and increasing costs.
[1 mark] Justifies 400–450°C as the optimal compromise using data: yield of 26–36% combined with fast equilibration time (<1 hour to moderate), giving commercially viable NH₃ production rate; unreacted gases can be recycled to compensate for the lower yield.
[1 mark] States correctly that K_eq changes with temperature (unlike concentration changes).
[1 mark] States the direction: as temperature increases from 300°C to 500°C, K_eq decreases (exothermic forward → higher T = smaller K_eq). Evidence from data: equilibrium yield falls monotonically from 63% to 17% across the table, reflecting declining K_eq.
[1 mark] Demonstrates a high-quality integrated answer linking LCP, K_eq, kinetics (rate–yield trade-off) and industrial practice coherently, or provides an additional nuanced point (e.g. the role of recycling, the cost of operating at extreme pressures vs moderate ones, or the specific role of the iron catalyst in lowering activation energy without affecting K_eq).

Sample response. Le Chatelier’s Principle states that when a system at dynamic equilibrium is disturbed, it shifts in the direction that minimises the effect of that disturbance. For the Haber process (N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ mol⁻¹), the forward reaction is exothermic. Increasing temperature adds thermal energy; LCP shifts the equilibrium in the endothermic direction (reverse), producing less NH₃. This is confirmed by the data: yield decreases monotonically from 63% at 300°C to 17% at 500°C at 300 atm. Because K_eq is the only equilibrium quantity affected by temperature, K_eq also decreases as temperature rises. At 300°C, yield is highest (63%) but time to equilibrium is “very slow (>days)” — insufficient collisions exceed the activation energy even with the iron catalyst, making throughput commercially inadequate. At 500°C, equilibration is very fast (<minutes) but the yield collapses to 17% and K_eq is very small, meaning only a small fraction of reactants become product per pass. Additionally, high temperatures deactivate the iron catalyst through sintering, increasing operating costs. At 400–450°C, the equilibrium yield (26–36%) is moderate but the reaction reaches equilibrium within hours, providing an acceptable production rate; any unreacted N₂ and H₂ are recycled, compensating for the lower per-pass yield. This 400–450°C range therefore represents the rate–yield trade-off optimum.

Q2 — Blood pH / rebreather mask (7 marks)

Marking criteria:

[1 mark] Defines Le Chatelier’s Principle and correctly identifies the initial disturbance as removal of CO₂(aq) from blood plasma by hyperventilation.
[1 mark] Traces shift in the first equilibrium: removing CO₂ (a reactant) causes the first equilibrium (CO₂ + H₂O ⇌ H₂CO₃) to shift left, reducing [H₂CO₃].
[1 mark] Traces shift in the second equilibrium: reduced [H₂CO₃] causes the second equilibrium (H₂CO₃ ⇌ H⁺ + HCO₃⁻) to shift left, reducing [H⁺] and [HCO₃⁻].
[1 mark] Uses quantitative data: [H⁺] falls from 40 nmol L⁻¹ (normal) to 28 nmol L⁻¹ (patient) — a 30% drop — corresponding to pH rise from 7.40 to 7.56; this reduction in [H⁺] explains both the pH increase (respiratory alkalosis) and symptoms (tingling/dizziness from altered nerve excitability).
[1 mark] Explains rebreather mask mechanism using LCP: the mask limits CO₂ loss by allowing re-inhalation of exhaled air, so [CO₂(aq)] in blood plasma increases back toward normal — a concentration disturbance that shifts both equilibria to the right (forward), increasing [H₂CO₃] and then [H⁺].
[1 mark] Explains how [H⁺] returning toward 40 nmol L⁻¹ restores blood pH toward 7.40, correcting the alkalosis.
[1 mark] States that K_eq does not change at any point in this process: all disturbances are concentration changes (CO₂ removal, then CO₂ restoration); body temperature remains approximately 37°C throughout, so K_eq is unchanged. Only temperature changes K_eq.

Sample response. Le Chatelier’s Principle states that when a closed system at equilibrium is disturbed, it shifts to minimise the effect of the disturbance. During hyperventilation, the patient expels CO₂ faster than it is produced metabolically, lowering [CO₂(aq)] in blood plasma. This is a concentration disturbance — removing a reactant from the first equilibrium (CO₂ + H₂O ⇌ H₂CO₃). LCP shifts this equilibrium left to partially replace the lost CO₂, consuming H₂CO₃. As [H₂CO₃] falls, the second equilibrium (H₂CO₃ ⇌ H⁺ + HCO₃⁻) also shifts left to partially replace H₂CO₃, consuming H⁺ and HCO₃⁻. This lowers [H⁺] from a normal value of 40 nmol L⁻¹ (pH 7.40) to 28 nmol L⁻¹ (pH 7.56) in this patient — a 30% reduction — producing respiratory alkalosis. The lower [H⁺] alters nerve excitability, causing the tingling and dizziness. The rebreather mask causes the patient to re-inhale some exhaled CO₂-rich air, raising [CO₂(aq)] back toward normal. LCP now shifts both equilibria to the right: more H₂CO₃ is formed, then more H⁺ and HCO₃⁻ are produced, raising [H⁺] back toward 40 nmol L⁻¹ and restoring pH toward 7.40. K_eq does not change throughout this process because all disturbances are concentration changes (CO₂ removal and restoration); body temperature remains constant at ∼37°C, so the thermodynamic basis of K_eq is unaltered.