S5 Chemistry: Industrial Processes — Tutor Lesson Plan (60 min, Haber and Contact, four errors)

2026-11-02

Tutor lesson-prep guides · Secondary chemistry (DSE)

Students can write both equations and still cannot say why the pressure is high in one plant and low in the other. This lesson makes every condition an answer to an equilibrium question, so the numbers stop being a list to memorise.

Objectives — the student can write the equations for the Haber and Contact processes, state the catalyst and the conditions for each, use Le Chatelier to justify each condition, calculate a mass yield from a sulfur feedstock, and explain why the operating temperature is a compromise between rate and equilibrium yield.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Le Chatelier	What happens to an equilibrium when pressure increases?	It shifts to the side with fewer gas molecules
Mole ratio	Nitrogen and hydrogen react 1 to 3. How much hydrogen for 2 moles of nitrogen?	6 moles
Molar mass	What is the molar mass of sulfuric acid?	98 grams per mole

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Hook	A bag of fertiliser and a bottle of acid	Guess what they share
8-20 min	Scene 1	Haber equation and the four volumes to two	Say which way pressure pushes it
20-32 min	Scene 2	Choosing 450 degrees and 200 atmospheres	Explain the compromise
32-44 min	Scene 3	Contact process in three steps	Say where the catalyst sits
44-54 min	Scene 4	Yield arithmetic from one tonne of sulfur	Get 3.06 tonnes
54-60 min	Exit check	Three items in section 08	Marked at once

03 Teaching script (read it out)

Scene 1: Two gases in, one gas out

Nitrogen plus three hydrogen gives two ammonia: four volumes of gas become two. That single fact answers the pressure question, because raising the pressure favours the side with fewer gas molecules, so high pressure favours ammonia. The reaction is also exothermic, about minus 92 kilojoules per mole of nitrogen, so a low temperature favours the product. Both statements come from Le Chatelier and neither needs the industrial number yet.

Ask: “If low temperature is better for yield, why not run the plant cold?” Because the rate collapses, and a plant that reaches equilibrium next week has produced nothing.

Scene 2: Four hundred and fifty degrees is a rate decision

The plant runs at about 450 degrees and 200 atmospheres over an iron catalyst. The temperature is high enough for a useful rate and low enough to keep a reasonable yield, roughly 15 per cent conversion per pass; unreacted gas is recycled, which is why a low single-pass figure is not a disaster. With a feed of 1 mole of nitrogen and 3 of hydrogen at 15 per cent conversion, 0.30 moles of ammonia form and the mixture holds 3.70 moles in total, so ammonia is about 8.1 per cent by amount. The pressure is as high as the vessel can take economically, and 200 atmospheres is a cost ceiling, not a chemistry optimum.

Warn: the catalyst does not change the yield. It only shortens the time taken to reach it.

Scene 3: The Contact process is three oxidations

Sulfur burns to sulfur dioxide, sulfur dioxide is oxidised to sulfur trioxide over vanadium pentoxide at about 450 degrees and one to two atmospheres, and the trioxide is absorbed in concentrated sulfuric acid to give more acid. The second step is the equilibrium one and it is exothermic, so again temperature is a compromise, but note the volumes: two of sulfur dioxide plus one of oxygen gives two of trioxide, so the mole count falls and high pressure would help. It is not used because the conversion is already about 98 per cent at ordinary pressure, and compressing gas costs money for almost no gain.

Ask: “Why is the pressure low here but high in the Haber plant?” Because 98 per cent is already good enough, and the Haber equilibrium is much less favourable.

Scene 4: Yield arithmetic in four lines

One tonne of sulfur is 1000 grams divided by 32, which is 31.25 kilomoles. Each sulfur atom ends up in one sulfuric acid molecule, so the mass is 31.25 kilomoles times 98 grams per mole, which is 3.06 tonnes of acid, assuming complete conversion. That ratio of 98 over 32 is worth remembering as roughly three, and it is the standard exam shortcut. If the question says 95 per cent efficiency, multiply the answer by 0.95 rather than redoing the chemistry.

Ask: “Where would you lose mass in the real plant?” In the absorber and in the sulfur burner, which is why the exam always tells you the efficiency.

04 Question types and worked steps

Type	Example	Steps	Answer
Equation	Write the equation for the Haber process.	N2 plus 3H2 in equilibrium with 2NH3	Exothermic, iron catalyst
Conditions	State the conditions used in the Haber process.	450 degrees, 200 atmospheres, iron catalyst	A compromise, not an optimum
Le Chatelier	Explain why high pressure is used in the Haber process.	Four gas volumes become two, so high pressure favours ammonia	Count molecules on each side
Yield	Find the mass of sulfuric acid from 1 tonne of sulfur.	1000 over 32 times 98	3.06 tonnes

Sanity check (teach this): Every condition answer must name the direction of the shift and the reason: fewer gas molecules, or exothermic, or rate too slow. Any answer that gives a number without the shift gets no credit. For arithmetic, the check is a mass balance: the mass of acid must be about three times the mass of sulfur, because the molar mass ratio is 98 to 32.

05 Independent practice (3 questions)

Question	Answer	What to watch
Write the Contact process equation for the oxidation of sulfur dioxide.	2SO2 plus O2 in equilibrium with 2SO3, vanadium pentoxide catalyst.	Exothermic and fewer moles
Explain why 450 degrees is used in the Contact process rather than a lower temperature.	A lower temperature gives a better yield but too slow a rate; 450 degrees is the economic compromise.	Say both halves
1 tonne of sulfur, 95 per cent efficiency. Find the mass of acid.	3.06 times 0.95 is 2.91 tonnes.	Multiply at the end
Why is the Haber catalyst not called a yield booster?	A catalyst changes only the rate of approach to equilibrium, not the position.	Equilibrium unchanged

06 Common mistakes and what to say

Mistake	Student writes	Say this
Saying the catalyst raises the yield	It reaches the same equilibrium faster	Ask what the equilibrium position depends on
Forgetting the recycle loop	A 15 per cent single-pass conversion is fine because unreacted gas returns	Ask where the other 85 per cent goes
Adding water directly to sulfur trioxide	That makes a mist; the trioxide is absorbed in concentrated acid	Ask about the absorber
Mixing up the two processes	Haber makes ammonia from nitrogen and hydrogen; Contact makes sulfur trioxide from sulfur dioxide	Ask for the feedstock first

07 Homework (paper, section and question numbers)

Secondary chemistry chapter index — Read this chapter with the equilibrium and the rates chapters
HKDSE Chemistry Paper 1 practice paper — Industrial process questions; redo section 05 questions 1 to 4 of this guide

08 Exit check (last 5 minutes)

Question	Answer	Decision
State the conditions for the Haber process.	450 degrees, 200 atmospheres, iron catalyst.	All correct: move to the arithmetic
Why does high pressure favour ammonia?	Four gas volumes become two, so the equilibrium shifts to the right.	Count the molecules
Find the mass of acid from 1 tonne of sulfur at 100 per cent.	3.06 tonnes.	Ratio 98 to 32

09 Tutor FAQ

Is 15 per cent yield a failure?

No, because the unreacted nitrogen and hydrogen are recycled, so overall conversion is high while each pass is modest.

Must students know the pressure of the Contact process?

Only roughly one to two atmospheres, and the reason it is low: conversion is already about 98 per cent.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.