S5 Chemistry: Industrial Processes prep guide | Haber, contact and chlor-alkali conditions and efficiency

2026-11-01

Tutor lesson-prep guides · Secondary Chemistry

Students memorise 200 atmospheres, 450 degrees and an iron catalyst, then cannot say why the pressure is not higher. This lesson puts the three routes on one table: conditions, catalyst, yield and atom economy.

Objectives — Students can: (1) write balanced equations for the Haber, contact and chlor-alkali processes; (2) justify each condition using Le Chatelier together with rate and cost; (3) compare the routes using atom economy and percentage yield.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Le Chatelier	Which side does pressure favour	The side with fewer gas moles
Equilibrium constant	Does a catalyst change K	No, it only speeds up approach
Mole conversion	How many moles in one tonne of ammonia	Convert grams first

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three routes	Write the equations and ask which is exothermic	Label enthalpy and states
8-22 min	Haber conditions	N2 plus 3H2 gives 2NH3, enthalpy -92 kJ per mol	200 atm, 450 C, iron
22-34 min	Contact and yield	2SO2 plus O2 gives 2SO3, 100 in and 96 out	96 percent
34-46 min	Atom economy	Haber 100 percent, chlor-alkali 46.4 percent	Name the by-products
46-54 min	Tonne scale	One tonne of ammonia needs 823.5 kg nitrogen and 176.5 kg hydrogen	5412 MJ released
54-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: The Haber compromise

N2 plus 3H2 giving 2NH3 releases 92 kilojoules per mole and halves the gas moles. Lower temperature and higher pressure both shift the equilibrium right, but a low temperature makes the rate useless and the cost of pressure rises steeply. Industry therefore runs at 450 degrees and 200 atmospheres over iron, accepting about 15 percent conversion per pass and recycling the unreacted gas to reach roughly 98 percent overall.
A catalyst only shortens the time to reach equilibrium and never raises the equilibrium yield.

Task: write one sentence each for why 800 degrees is rejected and why 1000 atmospheres is rejected, naming rate and cost.

Scene 2: Contact process and percentage yield

2SO2 plus O2 giving 2SO3 releases about 197 kilojoules per mole and runs at 420 to 450 degrees over vanadium pentoxide at 1 to 2 atmospheres, since pressure buys little once conversion is high. Percentage yield is actual product over theoretical product: 100 moles of sulfur dioxide should give 100 moles of sulfur trioxide, and 96 is obtained, so the yield is 96 percent.
Atom economy is target product mass over total reactant mass: the reactants weigh 160.2 g and the product 160.2 g, so it is 100 percent because no atoms leave as waste.

Task: explain in one line how yield and atom economy answer different questions.

Scene 3: Chlor-alkali: low atom economy, useful by-products

2NaCl plus 2H2O gives 2NaOH plus Cl2 plus H2 by electrolysis. The reactants weigh 153.0 g while the chlorine target is 71.0 g, so atom economy is 46.4 percent. Sodium hydroxide and hydrogen are both saleable, which is why a low atom economy does not make a route a bad one.
Tonne scale: one tonne of ammonia is 58824 moles, needing 29412 moles of nitrogen (823.5 kg) and 88235 moles of hydrogen (176.5 kg), releasing 5412 megajoules.

Warn: an atom economy answer without the fraction shown is treated as a memorised number and loses method marks.

04 Question types and worked steps

Type	Example	Steps	Answer
Writing	Balanced equation	Coefficients and state symbols	N2(g) plus 3H2(g) gives 2NH3(g)
Explanation	Condition choice	Le Chatelier, rate, cost	450 C and 200 atm
Calculation	Percentage yield	Actual over theoretical	96 percent
Calculation	Atom economy	Target over all reactants	Haber 100 percent

Sanity check (teach this): Atom economy cannot exceed 100 percent, and both Haber and contact routes sit at 100 percent because every reactant atom ends in the product; 120 percent means the fraction is upside down. Yield cannot exceed 100 percent either, so 96 percent is plausible while 110 percent is an arithmetic slip. Mass check: 34 g of reactants give 34 g of ammonia, and the tonne figures 823.5 plus 176.5 kg add up to slightly more than one tonne for that reason.

05 Independent practice (3 questions)

Question	Answer	What to watch
Atom economy of chlor-alkali taking chlorine as target	46.4 percent	Numerator 71, denominator 153
Why vanadium pentoxide is used	It speeds up approach without changing the yield	Catalyst property
Mass of sulfur trioxide from 100 moles at full conversion	8.0 kg	Moles times 80.1 g per mol
Why unreacted gas is recycled in Haber	To lift overall conversion to about 98 percent	Single pass is about 15 percent

06 Common mistakes and what to say

Mistake	Student writes	Say this
Catalyst raises yield	Writing that iron increases the equilibrium yield	“A catalyst only speeds things up.”
Confusing the two efficiencies	Quoting 96 percent as atom economy	“Yield measures what you get; atom economy measures waste.”
Higher pressure is always better	Writing that pressure should be maximised	“Plant cost and safety rise with pressure, so it is a trade-off.”
Missing state symbols	Writing the equation with no (g)	“Any question about gas moles needs state symbols.”

07 Homework (paper, section and question numbers)

Secondary chemistry chapter index — All chapters
DSE chemistry practice paper — Industrial chemistry, questions 5 to 9

08 Exit check (last 5 minutes)

Question	Answer	Decision
Atom economy of the Haber process	100 percent	All correct: move to reaction rates
Yield from 100 moles to 96 moles	96 percent	Wrong: recompute before next lesson
Effect of a catalyst on equilibrium	No change in position	Two wrong: restart act one

09 Tutor FAQ

Must all three routes be memorised

Learn the equations and conditions, but marks come from explaining the choice.

Is a low atom economy always bad

No, chlor-alkali sells sodium hydroxide and hydrogen, so the whole plant pays.

Why is the contact process not run at high pressure

Conversion is already 96 to 98 percent, so the extra cost is not repaid.

Find a tutor — free Tutor Registration →

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.