DSE Chemistry | Fossil Fuels and Air Pollution: Fractional Distillation, Combustion Calculations and Catalytic Converters (teacher prep) | from crude oil to three pollution equations

2026-11-05

Tutor lesson-prep guides · Secondary Chemistry

Students can say burning makes carbon dioxide, then freeze when asked how much carbon dioxide one kilogram of octane releases. This lesson runs from the distillation column to the converter.

Objectives — Students can: (1) name four fractions separated by boiling point and give a use for each; (2) write complete and incomplete combustion equations and calculate carbon dioxide from a mass of fuel; (3) write the converter equation turning CO and NO into carbon dioxide and nitrogen, and explain the unleaded requirement.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Alkane formula	What is the molecular formula of octane?	C8H18, from CnH2n+2
Moles and mass	How do you get moles from a mass?	Divide the mass by the molar mass
Redox	What kind of reaction is combustion?	Oxidation of the fuel

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Start with one barrel	Ask how petrol and bitumen come from the same crude oil	Silence or the word processing
5-15 min	Five cut points	From 20 C to above 350 C, longer chains boil higher	Five fractions named with uses
15-28 min	Complete and incomplete	Enough oxygen against too little	Where CO and soot come from
28-40 min	Combustion calculation	114 g of octane gives 352 g of carbon dioxide	1 kg gives 3.09 kg
40-45 min	Converter and exit check	2CO + 2NO gives 2CO2 + N2	Three out of three

03 Teaching script (read it out)

Scene 1: A distillation column has one principle: boiling point

Crude oil is heated to vapour and enters the column, hot at the bottom and cool at the top. Longer chains have higher boiling points, so they condense lower down. Five cut points: below 20 C the C1 to C4 gases, near 70 C petrol from C5 to C10, 180 C kerosene, 250 C diesel, and above 350 C lubricating oil and bitumen.
The common error is calling distillation a chemical reaction, so write on the board that the molecules are unchanged. Add the supply point: cars want petrol but petrol is only one fraction, which is why cracking exists.

Ask the student: are the petrol molecules the same as those in the crude oil?

Scene 2: Combustion has two roads: enough oxygen and too little

Complete combustion is C8H18 + 12.5 O2 giving 8 CO2 + 9 H2O, so each mole of octane needs 12.5 moles of oxygen; incomplete combustion gives CO or soot, which is the source of poisoning and visible smoke. The carbon-to-hydrogen ratio fixes the carbon dioxide: octane has a molar mass of 114 and gives 8 moles of carbon dioxide, which is 352 g, so 1 kg of octane releases about 3.09 kg of carbon dioxide.
Only then separate the three pollutants: CO from incomplete combustion, sulfur dioxide from sulfur in the fuel, and NO from nitrogen and oxygen combining at high temperature, not from the fuel. Keeping those three apart carries the long questions.

Ask the student: is black smoke from a diesel engine evidence of too much or too little oxygen?

Scene 3: A catalytic converter makes two poisons cancel out

Carbon monoxide in the exhaust is a reducing agent and nitrogen monoxide is an oxidising agent, so over platinum and rhodium they react: 2CO + 2NO gives 2CO2 + N2. Three points: the catalyst is not consumed, only speeds the reaction; unleaded fuel is required because lead poisons the catalyst; and the converter cannot touch carbon dioxide.
Add one number: coal with 1 percent sulfur contains 10 g of sulfur per kilogram, which is 0.312 mol, and complete combustion gives 20 g of sulfur dioxide, which is why coal plants need desulfurisation.

Ask the student: why can a catalytic converter not reduce carbon dioxide?

04 Question types and worked steps

Type	Example	Steps	Answer
Fraction matching	Match fractions to uses	Order by carbon number, then cut by boiling point	Petrol is C5 to C10
Combustion equation	Write complete or incomplete combustion	Balance carbon, then hydrogen, then oxygen	C8H18 + 12.5 O2
Emission calculation	Find carbon dioxide from a fuel mass	Mass to moles, multiply by eight, multiply by 44	1 kg gives 3.09 kg
Pollution control	Write the converter equation and explain it	CO is oxidised and NO is reduced in one equation	2CO + 2NO gives 2CO2 + N2

Sanity check (teach this): Three numbers must agree: octane at 114 g per mole, 352 g of carbon dioxide per mole, and 3.09 kg per kilogram of fuel. The sanity test is that the carbon dioxide mass must exceed the fuel mass, because oxygen has been added; anything under 1 kg is wrong. The sulfur line checks the same way: 10 g of sulfur gives 20 g of sulfur dioxide, a ratio of two, because the molar mass of the dioxide doubles the element.

05 Independent practice (3 questions)

Question	Answer	What to watch
What is the complete combustion equation of octane?	C8H18 + 12.5 O2 gives 8 CO2 + 9 H2O	Whether the oxygen coefficient balances
How much carbon dioxide does 1 kg of octane release?	About 3.09 kg	Whether moles were multiplied by eight and by 44
How much sulfur dioxide comes from 1 kg of coal with 1 percent sulfur?	20 g	Whether sulfur to dioxide was treated as one to one in moles

06 Common mistakes and what to say

Mistake	Student writes	Say this
Calling distillation a reaction	Writing that distillation breaks long chains	Distillation sorts by boiling point; cracking is the reaction.
Writing carbon dioxide for incomplete combustion	Showing 8 CO2 when oxygen is short	Count the oxygen available and let the student see it runs out.
Forgetting the carbon number	Scaling 114 g straight to 44 g	One mole of octane holds eight carbons, so multiply by eight.
Saying NO comes from nitrogen in the fuel	Writing that petrol contains nitrogen and therefore makes NO	NO forms from air at high temperature, independent of fuel composition.

07 Homework (paper, section and question numbers)

DSE Chemistry fossil fuels practice paper — Questions 1 to 3: draw the column with five fractions, and balance the combustion equation in question 3.
Same paper, emission calculation — Question 5: carbon dioxide from 1 kg of octane, then sulfur dioxide from coal with 1 percent sulfur, units throughout.

08 Exit check (last 5 minutes)

Question	Answer	Decision
Is distillation physical or chemical	Physical separation	Answering chemical means the molecules need reteaching
Two products of incomplete combustion	Carbon monoxide and carbon	Answering only CO means soot was missed
The converter equation	2CO + 2NO gives 2CO2 + N2	Failing this, return to CO being oxidised and NO reduced

09 Tutor FAQ

Why must the fuel be unleaded?

Lead poisons the catalyst, and the converter is the key emission device on a petrol car.

Is natural gas pollution free?

It produces almost no sulfur dioxide or particulates, but it is still a fossil fuel and releases carbon dioxide.

How does cracking differ from distillation?

Distillation separates existing molecules by boiling point; cracking breaks long chains to make more petrol.

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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.