Ch 3 · Fuels · IGCSE Chemistry 0620
Topic 3 of 8 in Organic Chemistry — IGCSE Chemistry 0620 — 47 lessons.
Introduction
Almost everything we burn for energy, and most of the plastics, paints and medicines we make, starts as a fossil fuel. Petroleum is useless as it comes out of the ground, so it is separated into fractions with different boiling ranges. This lesson covers the three fossil fuels, the fractional distillation of petroleum, the names and uses of the eight fractions, and the trends in their properties that examiners love to ask you to explain.
Key ideas — Fossil fuels
The three fossil fuels are coal, natural gas and petroleum (crude oil). They formed over millions of years from the remains of dead organisms buried under sediment and subjected to heat and pressure, so they are non-renewable: they are being used far faster than they form.
🧑🏫 Teacher's explanation
Think of fossil fuels like a massive, ancient savings account that took millions of years to build up. Every time we burn coal or oil, we are spending that 'capital' without making any new deposits. Because the earth takes millions of years to create these fuels, we call them non-renewable.
📖 Definitions
🔍 Worked example — Standard
A fraction of petroleum has a boiling point of 150 circC and is quite runny (low viscosity). If we compare this to a fraction with a boiling point of 350 circC, which one has a longer carbon chain length?
🔍 Worked example — Exam-style
In a fractional distillation column, the temperature decreases as you move up the column. If a hydrocarbon has a very low boiling point, where in the column will it be collected?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Name the process used to separate crude oil. [1] (b) Describe how the boiling point and viscosity of the fractions change as the carbon chain length increases. [2] (c) State one use for the fraction known as 'bitumen'. [1] [4 marks]
Key ideas — Hydrocarbons and petroleum
Hydrocarbons are compounds that contain hydrogen and carbon only. Alkanes and alkenes are hydrocarbons; ethanol and ethanoic acid are not, because they also contain oxygen.
🧑🏫 Teacher's explanation
Imagine you have a large jar filled with different sized marbles and grains of sand. If you want to separate them, you could shake the jar or pour it through a sieve; the different sizes allow them to separate based on physical properties. Petroleum is like that jar, but instead of sand and marbles, it is a mixture of hydrocarbon molecules of different lengths.
📖 Definitions
🔍 Worked example — Easy
Identify which of the following are hydrocarbons: CH_4, C_2H_6O, C_3H_8, C_2H_4O_2.
🔍 Worked example — Standard
A hydrocarbon fraction has a boiling point of 150^°C. Another fraction has a chain length of 2 carbons and a boiling point of -160^°C. Describe the trend in boiling point as the carbon chain length increases.
🔍 Worked example — Exam-style
In a fractional distillation column, a fraction is collected at the top where temperatures are lowest. This fraction is highly flammable and very volatile. Name this fraction and state its primary use.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the physical property that allows these fractions to be separated. [1] (b) A fraction used for road fuel (gasoline) is collected at a higher temperature than the fraction used for refinery gas. Explain why the gasoline fraction has a higher boiling point than the refinery gas fraction. [2] [3 marks]
Key ideas — Fractional distillation of petroleum
The petroleum is heated to about 350 °C in a furnace so that most of it vaporises, and the mixture of vapour and liquid enters near the bottom of a tall fractionating column. The column has a temperature gradient: it is hottest at the bottom and coolest at the top.
🧑🏫 Teacher's explanation
Imagine a crowded elevator in a skyscraper where people are jumping off at different floors. The people who are very energetic and light (short hydrocarbon chains) can jump the highest, while the heavy, slow-moving people (long hydrocarbon chains) can only manage the bottom floors.
📖 Definitions
🔍 Worked example — Easy
A hydrocarbon fraction contains molecules with 5 carbon atoms. If the boiling point of methane (CH_4) is -160 °C and the boiling point of octane (C_8H_18) is 125 °C, which fraction (gasoline or diesel oil) is this molecule more likely to be part of?
🔍 Worked example — Standard
A refinery produces 5.0 imes 10^5 kg of petroleum. If 15\% of this is refinery gas and 5\% is bitumen, calculate the mass of the remaining fractions in kg.
🔍 Worked example — Exam-style
Predict the trend in viscosity and boiling point as the carbon chain length increases from C_1 to C_40.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why the fractions are collected at different heights in the column. [2] (b) Describe how the viscosity of the fractions changes as the carbon chain length increases. [1] [3 marks]
Key ideas — How properties change with chain length
Going down the column, the fractions contain longer molecules, and four properties change in step:
🧑🏫 Teacher's explanation
Imagine a group of people trying to walk through a crowded hallway. If the people are small children (short-chain hydrocarbons), they can zip through the gaps easily and quickly. If the people are giant, long-limbed creatures (long-chain hydrocarbons), they get tangled up, move very slowly, and find it much harder to squeeze through the gaps.
📖 Definitions
🔍 Worked example — Easy
A hydrocarbon fraction has a boiling point of 40^°C. Another fraction has a boiling point of 350^°C. Which fraction has a longer carbon chain and why?
🔍 Worked example — Standard
In a refinery, two fractions are separated. Fraction A is a colorless, highly flammable liquid that evaporates quickly. Fraction B is a dark, thick liquid that is difficult to ignite. Predict which fraction has a higher viscosity.
🔍 Worked example — Exam-style
Calculate the percentage increase in boiling point if a hydrocarbon's boiling point increases from 50^°C to 150^°C due to an increase in chain length.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why the boiling points of the fractions increase as the carbon chain length increases. [2] (b) State one property that decreases as the chain length increases. [1] [3 marks]
Worked example
Question. Petroleum is separated into fractions in a fractionating column. (a) Explain why petroleum can be separated by fractional distillation. [2] (b) Kerosene is collected higher up the column than diesel oil. Explain why. [3] (c) State one use of naphtha and one use of bitumen. [2]
🔍 Worked example — Easy
Identify the main constituent of natural gas and state its molecular formula.
🔍 Worked example — Standard
A hydrocarbon fraction has a chain length of 12 carbons. Another has a chain length of 30 carbons. Which fraction has a higher boiling point and why?
🔍 Worked example — Exam-style
Predict the trend in viscosity and flammability as the carbon chain length increases in a petroleum fraction.
⚠️ Common errors
Common mistakes
Practice
Answers Coal, natural gas and petroleum [B1]; methane [B1]. A compound containing hydrogen and carbon only. [B1] Petroleum is heated so that it vaporises and the vapour enters the bottom of the column [B1]; the column is hottest at the bottom and coolest at the top [B1]; vapours rise and each condenses where the temperature is below its boiling point [B1]; long-chain hydrocarbons with high boiling points condense near the bottom and short-chain ones near the top or leave as gas [B1]. Boiling point and viscosity increase [B1]; volatility and flammability decrease [B1]. Kerosene (paraffin) [B1]; lubricating oil [B1].