Fossil Fuels and Carbon Compounds: Fossil Fuels and Introduction to Carbon Compounds
FoundationPetroleum is a mixture of hydrocarbons, fractionally distilled by boiling point into petroleum gas, petrol, diesel and other fractions. Alkanes are saturated (CₙH₂ₙ₊₂); alkenes carry a C=C double bond and decolourise bromine water. Burning fossil fuels emits carbon dioxide and carbon monoxide — warming and pollution.
- Fossil fuels = coal, petroleum, natural gas; formed from ancient remains over eons — non-renewable
- Petroleum is a mixture of hydrocarbons (carbon and hydrogen only); fractional distillation — a physical change — yields fractions from petroleum gas to bitumen
- Alkanes saturated, CₙH₂ₙ₊₂ (methane to butane); alkenes unsaturated, CₙH₂ₙ, with a C=C double bond
文體小知識：科學知識說明文——從石油的分餾塔認識化石燃料與烴，學習烷烴和烯烴的通式與溴水試驗，並討論燃燒帶來的環境問題。
§Text Passage
1Coal, petroleum and natural gas are the fossil fuels: remains of ancient organisms buried deep in the crust and transformed by heat and pressure over hundreds of millions of years. Reserves are finite — once burnt, gone. Petroleum is a dark, smelly, viscous liquid, a mixture of hydrocarbons — compounds of carbon and hydrogen only — ranging from methane with a single carbon atom to giant molecules of dozens, each with its own properties.
2Boiling points rise as the molecules grow, so industry separates petroleum in a fractionating column: crude oil is vaporised, the vapours rise, the column cools upwards; high-boiling components condense near the bottom, low-boiling ones climb to the top. From top to bottom come the fractions petroleum gas, petrol, naphtha, kerosene, diesel, fuel oil, lubricating oil and bitumen — bottled gas for cooking, petrol for cars, kerosene for lamps of old, diesel for lorries, bitumen for roads, each in its place.
3Hydrocarbons fall into two families. Alkanes contain only carbon-carbon single bonds and are saturated, with general formula CₙH₂ₙ₊₂ — methane, ethane, propane, butane holding one to four carbons. Alkenes contain one carbon-carbon double bond and are unsaturated, general formula CₙH₂ₙ — ethene and propene the standard-bearers. Add each to bromine water: alkanes leave the orange colour untouched, alkenes decolourise it quickly — the chemical test for unsaturation. Ethene is also the key feedstock of plastics, worth far beyond its role as fuel.
4Burning hydrocarbons releases much heat. With ample oxygen, combustion is complete, giving carbon dioxide and water — methane burns as CH₄ + 2O₂ → CO₂ + 2H₂O. Short of oxygen, combustion is incomplete, producing toxic carbon monoxide and black soot; gas water heaters must be well ventilated precisely to guard against carbon monoxide poisoning. Carbon dioxide from burning fossil fuels intensifies global warming, while sulfur dioxide from coal and sulfur-bearing fuels dissolves into acid rain — hence energy saving and alternative fuels brook no delay.
1Summary 重點摘要
Petroleum is a mixture of hydrocarbons, fractionally distilled by boiling point into petroleum gas, petrol, diesel and other fractions. Alkanes are saturated (CₙH₂ₙ₊₂); alkenes carry a C=C double bond and decolourise bromine water. Burning fossil fuels emits carbon dioxide and carbon monoxide — warming and pollution.
2Vocabulary cards Vocabulary
Fuels formed from ancient remains over geological time — coal, petroleum and natural gas; non-renewable.
A compound of carbon and hydrogen only, also called a carbon-hydrogen compound; the stuff of petroleum.
Separating mixture components by their different boiling points in a fractionating column; a physical change.
A group of hydrocarbons of similar boiling range from distillation, e.g. petrol, diesel, bitumen.
A saturated hydrocarbon with only single bonds, general formula CₙH₂ₙ₊₂, e.g. methane, butane.
An unsaturated hydrocarbon with one C=C double bond, general formula CₙH₂ₙ, e.g. ethene, propene.
A hydrocarbon whose carbons are joined only by single bonds, carrying the maximum number of hydrogens.
An orange solution of bromine, rapidly decolourised by alkenes — the test for unsaturation.
Burning with ample oxygen, converting a hydrocarbon fully to carbon dioxide and water with maximum energy.
Burning short of oxygen, giving toxic carbon monoxide and soot with less energy released.
3Step by step Steps
- Step 1: The pedigree of fossil fuelsMarine organisms and plants that died hundreds of millions of years ago were buried by sediment; deep in the crust, heat and pressure transformed them over immense ages into petroleum, natural gas and coal. For example, crude oil drawn from a platform may hold the remains of plankton from eons past. The keyword is non-renewable — humanity could burn through eons of savings within centuries, which is why the course stresses conservation and why solar and wind power are pursued worldwide.
- Step 2: Petroleum, a grand mixture of hydrocarbonsPetroleum is not one pure substance but a mixture of hundreds of hydrocarbons, each containing only carbon and hydrogen, differing in carbon count and chain length. The larger the molecule, the stronger the intermolecular attractions and the higher the boiling point. For example, methane, with one carbon, boils at about −162°C and is a gas at room temperature, while waxy molecules of twenty-odd carbons boil at several hundred. This ladder of boiling points is exactly what lets the fractionating column slice petroleum into useful fractions.
- Step 3: The temperature staircase of the columnCrude oil is heated to about 400°C and vaporised; the vapours rise from the bottom of the column, which grows cooler upwards. The highest-boiling components condense almost at once and collect at the base; lower-boiling ones climb before condensing. Top to bottom, the fractions run petroleum gas, petrol, naphtha, kerosene, diesel, fuel oil, lubricating oil and bitumen. Properties also trend with carbon count: colour deepens, viscosity thickens, volatility falls. For example, bitumen — black and sticky — paves roads while petroleum gas, light and clean, is bottled for cooking: an orderly tower from top to bottom.
- Step 4: Alkanes — the saturated familyIn alkanes every carbon-carbon link is a single bond and the hydrogens are maxed out: CₙH₂ₙ₊₂. Names follow carbon count: one carbon methane (CH₄), two ethane (C₂H₆), three propane (C₃H₈), four butane (C₄H₁₀), then pentane, hexane and so on. For example, bottled petroleum gas is chiefly propane and butane, liquefied under pressure in the cylinder and vaporised for burning. Alkanes are chemically stable; combustion is their signature reaction — the very reason they serve as fuels.
- Step 5: Alkenes and the bromine water testAlkenes carry two fewer hydrogens than the alkane of the same carbon count, with a carbon-carbon double bond: CₙH₂ₙ — ethene (C₂H₄), propene (C₃H₆). The double bond makes them reactive: it can 'open up' to add other atoms. The standard test: add the sample to bromine water and shake — rapid loss of the orange colour shows a C=C bond, hence unsaturation; alkanes cannot decolourise bromine water. For example, bubbling ethene through bromine water clears the colour within seconds; decolourised acidified potassium permanganate is further evidence. Ethene is also the feedstock of polythene plastics — immense industrial worth.
- Step 6: Combustion — from balancing to completenessThe pattern: carbon becomes carbon dioxide, hydrogen becomes water. Methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Butane takes a few moves: start with C₄H₁₀ + O₂ → 4CO₂ + 5H₂O; the right side holds 8 + 5 = 13 oxygens, so add 132 O₂, then double everything: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. Short of oxygen, incomplete combustion yields carbon monoxide and soot; carbon monoxide is colourless and odourless, binds haemoglobin and starves the body of oxygen. For example, gas water heaters in unventilated bathrooms have caused fatal carbon monoxide poisonings — a safety issue the course stresses.
- Step 7: The environmental bill of burningThree lines on the bill. Carbon dioxide, a greenhouse gas, drives global warming — melting glaciers, rising seas. Sulfur dioxide from coal and sulfur-bearing fuels dissolves into acid rain, corroding buildings and acidifying lakes. Incomplete combustion adds carbon monoxide and soot to the air. For example, Hong Kong's power stations fitted flue-gas desulfurisation and switched partly to natural gas, cutting sulfur dioxide sharply. Personal action counts too — saving electricity, using public transport, choosing efficient appliances — all real ways to burn less.
4Diagrams Diagrams
5Worked example Example
Data question: a refinery processes crude oil in a fractionating column while a laboratory receives an unlabelled colourless gas, suspected to be a component of petroleum gas. Answer: (1) why fractional distillation separates petroleum's components, and whether the process is physical or chemical; (2) the balanced equation for the complete combustion of butane (C₄H₁₀); (3) the gas rapidly decolourises bromine water — alkane or alkene, and why.
- Principle: petroleum is a mixture of hydrocarbons of different boiling points; vaporised, they condense in the column in order of boiling point, giving the fractions
- Type of change: only separation by boiling point, no new substance — a physical change
- Balance the combustion: C₄H₁₀ + O₂ → CO₂ + H₂O; fix carbon (4CO₂), then hydrogen (5H₂O), 13 oxygens on the right, 132 O₂ on the left, double the whole equation
- Balanced: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O; check C, H and O on both sides
- Bromine water: rapid decolourisation shows a C=C double bond — unsaturated, hence an alkene, not an alkane
- Answer: Petroleum is a mixture of hydrocarbons of differing boiling points; once vaporised, the components condense in the fractionating column in order of boiling point. No new substance forms, so it is a physical change. Butane burns completely by 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O. The colourless gas rapidly decolourises bromine water, showing a carbon-carbon double bond and unsaturation — it is an alkene, not an alkane (alkanes cannot decolourise bromine water).
6Common mistakes Mistakes
7Quick check Quick check
Q1Which three fuels are the fossil fuels?
Q2How do the boiling points of fractions change from the top to the bottom of the fractionating column?
Q3Write the general formulas of alkanes and alkenes.
Q4How do you distinguish an alkane from an alkene chemically?
Q5Write the equation for the complete combustion of methane.
8Recap Recap
- Fossil fuels = coal, petroleum, natural gas; formed from ancient remains over eons — non-renewable
- Petroleum is a mixture of hydrocarbons (carbon and hydrogen only); fractional distillation — a physical change — yields fractions from petroleum gas to bitumen
- Alkanes saturated, CₙH₂ₙ₊₂ (methane to butane); alkenes unsaturated, CₙH₂ₙ, with a C=C double bond
- Unsaturation test: bromine water loses its orange rapidly (or acidified permanganate decolourises) — an alkene
- The burning bill: complete combustion gives CO₂ and water; short of oxygen, toxic CO and soot; CO₂ warms the planet, SO₂ makes acid rain