5.2 — Hydrocarbons

Hydrocarbons from Natural Sources

• A hydrocarbon is an organic carbon compound made up of carbon and hydrogen only.

Formation of petroleum and natural gas

1. Remains of dead marine life are buried on the seabed.
2. Over millions of years, the remains become buried deeper under thick layers of rock and mud.
3. The combined effects of pressure, heat from the surroundings and decomposition by bacteria change the remains into petroleum and natural gas.
4. Natural gas occurs above petroleum beneath the seabed.

Formation of coal

1. Remains of dead plants are naturally buried underground.
2. Over millions of years, they become buried deeper under thick layers of rock.
3. The combined effects of pressure, heat from the surroundings and decomposition by bacteria change the plant fossils into coal.

Fractional Distillation of Petroleum

• Petroleum is a mixture of hydrocarbons.
• Petroleum fractions are separated by fractional distillation before use.
• The method works because petroleum fractions have different boiling points.

Activity 5.2 — Separating crude oil into four fractions

• Materials: crude oil, wooden splinter, ice, water and glass wool.
• Apparatus: measuring cylinder, boiling tube, retort stand, test tubes and rack, beaker, rubber stopper with delivery tube, thermometer (0°C–360°C), Bunsen burner and evaporating dishes.

1. Place 10 cm³ of crude oil and glass wool in a boiling tube; assemble the delivery tube and an ice-cooled receiving test tube.
2. Heat gently from room temperature to 80°C; collect about 1 cm³ as Fraction 1.
3. Continue with new test tubes to collect:
   • Fraction 2: 80°C–150°C
   • Fraction 3: 150°C–230°C
   • Fraction 4: 230°C–250°C
4. Compare each fraction’s colour, viscosity and flammability.
5. Fractional distillation separates the mixture according to the fractions’ different boiling points.

Safety: Crude oil and its vapour are highly flammable. Use crude oil only, heat gently, and wash skin exposed to crude oil with soap and water.

Saturated and Unsaturated Hydrocarbons

Type	Carbon–carbon bonding
Saturated hydrocarbon	Has only single covalent bonds, C–C; example: alkane
Unsaturated hydrocarbon	Has at least one double covalent bond, C=C, or triple covalent bond, C≡C; example: alkene

Homologous Series: Alkanes and Alkenes

• A homologous series is a specific group of organic compounds with similar chemical properties.

Alkanes

• Saturated hydrocarbons; every carbon atom forms single covalent bonds with other carbon atoms.
• General formula: ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+2}$, where $n=1,2,3,\dots$

Alkenes

• Unsaturated hydrocarbons; every molecule has at least one double covalent bond between two carbon atoms.
• General formula: ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n}$, where $n=2,3,\dots$

Number of carbon atoms, $n$	Alkane / alkene
1	Methane / —
2	Ethane / ethene
3	Propane / propene
4	Butane / butene
5	Pentane / pentene
6	Hexane / hexene

Alternative and Renewable Energy Sources

• Petroleum, coal and natural gas are non-renewable fossil fuels that are fast depleting.
• Alternative energy sources do not deplete easily; they include nuclear energy and renewable energy sources.
• Renewable sources: solar, geothermal, wind, tidal, hydroelectric, wave and biomass energy.
• The advantages and disadvantages of nuclear power stations must be evaluated before construction.
• Decay of organic waste in disposal sites releases carbon dioxide and methane; methane can be collected to generate electrical energy.
• Methane is highly flammable and must be collected carefully.