Hydrocarbons — Part 1: Alkanes (methane and ethane): visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

The fuels in the pipeline

Part 1 — Natural gas, biogas and CNG are mostly methane; LPG is mainly propane and butane. These are alkanes, the simplest family of hydrocarbons. We learn how methane and ethane are made in the laboratory and how they react.

In this lesson you will

Write the general formula of alkanes and explain why methane is a greenhouse gas of concern
Write equations with conditions for preparing methane and ethane by each listed method
Write equations for complete and incomplete combustion of methane and ethane
Write the stepwise substitution products of methane with chlorine in diffused sunlight

Table of three everyday fuels, the alkanes in them and where they are used
Fuel	Main alkanes	Where it is used
Natural gas, CNG	Methane (with some ethane)	Buses, autorickshaws, piped gas
Biogas	Methane (with CO₂)	Village cooking, from cow dung
LPG	Propane and butane	Kitchen cylinders

Concept

Alkanes: the saturated family

Alkanes are saturated hydrocarbons: every carbon–carbon bond is a single bond. General formula CₙH₂ₙ₊₂: methane CH₄ (n = 1), ethane C₂H₆ (n = 2), propane C₃H₈ (n = 3). With no double or triple bond to open up, alkanes are fairly unreactive: they burn, and they undergo substitution with chlorine, but they do not add other molecules.

Concept

Methane: a greenhouse gas of concern

Methane is released by paddy fields, cattle (digestion), landfills and rotting waste, coal mines and leaks from natural-gas pipelines. Like carbon dioxide, it absorbs infrared radiation given off by the Earth and so traps heat. Molecule for molecule it traps far more heat than carbon dioxide (roughly 25–30 times over a century), so even small amounts add to global warming.

Table of four sources of methane in the atmosphere and why methane forms there
Source of methane	Why methane forms
Flooded paddy fields	Bacteria break down plant matter without air
Cattle	Digestion of grass in the stomach
Landfills and sewage	Rotting of waste without air
Gas wells and pipelines	Leaks of natural gas

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