Methane

Methane is the first member of the alkanes and the main compound in natural gas. It doubles as the textbook simple-covalent molecule and as a fuel, so it turns up in bonding, organic and environmental questions across the whole course.

Key facts

Property	Detail
Formula	CH4
Relative molecular mass (Mr)	16 (C 12, H 1)
Homologous series	Alkanes, general formula CnH2n+2
Bonding / structure	Simple covalent molecule; tetrahedral; four C-H bonds
Saturation	Saturated (single bonds only)
Main exam roles	Fuel, combustion, greenhouse gas, covalent-bonding model

Where this compound appears in 0620

• Alkanes (Topic 12): a saturated hydrocarbon; combustion and substitution with chlorine.
• Simple molecules and covalent bonds (Topic 2): the dot-and-cross diagram, tetrahedral shape and low boiling point.
• Air quality and climate change (Topic 11): methane as a greenhouse gas.
• Fuels (Topic 12): natural gas as a fuel.

Bonding

Carbon (2,4) shares one electron with each of four hydrogen atoms, forming four single covalent bonds with no lone pairs. The dot-and-cross diagram shows four shared pairs around carbon, and the molecule is tetrahedral. Weak intermolecular forces between molecules explain the very low boiling point (-162 C).

Reactions you must know

Complete combustion (plenty of oxygen):

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)

Burns with a blue flame, releasing the most energy.

Incomplete combustion (limited oxygen):

2CH4(g) + 3O2(g) -> 2CO(g) + 4H2O(l) (toxic carbon monoxide) CH4(g) + O2(g) -> C(s) + 2H2O(l) (soot)

Substitution with chlorine (UV light):

CH4 + Cl2 -> CH3Cl + HCl

Because methane is saturated, it undergoes substitution — a chlorine atom replaces a hydrogen atom — and the reaction needs ultraviolet light. This is the key contrast with alkenes, which react by addition.

As a greenhouse gas

Methane absorbs and re-emits infrared radiation more effectively per molecule than carbon dioxide, so it contributes to the enhanced greenhouse effect despite its lower concentration. Its main sources are cattle, rice paddies, decomposing waste in landfill, and leaks from natural gas supplies.

Uses

Use	Detail
Fuel (natural gas)	Domestic heating, cooking and electricity generation
Chemical feedstock	Reacted with steam to make hydrogen: CH4 + H2O -> CO + 3H2

Worked exam question

Methane is the main compound in natural gas.

(a) Methane reacts with chlorine in the presence of ultraviolet light. Write the equation and name the type of reaction. (2 marks)

(b) Write the equation for the complete combustion of methane. (1 mark)

(c) A faulty heater burns methane in a limited air supply. Name the toxic gas produced and explain why it is harmful. (2 marks)

Mark scheme

• (a) CH4 + Cl2 -> CH3Cl + HCl [1]; substitution [1]
• (b) CH4 + 2O2 -> CO2 + 2H2O [1]
• (c) Carbon monoxide (CO) [1]; it binds to haemoglobin in the blood, reducing the blood’s ability to carry oxygen [1]

Examiner note: in (a) the reaction is substitution (alkanes do not undergo addition) and the UV-light condition is essential. A common slip in (b) is unbalanced water — four hydrogens require 2H2O.

Common exam mistakes

• Expecting methane to react by addition. Methane is saturated; it undergoes substitution with chlorine (needing UV light) and does not decolourise bromine water.
• Dropping the UV-light condition. Without ultraviolet light the substitution reaction with chlorine does not proceed.
• Unbalanced combustion equations. Complete combustion needs 2O2 and gives 2H2O; the incomplete-combustion equations have their own coefficients — count the atoms.
• Saying methane smells. Pure methane is odourless; the smell of “gas” is an odorant added deliberately so leaks are noticed.