Methane (CH4)

What Is Methane?

Methane is the simplest organic compound. Its molecular formula is CH₄, meaning one molecule has one carbon atom surrounded by four hydrogen atoms. The carbon atom sits in the center with the four hydrogen atoms arranged around it in a shape called tetrahedral. All the bonds are single covalent bonds.

Methane is a member of the alkane family and follows the general alkane formula C_nH_2n+2. When n = 1 (one carbon atom), the formula becomes C₁H_(2×1)+2 = CH₄.

In Nigeria, methane is the main gas we use for cooking. When you buy cooking gas from vendors or refill your gas cylinder, you are mostly buying methane mixed with small amounts of other gases like propane and butane.

Structure of Methane

The structure of methane is important for understanding its properties:

• Molecular formula: CH₄
• Structural formula: H-C-H with two more H atoms bonded to C
• Shape: Tetrahedral (like a pyramid with a triangular base)
• Bond angle: 109.5° between any two C-H bonds
• Bond type: Four single covalent bonds (C-H bonds)

The tetrahedral shape means the hydrogen atoms are as far apart from each other as possible. This arrangement gives methane its stability.

Laboratory Preparation of Methane

In the chemistry laboratory, methane is prepared by heating a mixture of anhydrous (water-free) sodium ethanoate and soda-lime. Soda-lime is a mixture of sodium hydroxide (NaOH) and calcium oxide (CaO).

Chemical equation:
CH₃COONa_(s) + NaOH_(s) → Na₂CO_3(s) + CH_4(g)

Procedure:

1. Mix dry sodium ethanoate powder with soda-lime in a test tube
2. Clamp the test tube at an angle (slightly tilted)
3. Heat the mixture strongly with a Bunsen burner
4. Collect the gas produced over water or by downward displacement of air
5. Test the gas by bringing a burning splint near it—it burns with a blue flame

Why collect over water? Methane is only slightly soluble in water, so collecting it over water ensures you get pure methane gas without air mixing in.

Safety note: Methane is highly flammable. Keep flames away from the collection area until you are ready to test the gas.

Physical Properties of Methane

Important note about cooking gas smell: Pure methane has no smell, but gas companies add a chemical called mercaptan (which smells like rotten eggs) so people can detect gas leaks. This is a safety measure to prevent explosions.

Chemical Properties of Methane

Methane undergoes several chemical reactions:

1. Combustion (Burning): Methane burns in oxygen to produce heat, carbon dioxide, and water. This is the reaction that happens when you cook with gas.

Complete combustion (plenty of oxygen):
CH₄ + 2O₂ → CO₂ + 2H₂O + heat (blue flame)

Incomplete combustion (limited oxygen):
2CH₄ + 3O₂ → 2CO + 4H₂O (yellow flame, produces poisonous carbon monoxide)

If there is very little oxygen:
CH₄ + O₂ → C + 2H₂O (produces black soot/carbon)

This is why your cooking pot turns black when the gas burner is not adjusted properly—it’s getting soot from incomplete combustion.

2. Substitution with halogens: In sunlight or UV light, chlorine gas reacts with methane. Hydrogen atoms are replaced (substituted) one by one with chlorine atoms.

CH₄ + Cl₂ → CH₃Cl + HCl (chloromethane + hydrogen chloride)

This continues further:
CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane)
CH₂Cl₂ + Cl₂ → CHCl₃ + HCl (trichloromethane/chloroform)
CHCl₃ + Cl₂ → CCl₄ + HCl (tetrachloromethane)

3. Reaction with steam: At very high temperature (around 1000°C) with a nickel catalyst, methane reacts with steam to produce hydrogen gas and carbon monoxide. This process is used industrially to make hydrogen.

CH₄ + H₂O → CO + 3H₂ (requires high temp + Ni catalyst)

Natural Sources of Methane

Methane is found in nature from several sources:

Natural gas deposits: Underground reserves contain mainly methane. In Nigeria, we have large natural gas deposits in the Niger Delta region. This is extracted and supplied to homes and industries.

Biogas from decomposition: When organic matter (animal dung, food waste, dead plants) decays without oxygen, bacteria produce methane. This is called biogas. Some Nigerian farms use biogas digesters to produce cooking gas from cow dung.

Wetlands and swamps: Marshy areas produce methane when plants decay underwater. This is sometimes called “swamp gas.”

Coal mines: Methane is trapped in coal deposits. Miners call it “firedamp” because it can cause explosions when it mixes with air and ignites.

Termite mounds: Termites produce methane in their digestive systems. Large termite populations release significant amounts of methane into the atmosphere.

Uses of Methane

Methane has many important uses in Nigeria and worldwide:

• Cooking fuel: The main component of cooking gas (LPG cylinders) used in homes, restaurants, and hotels
• Electricity generation: Power plants burn natural gas (methane) to produce electricity
• Vehicle fuel: Some vehicles in Lagos and Abuja run on Compressed Natural Gas (CNG), which is mainly methane. It is cheaper than petrol
• Industrial heating: Factories use methane for furnaces and heating processes
• Making chemicals: Methane is the starting material for producing methanol, formaldehyde, and other chemicals
• Making hydrogen: Industries convert methane to hydrogen gas for various uses
• Carbon black production: Incomplete burning of methane produces carbon black used in making tires and printer ink

Environmental Impact

Methane is a greenhouse gas—it traps heat in the atmosphere. It is about 25 times more powerful than carbon dioxide at trapping heat. Sources include:

• Leaking gas pipelines and cylinders
• Rice paddies (rice farms)
• Cattle and livestock digestion
• Landfills where waste decomposes

This is why it is important to fix gas leaks immediately and not waste cooking gas.

Common Exam Mistakes

WAEC examiners have noted these common errors about methane:

• Wrong chemical equation: Writing Na₂CO₂ instead of Na₂CO₃ in the preparation equation
• Omitting conditions: Not mentioning “sunlight” or “UV light” for halogenation reactions
• Confusing combustion types: Not knowing the difference between complete combustion (blue flame, CO₂) and incomplete combustion (yellow flame, CO or C)
• Wrong state symbols: Forgetting to show CH₄ as (g) for gas
• Incomplete answers: When asked to “describe” preparation, only writing the equation without mentioning apparatus or procedure
• Wrong molecular shape: Saying methane is linear or planar instead of tetrahedral

Practice Questions

Multiple Choice Questions

1. What is the molecular formula of methane?
a) CH₂
b) CH₃
c) CH₄ ✓
d) C₂H₆

2. Methane is prepared in the laboratory by heating sodium ethanoate with:
a) Dilute acid
b) Soda-lime ✓
c) Water
d) Sodium chloride

3. When methane burns in excess oxygen, the flame color is:
a) Yellow
b) Blue ✓
c) Red
d) Green

4. The shape of a methane molecule is:
a) Linear
b) Planar
c) Tetrahedral ✓
d) Octahedral

5. Which product is NOT formed during complete combustion of methane?
a) Carbon dioxide
b) Water
c) Carbon monoxide ✓
d) Heat energy

Essay Questions

1. (a) Write the molecular and structural formula of methane. (2 marks)
(b) Describe with a labeled diagram how methane can be prepared in the laboratory. (5 marks)
(c) State two physical properties of methane. (2 marks)

Examiner’s tip: For part (a), write both formulas clearly. In part (b), describe means give a detailed account—draw the apparatus, label it, and explain the steps. Don’t just write the equation. For part (c), state means give direct facts without explanation.

2. (a) What is meant by complete combustion? (2 marks)
(b) Write a balanced equation for the complete combustion of methane. (3 marks)
(c) Explain why cooking gas has a smell even though methane is odorless. (2 marks)
(d) State three uses of methane in Nigeria. (3 marks)

Examiner’s tip: In part (a), define and mention the conditions (excess oxygen). For part (b), balance carefully: CH₄ + 2O₂ → CO₂ + 2H₂O. Part (c) requires explanation (give reasons)—mention mercaptan added for safety. Part (d) needs practical Nigerian examples.

3. (a) Distinguish between complete and incomplete combustion of methane. (4 marks)
(b) Write equations for the reaction between methane and chlorine in sunlight to form: (i) chloromethane (ii) dichloromethane. (4 marks)
(c) Name two natural sources of methane. (2 marks)

Examiner’s tip: For part (a), distinguish means show clear differences in a table or point-by-point comparison. Compare oxygen supply, products, and flame color. In part (b), don’t forget to mention “sunlight” as the condition. Both equations should be balanced.

Memory Aids

Preparation reagents: “Soda-lime Heats Sodium Ethanoate” = SHSE

Complete vs Incomplete combustion:
“Blue is True (complete), Yellow is Mellow (incomplete)”

Combustion products:
“Complete = CO₂ + H₂O (both have 2!)”
“Incomplete = CO or C (only 1 oxygen or none!)”

Molecular shape: “TETRA = 4” (tetrahedral = 4 hydrogen atoms around carbon)

Why gas smells: “Mercaptan Makes Methane Smelly” (added for safety)

Related Topics

• Alkane – Learn about the homologous series methane belongs to
• Physical properties of Methane – Detailed study of methane’s physical characteristics
• Chemical properties of methane – More detailed reactions of methane
• Use Of Methane – Comprehensive applications of methane
• Hydrocarbon – Understand the broader family of compounds