Alkane

What Are Alkanes?

Alkanes are organic compounds made up of carbon and hydrogen atoms only. They belong to a group called saturated hydrocarbons because all the carbon atoms are bonded to the maximum number of hydrogen atoms possible. There are no double or triple bonds in alkanes—only single bonds.

The word “saturated” means the carbon atoms cannot hold any more hydrogen atoms. Think of a sponge that has absorbed all the water it can hold. That’s how carbon atoms are in alkanes—fully bonded with hydrogen.

Every alkane follows the general formula C_nH_2n+2. If you know the number of carbon atoms (n), you can calculate the number of hydrogen atoms. For example, if an alkane has 5 carbon atoms (n = 5), it will have (2×5) + 2 = 12 hydrogen atoms. The formula would be C₅H₁₂.

Homologous Series of Alkanes

Alkanes form a homologous series—a family of compounds with similar chemical properties. Each member differs from the next by a CH₂ group. Here are the first ten alkanes:

Physical Properties of Alkanes

The physical properties of alkanes change as the molecules get bigger:

State at room temperature: The first four alkanes (methane to butane) are gases. Pentane to C₁₇H₃₆ are liquids. Alkanes with 18 or more carbon atoms are solids.

Boiling point: As the number of carbon atoms increases, the boiling point also increases. This happens because larger molecules have stronger Van der Waals forces between them. More energy is needed to separate the molecules.

Density: Alkanes are less dense than water. They float on water. This is why oil spills float on ocean water.

Solubility: Alkanes do not dissolve in water because they are non-polar, while water is polar. However, alkanes dissolve in non-polar solvents like benzene and tetrachloromethane.

Chemical Properties of Alkanes

Alkanes are generally unreactive compared to other organic compounds. This is because the C-C and C-H bonds are strong and difficult to break. However, alkanes do undergo some important reactions:

1. Combustion (Burning): All alkanes burn in oxygen to produce carbon dioxide and water. This reaction releases a lot of energy, which is why alkanes are used as fuels.

Complete combustion (excess oxygen):
CH₄ + 2O₂ → CO₂ + 2H₂O + heat

Incomplete combustion (limited oxygen) produces carbon monoxide or carbon (soot):
2CH₄ + 3O₂ → 2CO + 4H₂O

2. Substitution reaction with halogens: In the presence of sunlight or UV light, alkanes react with halogens (chlorine, bromine) where hydrogen atoms are replaced by halogen atoms.

CH₄ + Cl₂ → CH₃Cl + HCl (in sunlight)

This can continue with more chlorine to form CH₂Cl₂, CHCl₃, and CCl₄.

3. Cracking: Large alkane molecules can be broken down into smaller, more useful molecules when heated at high temperature with a catalyst. This process is important in petroleum refineries.

C₁₀H₂₂ → C₅H₁₂ + C₅H₁₀ (pentane + pentene)

Sources of Alkanes

Alkanes are found in several natural sources:

Natural gas: Mainly methane (about 95%) with small amounts of ethane, propane, and butane. It is used for cooking and heating in Nigerian homes.

Petroleum (crude oil): A mixture of many alkanes and other hydrocarbons. Through fractional distillation at refineries like the Port Harcourt Refinery, different alkanes are separated for various uses—petrol for vehicles, kerosene for stoves and lamps, diesel for generators and trucks.

Biogas: Produced from decomposing organic matter (animal waste, food waste). It contains methane and is used in some Nigerian rural areas for cooking.

Uses of Alkanes

Alkanes have many practical uses in daily life:

• Fuels: Methane for cooking gas, petrol for cars and motorcycles, diesel for generators and buses, kerosene for stoves
• Solvents: Hexane is used to extract vegetable oil from seeds
• Lubricants: Long-chain alkanes are used as engine oil and grease
• Raw materials: Starting materials for making plastics, detergents, and other chemicals
• Wax: Long-chain solid alkanes are used in candles and polishes

Common Exam Mistakes

WAEC examiners have noted these common errors students make about alkanes:

• Wrong general formula: Students write C_nH_2n (which is for alkenes) instead of C_nH_2n+2
• Confusing saturated and unsaturated: Saying alkanes are unsaturated or can undergo addition reactions
• Incomplete combustion equations: Forgetting that limited oxygen produces CO or C, not just CO₂
• Substitution vs addition: Saying alkanes undergo addition reactions instead of substitution
• Naming errors: Mixing up the prefixes (meth-, eth-, prop-, but-) or spelling them wrongly
• Not stating conditions: Omitting “sunlight” or “UV light” for halogenation reactions

Practice Questions

Multiple Choice Questions

1. Which of the following is the general formula for alkanes?
a) C_nH_2n
b) C_nH_2n+2 ✓
c) C_nH_2n-2
d) C_nH_n

2. What type of bond is found in alkanes?
a) Double covalent bonds only
b) Triple covalent bonds only
c) Single covalent bonds only ✓
d) Ionic bonds

3. The reaction between methane and chlorine in sunlight is an example of:
a) Addition reaction
b) Substitution reaction ✓
c) Elimination reaction
d) Polymerization

4. Which of these alkanes is a gas at room temperature?
a) Hexane
b) Pentane
c) Propane ✓
d) Decane

Essay Questions

1. (a) What are alkanes? (2 marks)
(b) State three physical properties of alkanes. (3 marks)
(c) Write a balanced equation for the complete combustion of propane (C₃H₈). (3 marks)

Examiner’s tip: In part (a), define and mention the general formula. In part (b), state means give direct points—don’t explain unless asked. In part (c), balance the equation carefully: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

2. (a) Explain why alkanes are described as saturated hydrocarbons. (3 marks)
(b) Distinguish between complete and incomplete combustion of alkanes. (4 marks)
(c) An alkane has 7 carbon atoms. What is its molecular formula? (3 marks)

Examiner’s tip: For part (a), explain means give reasons. Mention single bonds and maximum hydrogen atoms. In part (b), distinguish means show differences—compare oxygen supply and products formed. For part (c), use the formula: C₇H_(2×7)+2 = C₇H₁₆.

3. (a) State the first four members of the alkane series with their molecular formulas. (4 marks)
(b) Describe how alkanes are obtained from petroleum. (3 marks)
(c) Give three uses of alkanes in Nigeria. (3 marks)

Examiner’s tip: In part (a), list methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀). For part (b), describe means give a brief account—mention fractional distillation process. Part (c) requires practical Nigerian examples.

Memory Aids

Remember the first 10 alkanes using: “My Enormous Penguin Barely Protects His House Of New Dogs”

• Methane
• Ethane
• Propane
• Butane
• Pentane
• Hexane
• Heptane
• Octane
• Nonane
• Decane

General formula: Think “2n PLUS 2” = C_nH_2n+2

Reactivity: “Saturated = Stable” (single bonds make alkanes less reactive)

Related Topics

• Hydrocarbon – Learn about the broader family of organic compounds
• Methane (CH₄) – Detailed study of the simplest alkane
• Physical properties of Methane – Specific properties of methane
• Chemical properties of methane – Reactions of methane
• Use Of Methane – Applications of methane in daily life