Alkanes

This page covers content standard 10.3 of the Form 5 Carbon Compounds chapter: alkanes. Alkanes are the simplest homologous series and the fuels you use every day, natural gas, petrol and diesel are mostly alkanes. Learning their formula, their property trends and their two examinable reactions gives you a clear template for every family that follows.

What are alkanes?

Alkanes are saturated hydrocarbons: they contain only carbon and hydrogen (hydrocarbons) and only single carbon–carbon bonds (saturated). Each carbon atom is bonded to the maximum possible number of hydrogen atoms, which is what “saturated” means.

General formula: CₙH₂ₙ₊₂. Substitute a value of n to get any member. For n = 1, CH₄; for n = 2, C₂H₆; for n = 3, C₃H₈; for n = 4, C₄H₁₀.

The first four members you must know are:

Name	Molecular formula	n
Methane	CH₄	1
Ethane	C₂H₆	2
Propane	C₃H₈	3
Butane	C₄H₁₀	4

Physical properties

Alkanes follow the usual homologous-series trend:

• State. The first members (methane to butane) are gases at room temperature; the middle members are liquids; the higher members are solids.
• Boiling and melting points increase down the series, because larger molecules have stronger van der Waals forces between them, so more energy is needed to separate them.
• Solubility. Alkanes are insoluble in water but dissolve in organic solvents.
• Density is low and increases gradually down the series.

Chemical properties

Because alkanes have only strong single C–C and C–H bonds and no reactive functional group, they are relatively unreactive. There are two reactions the syllabus examines.

1. Combustion

Alkanes are excellent fuels. In a plentiful supply of oxygen, alkanes undergo complete combustion to give carbon dioxide and water and release a large amount of heat:

CH₄ + 2O₂ → CO₂ + 2H₂O

In a limited supply of oxygen, incomplete combustion occurs, producing carbon monoxide (CO), a toxic gas, and/or soot (carbon, C) together with water. This is why fuel-burning appliances must be well ventilated.

2. Substitution with halogens

In the presence of ultraviolet light (sunlight), an alkane reacts with a halogen such as chlorine by substitution: a hydrogen atom is replaced by a halogen atom, and hydrogen chloride is also formed:

CH₄ + Cl₂ → CH₃Cl + HCl

This is a substitution reaction because one atom takes the place of another. Note that ultraviolet light is required, this is a favourite detail in exam questions.

Worked example

Question. Write the balanced equation for the complete combustion of propane (C₃H₈), and name the two products.

Step 1, Write the reactants and products. Complete combustion means propane reacts with oxygen to give carbon dioxide and water:

C₃H₈ + O₂ → CO₂ + H₂O

Step 2, Balance carbon. Propane has 3 carbons, so we need 3 CO₂:

C₃H₈ + O₂ → 3CO₂ + H₂O

Step 3, Balance hydrogen. Propane has 8 hydrogens, so we need 4 H₂O (4 × 2 = 8 H):

C₃H₈ + O₂ → 3CO₂ + 4H₂O

Step 4, Balance oxygen. The right side has (3 × 2) + 4 = 10 oxygen atoms, so we need 5 O₂:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Answer. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. The two products are carbon dioxide and water.

Practice question

Question. Explain why alkanes are described as relatively unreactive, and state the condition needed for an alkane to react with chlorine.

Answer. Alkanes are saturated: every carbon–carbon bond is a single bond and there is no reactive functional group (no double bond). The single C–C and C–H bonds are strong and are not easily broken, so alkanes are relatively unreactive. For an alkane to react with chlorine, ultraviolet light (sunlight) is needed; the reaction is a substitution, in which a hydrogen atom is replaced by a chlorine atom, for example CH₄ + Cl₂ → CH₃Cl + HCl.

Exam tip

Two answers earn marks reliably. For combustion, be precise about the supply of oxygen: plentiful oxygen gives complete combustion (CO₂ + water); limited oxygen gives incomplete combustion (CO and/or soot + water). For the halogen reaction, always name it as substitution and always state the condition, ultraviolet light. A very common trap is to confuse this with the addition reaction of alkenes: alkanes are saturated, so they can only substitute, never add. Balancing combustion equations is easiest if you balance C, then H, then O last.

How this connects

Alkanes are your reference point for the whole chapter. The moment you meet alkenes (standard 10.4), the key contrast is that alkenes are unsaturated and undergo addition, whereas alkanes only substitute. Combustion reappears when you compare the sootier flame of unsaturated compounds. Because combustion and balancing equations are tested throughout SPM Chemistry, the practice you do here carries far beyond this one standard.

Where this fits

This is content standard 10.3 of the Carbon Compounds chapter, the first individual family after the general introduction. Work through more balanced equations in the chapter worked examples, and check you avoid the substitution-versus-addition slip using the common mistakes page. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers rehearse combustion balancing and the substitution condition until both are automatic, because they are among the most frequently examined points in this chapter.

Quick recap

• Alkanes are saturated hydrocarbons, general formula CₙH₂ₙ₊₂.
• First members: methane, ethane, propane, butane (CH₄, C₂H₆, C₃H₈, C₄H₁₀).
• Boiling point rises down the series (stronger forces between larger molecules).
• Complete combustion → CO₂ + H₂O; incomplete combustion → CO/soot + H₂O.
• Substitution with halogens needs ultraviolet light: CH₄ + Cl₂ → CH₃Cl + HCl.
• Relatively unreactive because they are saturated with strong single bonds.