What you'll learn
Alkanes form the foundation of organic chemistry in the CXC CSEC Chemistry syllabus. This topic covers the structure, physical and chemical properties, reactions (combustion and substitution), and practical applications of saturated hydrocarbons. Expect direct questions on naming, balancing combustion equations, and explaining reaction mechanisms in Paper 2 Section A and structured questions in Paper 2 Section B.
Key terms and definitions
Alkanes — saturated hydrocarbons containing only single carbon-carbon bonds with the general formula CₙH₂ₙ₊₂.
Saturated hydrocarbon — an organic compound containing only carbon and hydrogen atoms joined by single covalent bonds, with no double or triple bonds present.
Homologous series — a family of organic compounds with the same general formula, similar chemical properties, and successive members differing by CH₂.
Combustion — a chemical reaction in which a substance reacts rapidly with oxygen, releasing energy in the form of heat and light.
Complete combustion — combustion in excess oxygen producing carbon dioxide and water only.
Incomplete combustion — combustion in limited oxygen producing carbon monoxide and/or carbon (soot) along with water.
Substitution reaction — a reaction where one atom or group of atoms in a molecule is replaced by another atom or group of atoms.
Cracking — the thermal decomposition of long-chain alkanes into shorter-chain alkanes and alkenes at high temperatures, often in the presence of a catalyst.
Core concepts
Structure and bonding in alkanes
Alkanes are aliphatic hydrocarbons where each carbon atom forms four single covalent bonds arranged tetrahedrally with bond angles of approximately 109.5°. The carbon-carbon single bond allows free rotation, giving alkanes flexibility in three-dimensional space.
The first ten members of the alkane homologous series you must know for CXC CSEC Chemistry are:
- Methane (CH₄)
- Ethane (C₂H₆)
- Propane (C₃H₈)
- Butane (C₄H₁₀)
- Pentane (C₅H₁₂)
- Hexane (C₆H₁₄)
- Heptane (C₇H₁₆)
- Octane (C₈H₁₈)
- Nonane (C₉H₂₀)
- Decane (C₁₀H₂₂)
Each successive member adds one carbon atom and two hydrogen atoms (CH₂), maintaining the general formula CₙH₂ₙ₊₂. This pattern is critical for calculations involving molecular formulae in exam questions.
Physical properties of alkanes
State at room temperature: The first four alkanes (methane through butane) are gases at room temperature and pressure. Pentane through approximately C₁₇ are liquids, and alkanes with more than 17 carbon atoms are solids.
Boiling points: Alkanes show a gradual increase in boiling point as molecular mass increases. This occurs because larger molecules have greater surface area for van der Waals forces (weak intermolecular forces) to act between molecules. More energy is required to overcome these forces in larger alkanes.
For example:
- Methane: -162°C
- Ethane: -89°C
- Propane: -42°C
- Butane: -0.5°C
- Pentane: 36°C
- Octane: 126°C
Density: All alkanes are less dense than water (density less than 1 g/cm³), causing them to float on water surfaces. This property is important in understanding oil spills in Caribbean waters, such as incidents affecting Trinidad and Tobago's coastal regions.
Solubility: Alkanes are non-polar molecules because the electronegativity difference between carbon and hydrogen is very small. Water is polar, so alkanes are insoluble in water but dissolve readily in non-polar solvents like tetrachloromethane or other hydrocarbons.
Viscosity: Shorter-chain alkanes are runny liquids with low viscosity, while longer-chain alkanes become increasingly viscous (thick and sticky) due to stronger intermolecular forces.
Chemical properties and reactivity
Alkanes are relatively unreactive compared to other organic compounds because:
- C-C and C-H bonds are strong and require significant energy to break
- The molecules are non-polar, making them resistant to attack by ionic reagents
- They contain no functional groups that can undergo typical organic reactions
The two main reactions of alkanes tested in CXC CSEC Chemistry are combustion and substitution.
Combustion reactions of alkanes
Complete combustion occurs when alkanes burn in excess (plentiful) oxygen:
General equation: CₙH₂ₙ₊₂ + excess O₂ → nCO₂ + (n+1)H₂O + energy
Specific examples:
- CH₄ + 2O₂ → CO₂ + 2H₂O
- C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
- 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
Complete combustion produces a clean blue flame and releases maximum energy. This is the desirable combustion for cooking gas (liquefied petroleum gas containing propane and butane) used throughout the Caribbean.
Incomplete combustion occurs in limited oxygen supply:
Possible products include:
- Carbon monoxide (CO): 2CH₄ + 3O₂ → 2CO + 4H₂O
- Carbon (soot): CH₄ + O₂ → C + 2H₂O
- Mixture of products: 2C₃H₈ + 7O₂ → 2C + 4CO + 2CO₂ + 8H₂O
Incomplete combustion produces a yellow or orange smoky flame and releases less energy than complete combustion. Carbon monoxide is a toxic, colourless, odourless gas that binds irreversibly to haemoglobin, preventing oxygen transport in blood. This hazard is particularly relevant to poorly ventilated cooking areas in Caribbean homes.
Substitution reactions with halogens
Alkanes undergo substitution reactions with chlorine or bromine in the presence of ultraviolet (UV) light. This is a photochemical reaction where UV light provides energy to break halogen molecules into reactive atoms.
Mechanism for chlorination of methane:
Step 1 (Initiation): Cl₂ → 2Cl• (UV light breaks Cl-Cl bond forming free radicals)
Step 2 (Propagation):
- CH₄ + Cl• → •CH₃ + HCl
- •CH₃ + Cl₂ → CH₃Cl + Cl•
Step 3 (Termination): Two radicals combine
- Cl• + Cl• → Cl₂
- •CH₃ + •CH₃ → C₂H₆
- •CH₃ + Cl• → CH₃Cl
The primary product is chloromethane (CH₃Cl), but further substitution can occur:
- CH₃Cl + Cl₂ → CH₂Cl₂ (dichloromethane) + HCl
- CH₂Cl₂ + Cl₂ → CHCl₃ (trichloromethane/chloroform) + HCl
- CHCl₃ + Cl₂ → CCl₄ (tetrachloromethane) + HCl
This reaction produces a mixture of products, which is a disadvantage in industrial synthesis. The UV light requirement explains why containers of chlorine and alkane mixtures are stored in dark bottles.
Industrial uses and economic importance
Fuel sources: Alkanes are the primary components of fossil fuels. Natural gas (mainly methane) and petroleum (mixture of alkanes and other hydrocarbons) power Caribbean economies. Trinidad and Tobago's petroleum industry at Point Lisas and Pointe-à-Pierre processes crude oil into various alkane fractions.
Liquefied Petroleum Gas (LPG): Propane and butane mixtures are used for cooking and heating throughout Jamaica, Barbados, and other Caribbean islands. LPG is stored under pressure in metal cylinders.
Petrol (gasoline): Contains alkanes from C₅ to C₁₀, used as motor vehicle fuel. Octane ratings measure fuel quality and resistance to knocking in engines.
Diesel fuel: Contains alkanes from C₁₂ to C₂₀, used in heavier vehicles and generators common in Caribbean industries.
Kerosene: Contains alkanes around C₁₀ to C₁₆, used as jet fuel and lamp fuel in rural Caribbean communities.
Lubricating oils and waxes: Longer-chain alkanes (C₂₀+) provide lubrication for machinery and are used in candle manufacturing.
Solvents: Hexane and heptane are used as non-polar solvents in laboratories and industrial processes.
Chemical feedstock: Cracking of long-chain alkanes produces shorter alkanes and alkenes (unsaturated hydrocarbons). Alkenes serve as raw materials for plastics, detergents, and other chemicals manufactured at industrial estates like Jamaica's Kingston Harbour area.
Cracking of alkanes
Cracking breaks long-chain alkanes into more useful shorter-chain alkanes and alkenes:
Conditions: High temperature (450-900°C) and sometimes a catalyst (aluminium oxide or silicon dioxide)
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)
This process increases the yield of petrol-range alkanes from crude oil and produces alkenes for the petrochemical industry. Caribbean refineries use cracking to optimize product distribution for regional markets.
Worked examples
Example 1: Write a balanced equation for the complete combustion of pentane (C₅H₁₂). Calculate the volume of oxygen required to completely burn 0.5 moles of pentane at room temperature and pressure (RTP: 1 mole of gas = 24 dm³).
Solution: Step 1: Write the unbalanced equation C₅H₁₂ + O₂ → CO₂ + H₂O
Step 2: Balance the equation C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O (5 carbons give 5CO₂; 12 hydrogens give 6H₂O; count oxygen: 5×2 + 6 = 16 oxygen atoms needed, so 8O₂)
Step 3: Use mole ratio From equation: 1 mole C₅H₁₂ requires 8 moles O₂ Therefore: 0.5 moles C₅H₁₂ requires 0.5 × 8 = 4 moles O₂
Step 4: Calculate volume Volume = moles × 24 dm³ Volume = 4 × 24 = 96 dm³
Answer: 96 dm³ of oxygen required (3 marks: 1 for balanced equation, 1 for mole calculation, 1 for volume)
Example 2: A hydrocarbon burns in excess oxygen to produce 8.8 g of carbon dioxide and 5.4 g of water. Determine the empirical formula of the hydrocarbon. (Relative atomic masses: C = 12, H = 1, O = 16)
Solution: Step 1: Calculate moles of CO₂ and H₂O Moles of CO₂ = 8.8 ÷ 44 = 0.2 mol Moles of H₂O = 5.4 ÷ 18 = 0.3 mol
Step 2: Find moles of C and H atoms Moles of C = 0.2 mol (1 C per CO₂) Moles of H = 0.3 × 2 = 0.6 mol (2 H per H₂O)
Step 3: Find ratio C : H = 0.2 : 0.6 = 1 : 3
Answer: Empirical formula is CH₃ (3 marks: 1 for moles calculation, 1 for atom moles, 1 for ratio)
Note: The molecular formula could be C₂H₆ (ethane) since (CH₃)₂ gives the correct empirical formula.
Example 3: Methane reacts with chlorine in sunlight. Name the type of reaction and write equations for the formation of two possible products.
Solution: Type of reaction: Substitution reaction (or photochemical substitution)
First product: CH₄ + Cl₂ → CH₃Cl + HCl (chloromethane formed)
Second product: CH₃Cl + Cl₂ → CH₂Cl₂ + HCl (dichloromethane formed)
Answer: Substitution; equations as shown (3 marks: 1 for reaction type, 1 mark per equation)
Common mistakes and how to avoid them
• Mistake: Writing combustion products incorrectly, such as C₃H₈ + 5O₂ → 3C + 4H₂O instead of → 3CO₂ + 4H₂O. Correction: Complete combustion ALWAYS produces CO₂ and H₂O only. Carbon (soot) and CO only form in incomplete combustion with limited oxygen. Read the question carefully for "excess oxygen" or "limited oxygen."
• Mistake: Confusing alkanes with alkenes by writing unsaturated structures or assuming alkanes undergo addition reactions. Correction: Alkanes contain only C-C single bonds (saturated) and undergo substitution, not addition. Alkenes have C=C double bonds and undergo addition. Check the general formula: alkanes are CₙH₂ₙ₊₂.
• Mistake: Incorrectly balancing combustion equations, especially miscounting hydrogen atoms in water. Correction: Remember water has TWO hydrogen atoms. For CₙH₂ₙ₊₂, you need (n+1) water molecules, not (2n+2). For C₃H₈, you need 4H₂O because 8÷2 = 4, not 8.
• Mistake: Stating alkanes dissolve in water because "everything dissolves a little." Correction: Alkanes are non-polar and water is polar. Non-polar substances do NOT dissolve in polar solvents—this is a fundamental principle. State clearly: "Alkanes are insoluble in water but soluble in non-polar solvents."
• Mistake: Writing substitution reactions without UV light or stating heat is required. Correction: Alkane substitution with halogens requires UV light specifically, not just any energy source. Always write "UV light" or "sunlight" above the arrow in your equation.
• Mistake: Claiming alkanes are very reactive or undergo many chemical reactions. Correction: Alkanes are relatively unreactive (chemically stable) due to strong C-C and C-H bonds. They undergo only combustion and substitution under specific conditions. This low reactivity is why alkanes are good fuels and storage compounds.
Exam technique for Alkanes: Properties, Reactions and Uses
• Balancing combustion equations: CXC CSEC examiners award 1 mark for correct balancing even if your formula is wrong. Show working by balancing carbon first, then hydrogen, finally oxygen. Double-check that oxygen appears only on the left side as O₂, not as O.
• Command word "Explain": When asked to explain differences in physical properties (boiling point, viscosity), you must state the property trend AND the reason. For example: "Octane has a higher boiling point than butane because octane molecules have greater surface area, resulting in stronger van der Waals forces between molecules that require more energy to overcome." This scores full marks (2-3 marks); stating only the trend scores 1 mark.
• Substitution reaction questions: If asked for the equation, include the halogen (Cl₂ or Br₂), the organic product, AND hydrogen halide (HCl or HBr). Missing HCl loses a mark. Always write "UV light" or "sunlight" as the condition above the arrow.
• Caribbean context questions: Recent papers include scenarios about LPG delivery in Jamaica or natural gas production in Trinidad. Extract the chemistry principle being tested (usually combustion or uses) and apply standard alkane knowledge. Don't be distracted by unfamiliar place names—the chemistry remains the same.
Quick revision summary
Alkanes are saturated hydrocarbons (CₙH₂ₙ₊₂) with only single C-C bonds. Physical properties (boiling point, viscosity) increase with chain length due to stronger van der Waals forces. Alkanes are non-polar, insoluble in water, and chemically unreactive. Complete combustion in excess oxygen produces CO₂ and H₂O; incomplete combustion in limited oxygen produces CO and/or C. Substitution reactions with halogens require UV light, producing haloalkanes and hydrogen halides. Alkanes serve as fuels (LPG, petrol, diesel) and chemical feedstocks after cracking into shorter alkanes and alkenes, supporting Caribbean petrochemical industries.