11.4 Alkanes - Revision Notes
1. Overview
Alkanes are the simplest family of hydrocarbons and form a homologous series with the general formula CₙH₂ₙ₊₂. They are the primary components of fossil fuels like natural gas and petroleum, making them essential for global energy production.
Key Definitions
- Hydrocarbon: A compound containing only carbon and hydrogen atoms.
- Saturated Hydrocarbon: A hydrocarbon in which all carbon-carbon bonds are single covalent bonds.
- Alkanes: A homologous series of saturated hydrocarbons.
- Homologous Series: A family of similar compounds with the same functional group and general formula, showing a gradation in physical properties.
- Substitution Reaction: A reaction in which one atom or group of atoms is replaced by another atom or group of atoms.
Core Content
Bonding and Structure
- Alkanes consist of carbon and hydrogen atoms held together by single covalent bonds.
- They are described as saturated because they contain no carbon-carbon double bonds (C=C). Every carbon atom is bonded to the maximum possible number of hydrogen atoms.
- Displayed formula of Methane (CH₄) showing four single lines from C to H, and Ethane (C₂H₆) showing a single line between two C atoms and three H atoms on each C.
Chemical Properties
Alkanes are generally unreactive compounds. This is because the C-C and C-H bonds are very strong and difficult to break. However, they undergo two key types of reactions:
A. Combustion Alkanes react with oxygen to release energy (exothermic).
- Complete Combustion: Occurs in excess oxygen, producing carbon dioxide and water.
- Word Equation: methane + oxygen → carbon dioxide + water
- Symbol Equation: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
- Incomplete Combustion: Occurs when oxygen is limited, producing carbon monoxide and/or soot (carbon) and water.
- Word Equation: ethane + oxygen (limited) → carbon monoxide + water
- Symbol Equation: 2C₂H₆(g) + 5O₂(g) → 4CO(g) + 6H₂O(l)
B. Substitution (General) Alkanes can react with halogens (like chlorine) under specific conditions to swap a hydrogen atom for a halogen atom.
Extended Content (Extended Curriculum Only)
Photochemical Substitution
The reaction between alkanes and halogens (e.g., chlorine) is a photochemical reaction.
- Requirement: Ultraviolet (UV) light.
- Role of UV: The UV light provides the activation energy ($E_a$) required to break the Cl-Cl bond to start the reaction.
Monosubstitution of Methane
In a monosubstitution reaction, only one hydrogen atom is replaced.
- Word Equation: methane + chlorine → chloromethane + hydrogen chloride
- Symbol Equation: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g)
Structural Formulas of Products:
Chloromethane (CH₃Cl): H | H—C—Cl | H
Hydrogen Chloride (HCl): H—Cl
Key Equations
| Reaction Type | General/Specific Equation | Symbols/Units |
|---|---|---|
| General Formula | CₙH₂ₙ₊₂ | n = number of carbon atoms |
| Complete Combustion | C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l) | Propane combustion |
| Substitution | CH₄(g) + Cl₂(g) $\xrightarrow{UV}$ CH₃Cl(g) + HCl(g) | Requires UV light |
Common Mistakes to Avoid
- ❌ Wrong: Defining a hydrocarbon as a molecule made of carbon and hydrogen.
- ✓ Right: A molecule made of only carbon and hydrogen (the word "only" is essential for marks).
- ❌ Wrong: Forgetting to include the second product (HCl) in substitution reactions.
- ✓ Right: Always show the inorganic byproduct (e.g., HCl or HBr) in substitution equations.
- ❌ Wrong: Writing the formula for ethane as C₂H₄.
- ✓ Right: Alkanes follow CₙH₂ₙ₊₂, so ethane is C₂H₆ (C₂H₄ is an alkene).
Exam Tips
- Command Word "State": If a question asks you to "state the bonding in alkanes," simply write "single covalent bonds."
- Command Word "Name": If asked to name the condition for substitution, "UV light" or "ultraviolet radiation" is the required answer.
- Contextual Questions: Be prepared to discuss the environmental impact of combustion (CO₂ as a greenhouse gas or CO as a toxic gas).
- High Frequency: Substitution reactions and the definition of "saturated" appear very frequently. Memorize the "one atom replaced by another" definition for substitution exactly.
- Numerical Check: If asked to identify an unknown alkane from its mass or formula, always check if it fits the $C_nH_{2n+2}$ pattern. (e.g., If n=5, H must be 12).