GCSE WJEC Chemistry: Alkanes Revision | GCSE WJEC 化学：烷烃考点精讲

📚 GCSE WJEC Chemistry: Alkanes Revision | GCSE WJEC 化学：烷烃考点精讲

Alkanes form the simplest family of hydrocarbons and are a core topic in the WJEC GCSE Chemistry specification. They appear in questions on fossil fuels, combustion, substitution, and environmental chemistry. This revision guide covers everything you need: structure, naming, reactions, properties, isomerism, and exam-style tips — all presented bilingually so you can learn key terms in both English and Chinese.

烷烃是最简单的碳氢化合物家族，也是WJEC GCSE化学大纲的核心主题。它们常出现在与化石燃料、燃烧、取代反应和环境化学相关的考题中。本复习指南涵盖你需要掌握的所有内容：结构、命名、反应、性质、同分异构现象以及答题技巧——全部以中英双语呈现，帮助你同步掌握中英文关键术语。

1. What Are Alkanes? | 什么是烷烃？

Alkanes are saturated hydrocarbons. The term ‘saturated’ means that they contain only single covalent bonds between carbon atoms, and each carbon atom is bonded to the maximum possible number of hydrogen atoms. Alkanes are the main components of natural gas and crude oil. The simplest alkane is methane, CH₄.

烷烃是饱和烃。“饱和”一词表示它们只含有碳原子之间的单共价键，每个碳原子都与最大可能数量的氢原子键合。烷烃是天然气和原油的主要成分。最简单的烷烃是甲烷，化学式为CH₄。

2. General Formula and Homologous Series | 通式和同系列

The alkanes follow the general molecular formula CₙH₂ₙ₊₂, where n is the number of carbon atoms. They belong to a homologous series — a family of compounds that share the same functional group (for alkanes, the C–C single bond is the only feature), have similar chemical properties, and show a gradual trend in physical properties. Each member differs from the next by a –CH₂– unit.

烷烃的通式为CₙH₂ₙ₊₂，其中n是碳原子的个数。它们属于同系列——即一组具有相同官能团（对于烷烃，仅有的特征就是C–C单键）、化学性质相似、物理性质呈现规律性变化的化合物。相邻成员之间相差一个–CH₂–单元。

3. Structure and Bonding | 结构与键合

In an alkane molecule, each carbon atom forms four single covalent bonds. For methane, one carbon atom bonds to four hydrogen atoms, giving a tetrahedral shape with bond angles of 109.5°. In ethane, C₂H₆, two carbon atoms share one pair of electrons (a C–C single bond), and each carbon bonds to three hydrogen atoms. All bonds in alkanes are sigma bonds (σ), allowing free rotation around the C–C axis.

在烷烃分子中，每个碳原子形成四个单共价键。甲烷中，一个碳原子与四个氢原子结合，形成键角为109.5°的四面体形状。在乙烷C₂H₆中，两个碳原子共用一对电子（C–C单键），每个碳原子上另外连接三个氢原子。烷烃中的所有键都是σ键，可以围绕C–C轴自由旋转。

4. Naming Alkanes | 烷烃的命名

WJEC expects you to name the first ten straight-chain alkanes and recognise basic branched structures. The names follow the IUPAC system: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C), hept- (7C), oct- (8C), non- (9C), dec- (10C). All end with ‘-ane’. For branched alkanes, the longest continuous chain gives the stem name, and alkyl side chains (e.g., methyl –CH₃) are named with position numbers. Example: 2-methylpropane.

WJEC要求你能够命名前十种直链烷烃并识别基本的支链结构。命名遵循IUPAC系统：meth-（1个碳）、eth-（2个碳）、prop-（3个碳）、but-（4个碳）、pent-（5个碳）、hex-（6个碳）、hept-（7个碳）、oct-（8个碳）、non-（9个碳）、dec-（10个碳）。所有名字都以“-ane”结尾。对于支链烷烃，最长的连续碳链给出主干名称，烷基侧链（如甲基–CH₃）用位次编号表示。例如：2-甲基丙烷。

5. Physical Properties | 物理性质

As the number of carbon atoms increases, the boiling point, viscosity, and melting point of alkanes increase, while volatility (ease of evaporation) decreases. This is because larger molecules have stronger London dispersion forces (intermolecular forces) due to greater electron cloud distortion. Short-chain alkanes (methane to butane) are gases at room temperature; pentane to hexadecane are liquids; longer chains are waxy solids. Alkanes are insoluble in water but dissolve in non-polar solvents.

随着碳原子数增加，烷烃的沸点、黏度、熔点升高，而挥发性（蒸发的容易程度）降低。这是因为更大的分子由于电子云形变更大，具有更强的伦敦色散力（分子间作用力）。短链烷烃（甲烷到丁烷）在室温下为气体；戊烷到十六烷是液体；更长的链是蜡状固体。烷烃不溶于水，但可溶于非极性溶剂。

6. Combustion of Alkanes | 烷烃的燃烧

Alkanes readily undergo complete combustion in excess oxygen to produce carbon dioxide and water, releasing a large amount of energy. This makes them excellent fuels. The general equation is:

CₓHᵧ + (x+y/4) O₂ → x CO₂ + (y/2) H₂O

For example, methane: CH₄ + 2O₂ → CO₂ + 2H₂O. In limited oxygen, incomplete combustion occurs, producing carbon monoxide (CO) — a toxic gas — or solid carbon (soot), reducing energy output. In the exam, you must be able to write balanced equations for complete combustion of simple alkanes and explain the dangers of incomplete combustion in faulty gas appliances.

烷烃在过量氧气中容易发生完全燃烧，生成二氧化碳和水，同时释放大量能量。这使得它们成为极佳的燃料。通式如下：

CₓHᵧ + (x+y/4) O₂ → x CO₂ + (y/2) H₂O

例如甲烷：CH₄ + 2O₂ → CO₂ + 2H₂O。在氧气不足时，发生不完全燃烧，产生有毒气体一氧化碳（CO）或固体碳（烟灰），能量输出降低。考试中，你必须能够配平简单烷烃完全燃烧的方程式，并解释燃气设备故障时不完全燃烧的危害。

7. Substitution Reactions with Halogens | 与卤素的取代反应

Alkanes react with halogens (e.g., chlorine or bromine) in the presence of ultraviolet (UV) light through a free-radical substitution mechanism. One hydrogen atom is replaced by a halogen atom, producing a haloalkane and a hydrogen halide. For example, methane reacts with chlorine:

CH₄ + Cl₂ → (UV light) CH₃Cl + HCl

This reaction can continue, substituting further hydrogens to form dichloromethane, trichloromethane, and tetrachloromethane. The reaction is slow in the dark at room temperature but rapid under sunlight. WJEC does not require the full radical mechanism at GCSE, but you should recall that UV light is necessary to break the Cl–Cl bond and start the reaction, and that it results in a mixture of products.

烷烃在紫外光（UV）存在下与卤素（如氯气或溴）发生自由基取代反应。一个氢原子被卤素原子取代，生成卤代烷和卤化氢。例如，甲烷与氯气反应：

CH₄ + Cl₂ → （紫外光） CH₃Cl + HCl

这个反应可以继续进行，进一步取代氢原子，生成二氯甲烷、三氯甲烷和四氯甲烷。在室温黑暗中反应缓慢，但在阳光下反应迅速。GCSE阶段不要求详细自由基机理，但你应记住紫外光是打断Cl–Cl键启动反应所必需的，并且反应产物为混合物。

8. Cracking and the Environmental Impact | 裂解与环境影响

Long-chain alkanes from crude oil have low demand but high boiling points. Cracking breaks them into shorter, more useful alkanes and alkenes. There are two types: thermal cracking (high temperature and pressure) and catalytic cracking (using a zeolite catalyst at lower temperatures). The products include branched alkanes for better fuels and alkenes for making polymers. Combustion of alkanes produces CO₂, a greenhouse gas; incomplete combustion produces CO and particulates. Unburnt hydrocarbons and NOₓ from engines contribute to photochemical smog. In exams, link the properties and reactivity of alkanes to these environmental challenges.

原油中的长链烷烃需求低但沸点高。裂解将它们分解成更短、更有用的烷烃和烯烃。裂解有两种类型：热裂解（高温高压）和催化裂解（在较低温度下使用沸石催化剂）。产物包括用于优质燃料的支链烷烃以及用于制造聚合物的烯烃。烷烃的燃烧产生温室气体CO₂；不完全燃烧产生CO和颗粒物。发动机排放的未燃烧烃类与NOₓ导致光化学烟雾。考试中，要将烷烃的性质和反应性与这些环境挑战联系起来。

9. Isomerism in Alkanes | 烷烃的同分异构现象

Isomers are molecules with the same molecular formula but different structural arrangements. For alkanes, isomerism starts with butane (C₄H₁₀), which has two isomers: butane (straight chain) and 2-methylpropane (branched). Pentane (C₅H₁₂) has three isomers. Branched isomers generally have lower boiling points because they pack less efficiently, reducing intermolecular forces. The WJEC specification expects you to recognise and draw structural isomers up to C₅H₁₂, and explain how branching affects boiling point.

同分异构体是指分子式相同但结构排列不同的分子。烷烃的同分异构现象从丁烷（C₄H₁₀）开始，它有两种异构体：丁烷（直链）和2-甲基丙烷（支链）。戊烷（C₅H₁₂）有三种异构体。支链异构体通常沸点较低，因为分子无法紧密堆积，分子间作用力减小。WJEC大纲要求你识别并画出最多到C₅H₁₂的结构异构体，并解释支链如何影响沸点。

10. Uses of Alkanes | 烷烃的用途

Short-chain alkanes like methane are used as domestic fuels (natural gas). Propane and butane are liquefied and stored as LPG for heating and cooking. Petrol (gasoline) contains alkanes from C₅ to C₁₂, while diesel contains C₁₂ to C₂₀. Bitumen, a residue from crude oil distillation, is used for roads and roofing. Alkanes also serve as raw materials (feedstock) in the petrochemical industry to produce solvents, lubricants, and via cracking to produce alkenes for plastics.

短链烷烃如甲烷被用作家庭燃料（天然气）。丙烷和丁烷被液化储存为LPG，用于取暖和烹饪。汽油（petrol）含有C₅到C₁₂的烷烃，而柴油含有C₁₂到C₂₀。沥青是原油蒸馏后的残余物，用于铺路和屋顶材料。烷烃还作为原料（给料）在石化工业中用于生产溶剂、润滑剂，并通过裂解产生烯烃用于制造塑料。

11. Common Exam Questions and Tips | 常见考题与答题技巧

Typical WJEC questions ask you to: (a) give the general formula of alkanes; (b) write balanced equations for combustion; (c) describe the reaction of methane with chlorine and state the necessary condition (UV light); (d) explain the trend in boiling points or viscosity; (e) draw isomers of a given molecular formula and comment on boiling points; (f) discuss environmental issues related to alkane fuels. Use keywords like ‘saturated’, ‘single bonds’, ‘homologous series’, ‘complete/incomplete combustion’, ‘London dispersion forces’, and ‘free-radical substitution’. Always show state symbols in equations when asked, and remember that cracking produces alkenes which can be tested with bromine water.

WJEC典型考题会要求你：(a) 给出烷烃的通式；(b) 书写燃烧反应的配平方程式；(c) 描述甲烷与氯气的反应并说明必要条件（紫外光）；(d) 解释沸点或黏度的变化趋势；(e) 画出给定分子式的异构体并比较沸点；(f) 讨论与烷烃燃料相关的环境问题。答题时要使用关键词，如“饱和”、“单键”、“同系列”、“完全/不完全燃烧”、“伦敦色散力”、“自由基取代”。当题目要求时，记得在方程式中标出状态符号，并记住裂解产生的烯烃可用溴水进行检验。

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GCSE WJEC Chemistry: Alkanes Revision | GCSE WJEC 化学：烷烃 考点精讲