Alkanes in IGCSE Edexcel Chemistry | IGCSE Edexcel 化学：烷烃考点精讲

📚 Alkanes in IGCSE Edexcel Chemistry | IGCSE Edexcel 化学：烷烃考点精讲

Alkanes are the simplest family of organic compounds and a core topic in IGCSE Edexcel Chemistry. Understanding their structure, naming, physical trends and characteristic reactions – especially combustion and substitution – is essential for exam success. This article breaks down every key learning point you need, with paired English–Chinese explanations to reinforce your revision.

烷烃是最简单的有机化合物家族，也是IGCSE Edexcel化学的核心主题。掌握烷烃的结构、命名、物理性质变化趋势以及特征化学反应——尤其是燃烧反应与取代反应——是考试成功的关键。本文逐一拆解所有关键考点，以中英双语的对照讲解帮助你巩固复习。

1. What Are Alkanes? | 什么是烷烃？

Alkanes are hydrocarbons that contain only carbon and hydrogen atoms, with all carbon–carbon bonds being single bonds. They are obtained from crude oil and natural gas. Because every carbon atom forms four single bonds, alkanes are described as saturated hydrocarbons.

烷烃是仅由碳和氢原子组成的碳氢化合物，其中所有碳–碳键均为单键。烷烃从原油和天然气中获取。由于每个碳原子都形成四个单键，烷烃被称为饱和烃。

The simplest alkane is methane (CH₄), followed by ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀). These names follow a systematic pattern that forms the homologous series.

最简单的烷烃是甲烷（CH₄），其次是乙烷（C₂H₆）、丙烷（C₃H₈）和丁烷（C₄H₁₀）。这些名称遵循系统的规律，构成了同系物系列。

2. Homologous Series and General Formula | 同系物与通式

Alkanes belong to a homologous series – a family of compounds with the same functional group (in alkanes, the C–C and C–H bonding pattern), similar chemical properties, and a gradual change in physical properties. Each member differs from the next by a –CH₂– unit.

烷烃属于同系物——同一系列中的化合物具有相同的官能团（对烷烃而言是C–C和C–H的成键模式）、相似的化学性质以及渐变的物理性质。相邻成员之间相差一个 –CH₂– 单元。

The general formula for alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms. For example, when n = 2 (ethane), the formula is C₂H₆. This formula allows you to predict the molecular formula of any straight‑chain alkane.

烷烃的通式为 CₙH₂ₙ₊₂，其中n为碳原子个数。例如当n = 2（乙烷）时，分子式为C₂H₆。利用该通式可以推导出任何直链烷烃的分子式。

Methane: n=1, CH₄

甲烷：n=1，CH₄
Ethane: n=2, C₂H₆

乙烷：n=2，C₂H₆
Propane: n=3, C₃H₈

丙烷：n=3，C₃H₈
Butane: n=4, C₄H₁₀

丁烷：n=4，C₄H₁₀
Pentane: n=5, C₅H₁₂

戊烷：n=5，C₅H₁₂

3. Naming and Representing Alkanes | 烷烃的命名与表示

IUPAC rules give each alkane a name based on the number of carbon atoms in the longest continuous chain. The prefix indicates the number of carbons (meth‑ = 1, eth‑ = 2, prop‑ = 3, but‑ = 4, pent‑ = 5), and the suffix ‘‑ane’ shows the compound is an alkane.

根据IUPAC命名规则，烷烃的名称由最长连续碳链的碳原子数决定。前缀表示碳原子数（meth‑ = 1，eth‑ = 2，prop‑ = 3，but‑ = 4，pent‑ = 5），后缀“‑ane”表明该化合物属于烷烃。

You must be able to draw and interpret four types of formula for alkanes up to five carbons:

你必须能够画出并识别含五个以内碳原子的烷烃的四种表示法：

Molecular formula: shows the actual number of each type of atom, e.g. C₃H₈

分子式：表示各原子的实际数目，如 C₃H₈
General formula: CₙH₂ₙ₊₂, which applies to all alkanes

通式：CₙH₂ₙ₊₂，适用于所有烷烃
Structural formula: written in a single line showing how groups are arranged, e.g. CH₃–CH₂–CH₃ for propane

结构简式：用一条线写出原子团的排列方式，如丙烷为 CH₃–CH₂–CH₃
Displayed formula: shows every atom and every bond, with lines representing covalent bonds

显示式（完整结构式）：画出所有的原子和化学键，用线段表示共价键

4. Why Alkanes Are Saturated | 烷烃为何是饱和烃

Saturated means that the molecule contains only single covalent bonds and no double or triple bonds. In alkanes, each carbon atom uses its four outer electrons to form four single bonds, either C–C or C–H. There are no spare bonds to accept additional atoms, which makes alkanes relatively unreactive compared to alkenes.

饱和是指分子中只含有单键共价键，没有双键或三键。在烷烃中，每一个碳原子都使用其四个最外层电子形成四个单键，可以是C–C键或C–H键。由于没有多余的价键可以接纳额外的原子，烷烃与烯烃相比相对不活泼。

Being saturated explains why alkanes do not decolourise bromine water in the dark – a key test that distinguishes them from unsaturated compounds like alkenes.

烷烃的饱和特性可以解释为什么它们在黑暗条件下不能使溴水褪色——这是区分烷烃与不饱和化合物（如烯烃）的关键检测方法。

5. Physical Properties: Boiling Point Trend | 物理性质：沸点变化趋势

As the number of carbon atoms increases, the boiling points of alkanes rise. This trend is caused by increasing molecular size and larger intermolecular forces (London dispersion forces) between molecules. More energy is needed to overcome these forces during boiling.

随着碳原子数增加，烷烃的沸点逐渐升高。这一趋势是由于分子体积增大，分子间作用力（伦敦色散力）增强。沸腾时需要更多的能量来克服这些分子间作用力。

Alkane	Molecular Formula	Boiling Point (°C)
Methane	CH₄	-164
Ethane	C₂H₆	-89
Propane	C₃H₈	-42
Butane	C₄H₁₀	-1
Pentane	C₅H₁₂	36

烷烃表格 / Alkane data

Short‑chain alkanes (1–4 carbons) are gases at room temperature; longer chains are liquids or solids. This property is used to separate crude oil into fractions during fractional distillation.

短链烷烃（1–4个碳）在室温下为气体；碳链较长的烷烃则为液体或固体。这一性质被用于原油的分馏，将不同沸点的馏分分离出来。

6. Chemical Reactions: Combustion | 化学反应：燃烧

Alkanes burn readily in a plentiful supply of oxygen to release energy. Complete combustion produces carbon dioxide and water vapour. The general word equation is:

烷烃在氧气充足的条件下容易燃烧并释放能量。完全燃烧生成二氧化碳和水蒸气。通用的文字方程式为：

alkane + oxygen → carbon dioxide + water

烷烃 + 氧气 → 二氧化碳 + 水

Example: CH₄ + 2O₂ → CO₂ + 2H₂O

例如：CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion occurs when the oxygen supply is limited. It produces carbon monoxide (CO, a toxic gas) or carbon (soot) in addition to water. This is dangerous because carbon monoxide binds to haemoglobin and reduces the blood’s oxygen‑carrying capacity.

不完全燃烧发生在氧气供应不足的情况下。此时除了水，还会生成一氧化碳（CO，一种有毒气体）或碳（炭黑）。这是危险的，因为一氧化碳与血红蛋白结合，降低了血液的携氧能力。

alkane + limited O₂ → CO + H₂O or C + H₂O

烷烃 + 有限O₂ → CO + H₂O 或 C + H₂O

In the exam, you may be asked to balance combustion equations. Always balance carbon first, then hydrogen, then oxygen.

考试中可能会要求配平燃烧方程式。记住配平顺序：先平衡碳，再氢，最后氧。

7. Chemical Reactions: Substitution with Halogens | 化学反应：与卤素的取代反应

Alkanes react with halogens (e.g. chlorine or bromine) in the presence of ultraviolet (UV) light. This is a substitution reaction because a hydrogen atom is replaced by a halogen atom. The UV light provides the energy to break covalent bonds and start the reaction.

烷烃在紫外线（UV）照射下与卤素（如氯或溴）反应。这是一种取代反应，因为氢原子被卤素原子取代。紫外线提供打破共价键并启动反应所需的能量。

The substitution of methane with chlorine can be shown stepwise. The overall equation for mono‑substitution is:

甲烷与氯的取代反应可以分步表示。单取代的总反应方程式为：

CH₄ + Cl₂ → CH₃Cl + HCl

The reaction can continue, replacing more hydrogen atoms to produce CH₂Cl₂, CHCl₃ and CCl₄. For IGCSE, you only need to know mono‑substitution, though you should recognise that further substitution is possible.

该反应可以继续进行，取代更多的氢原子，生成CH₂Cl₂、CHCl₃和CCl₄。在IGCSE阶段，你只需要掌握单取代反应，但应了解进一步取代的可能性。

Key observation: in UV light, a mixture of alkane and bromine water turns colourless slowly as substitution occurs. This is different from alkenes, which decolourise bromine water instantly without UV light.

关键现象：在紫外光下，烷烃与溴水的混合物会缓慢褪色，取代反应发生。这不同于烯烃，烯烃无需紫外线就能瞬间使溴水褪色。

8. Environmental and Practical Considerations | 环境与实际应用考量

Alkanes are excellent fuels. Methane is the main component of natural gas; propane and butane are used in portable gas cylinders; larger alkanes are found in petrol, diesel and aviation fuel. Their combustion is highly exothermic, which makes them valuable energy sources.

烷烃是优良的燃料。甲烷是天然气的主要成分；丙烷和丁烷用于便携式气瓶；较大的烷烃存在于汽油、柴油和航空燃料中。烷烃的燃烧是高度放热的，这使它们成为宝贵的能源。

However, burning fossil fuels releases CO₂, a greenhouse gas that contributes to global warming. Incomplete combustion releases CO and particulate carbon, causing health problems and smog. Therefore, modern appliances are designed to ensure complete combustion by providing plenty of air, and catalytic converters reduce CO emissions from vehicles.

然而，燃烧化石燃料会释放CO₂，这是一种导致全球变暖的温室气体。不完全燃烧会释放CO和碳颗粒，引发健康问题和雾霾。因此，现代设备通过提供充足空气来确保完全燃烧，汽车上的催化转化器则减少了CO的排放。

You should also understand that alkanes from crude oil are non‑renewable. Chemists use cracking to break long‑chain alkanes into shorter, more useful alkanes and alkenes, maximising the utility of each barrel of oil.

你还需要明白，原油中的烷烃是不可再生的。化学家通过裂解将长链烷烃断裂为更短、更有用的烷烃和烯烃，以最大限度地利用每一桶石油。

9. Exam Tips for Alkanes | 烷烃考试技巧

Always write formula subscripts correctly: CH₄ not CH4, C₂H₆ not C2H6. Use the general formula CₙH₂ₙ₊₂ to check your molecular formulae. When describing physical trends, always link to intermolecular forces – simply saying ‘bigger molecules’ is not enough.

务必正确书写下标的分子式：CH₄而不是CH4，C₂H₆而不是C2H6。利用通式CₙH₂ₙ₊₂检查你的分子式。在描述物理变化趋势时，一定要与分子间作用力联系起来——仅仅说“分子更大”是不够的。

For substitution reactions, state that UV light is necessary. Write balanced chemical equations showing HCl as a second product. If asked to compare combustion types, give a clear difference: complete combustion produces CO₂, incomplete produces CO or C.

对于取代反应，要说明紫外线是必要条件。写出平衡的化学方程式，表明HCl是第二个产物。如果要求比较燃烧类型，要给出明确的区别：完全燃烧生成CO₂，不完全燃烧生成CO或C。

Finally, memorise the systematic names for the first five alkanes and their structural/displayed formulae. They are frequently tested in multiple‑choice and structured questions.

最后，记住前五个烷烃的系统命名及其结构简式和显示式。它们在选择题和结构题中经常出现。

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Alkanes in IGCSE Edexcel Chemistry | IGCSE Edexcel 化学：烷烃考点精讲