Alkanes | IGCSE AQA 化学：烷烃考点精讲

📚 Alkanes | IGCSE AQA 化学：烷烃考点精讲

Alkanes are one of the cornerstone topics in IGCSE AQA Chemistry, forming the basis for understanding organic chemistry. This comprehensive guide covers the general formula, homologous series, nomenclature, physical properties, combustion, substitution reactions, and real-world applications of alkanes. Whether you are preparing for your exams or strengthening your core knowledge, this article breaks down every key concept with clear explanations and bilingual notes to help you succeed.

烷烃是 IGCSE AQA 化学中的基石专题之一，为理解有机化学打下基础。本篇全面讲解烷烃的通式、同系物、命名法、物理性质、燃烧反应、取代反应及其实际应用。无论你是在备考还是巩固核心知识，本文都将通过清晰的双语讲解逐一梳理每一个关键概念，助你取得好成绩。

1. What Are Alkanes? | 什么是烷烃？

Alkanes are saturated hydrocarbons, meaning they contain only carbon and hydrogen atoms and all carbon-carbon bonds are single covalent bonds. The term ‘saturated’ indicates that no more hydrogen atoms can be added to the molecule. Alkanes are the simplest family of organic compounds and are found abundantly in crude oil and natural gas.

烷烃是饱和烃，这意味着它们只含有碳和氢原子，并且所有的碳-碳键都是单共价键。“饱和”一词表示分子中无法再加入更多的氢原子。烷烃是最简单的有机化合物家族，广泛存在于原油和天然气中。

The general formula for a straight-chain or branched-chain alkane is CₙH₂ₙ₊₂, where n represents the number of carbon atoms. For example, when n = 1, the formula is CH₄ (methane); for n = 2, it is C₂H₆ (ethane). This formula applies to non-cyclic alkanes only.

直链或支链烷烃的通式为 CₙH₂ₙ₊₂，其中 n 代表碳原子数。例如，当 n=1 时，分子式为 CH₄（甲烷）；当 n=2 时，为 C₂H₆（乙烷）。此通式仅适用于非环状烷烃。

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

Alkanes form a homologous series: a family of compounds that share the same general formula, have similar chemical properties, and show a gradual change in physical properties as the chain length increases. The defining feature of the alkane homologous series is the general formula CₙH₂ₙ₊₂.

烷烃构成一个同系列：这是一族化合物，它们具有相同的通式、相似的化学性质，并且随着碳链增长，物理性质呈现规律性变化。烷烃同系物的特征性标志就是通式 CₙH₂ₙ₊₂。

Each successive member differs from the previous one by a –CH₂– unit. This means the molecular mass increases by 14 each time you go from CH₄ to C₂H₆ to C₃H₈ and so on. In IGCSE exams, you must be able to identify the next alkane in the series and write its molecular and structural formulae.

每一个后续成员都比前一个多一个 –CH₂– 单元。这意味着从 CH₄ 到 C₂H₆ 再到 C₃H₈ 等，分子量每次增加 14。在 IGCSE 考试中，你必须能识别同系列中的下一个烷烃，并写出其分子式和结构式。

3. Naming Straight-Chain Alkanes | 直链烷烃命名

The names of straight-chain alkanes follow IUPAC rules and use a prefix that indicates the number of carbon atoms, ending with the suffix ‘-ane’. The first ten alkanes (methane to decane) are essential to memorise because they appear frequently in exam questions.

直链烷烃的命名遵循 IUPAC 规则，使用表示碳原子数的前缀，并以“-ane”结尾。前十种烷烃（甲烷到癸烷）必须牢记，因为它们在考试中频繁出现。

Number of C atoms	Prefix	Alkane name	Molecular formula
1	meth-	methane	CH₄
2	eth-	ethane	C₂H₆
3	prop-	propane	C₃H₈
4	but-	butane	C₄H₁₀
5	pent-	pentane	C₅H₁₂
6	hex-	hexane	C₆H₁₄
7	hept-	heptane	C₇H₁₆
8	oct-	octane	C₈H₁₈
9	non-	nonane	C₉H₂₀
10	dec-	decane	C₁₀H₂₂

You should be able to deduce the prefix and formula quickly. For example, an alkane with 7 carbon atoms is heptane, C₇H₁₆. The general formula can be used to check your work: for C₇H₁₆, 2n+2 = 2×7+2 = 16, which matches.

你应该能迅速推断出前缀和分子式。例如，有 7 个碳原子的烷烃是庚烷 C₇H₁₆。可以用通式来检验：对于 C₇H₁₆，2n+2 = 2×7+2 = 16，吻合无误。

4. Branched Alkanes and Structural Isomerism | 支链烷烃和结构异构

When an alkane has four or more carbon atoms, it is possible to arrange the carbon skeleton in different ways while keeping the same molecular formula. These different structures are called structural isomers. The simplest example is butane (C₄H₁₀), which has two isomers: straight-chain butane and branched-chain 2-methylpropane (also known as isobutane).

当烷烃含有四个或更多碳原子时，可以用不同的方式排列碳骨架而保持相同的分子式。这些不同的结构称为结构异构体。最简单的例子是丁烷 (C₄H₁₀)，它有两种异构体：直链丁烷和支链 2-甲基丙烷（也称异丁烷）。

Pentane (C₅H₁₂) has three structural isomers: pentane, 2-methylbutane and 2,2-dimethylpropane. In IGCSE, you are expected to recognise and draw the structural formulae of these isomers, using either full displayed formulae or simplified representations. Remember that branching will slightly alter physical properties such as boiling point because branched isomers are more compact and have weaker intermolecular forces.

戊烷 (C₅H₁₂) 有三种结构异构体：戊烷、2-甲基丁烷和 2,2-二甲基丙烷。在 IGCSE 考试中，要求你识别并画出这些异构体的结构式，可用完整的显示式或简化表示。请记住，支链会略微改变沸点等物理性质，因为支链异构体结构更紧凑，分子间作用力更弱。

5. Physical Properties: Boiling Points and Viscosity | 物理性质：沸点与粘度

The physical properties of alkanes change predictably with increasing molecular size. As the number of carbon atoms increases, the boiling point rises, viscosity increases, and flammability (ease of ignition) decreases. This trend is due to stronger London dispersion forces between larger molecules, which require more energy to overcome.

烷烃的物理性质随着分子尺寸的增大而有规律地变化。随着碳原子数增加，沸点升高、粘度增大，而可燃性（着火容易度）降低。这一趋势归因于较大分子间更强的伦敦色散力，需要更多的能量才能克服。

Short-chain alkanes like methane and ethane are gases at room temperature, while medium-chain alkanes such as pentane and hexane are volatile liquids, and very long-chain alkanes are waxy solids. Volatility decreases as chain length increases; hence crude oil can be separated into fractions based on boiling points in a fractionating column.

短链烷烃如甲烷和乙烷在室温下是气体，中链烷烃如戊烷和己烷是挥发性液体，而很长链的烷烃则是蜡状固体。挥发性随链长增加而降低；因此，原油可以在分馏塔中根据沸点差异被分离成不同馏分。

Viscosity also increases with chain length: long hydrocarbons are thicker and flow less easily. This is important when considering fuels – diesel (longer chains) is more viscous than petrol (shorter chains).

粘度也随链长增加而增大：长链烃更粘稠，流动性更差。这在考虑燃料时很重要——柴油（较长碳链）比汽油（较短碳链）更粘稠。

6. Combustion Reactions | 燃烧反应

Alkanes are excellent fuels because they undergo exothermic combustion reactions, releasing large amounts of energy. Complete combustion occurs when there is a plentiful supply of oxygen. The products are carbon dioxide and water. For example, the equation for the complete combustion of methane is:

烷烃是优良的燃料，因为它们发生放热燃烧反应，释放大量能量。当氧气供应充足时发生完全燃烧，产物是二氧化碳和水。例如，甲烷完全燃烧的方程式为：

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion happens when the oxygen supply is limited. This produces carbon monoxide (a toxic gas) or solid carbon (soot) along with water. The equations for propane illustrate this:

当氧气供应不足时会发生不完全燃烧，生成一氧化碳（有毒气体）或固态碳（炭黑）以及水。以丙烷为例：

2C₃H₈ + 7O₂ → 6CO + 8H₂O (incomplete, CO formed)
C₃H₈ + 2O₂ → 3C + 4H₂O (incomplete, carbon formed)

In exam answers, you must state that incomplete combustion releases less energy than complete combustion and produces harmful products. Carbon monoxide reduces the blood’s ability to carry oxygen, and soot causes respiratory problems and blackens buildings.

在考试作答中，你必须说明不完全燃烧释放的能量比完全燃烧少，并产生有害产物。一氧化碳会降低血液携氧能力，碳粒则导致呼吸问题并使建筑物变黑。

7. Substitution Reactions with Chlorine/Bromine | 与氯/溴的取代反应

Although alkanes are generally unreactive due to the strong C–C and C–H bonds, they do undergo substitution reactions with halogens in the presence of ultraviolet (UV) light. In a substitution reaction, a hydrogen atom in the alkane is replaced by a halogen atom, producing a haloalkane and a hydrogen halide.

虽然烷烃由于牢固的 C–C 和 C–H 键通常不活泼，但在紫外光存在下，它们确实能与卤素发生取代反应。在取代反应中，烷烃中的一个氢原子被卤素原子取代，生成卤代烷和卤化氢。

For instance, methane reacts with chlorine when exposed to UV light:

例如，甲烷在紫外光照射下与氯气反应：

CH₄ + Cl₂ → CH₃Cl + HCl

The reaction does not stop at one substitution; further substitution can produce dichloromethane (CH₂Cl₂), trichloromethane (CHCl₃) and tetrachloromethane (CCl₄). In the exam, you may be asked to write the equation for the reaction between ethane and bromine.

反应不会停在一次取代上；进一步取代可生成二氯甲烷 (CH₂Cl₂)、三氯甲烷 (CHCl₃) 和四氯化碳 (CCl₄)。在考试中，你可能会被要求写出乙烷与溴反应的方程式。

Key point: UV light provides the energy to break the Cl–Cl or Br–Br bond to form reactive halogen atoms (free radicals). You do not need to describe the mechanism in detail at IGCSE, but you should know the condition (UV light) and the fact that it is a substitution reaction, not an addition reaction.

关键点：紫外光提供能量来断裂 Cl–Cl 或 Br–Br 键，生成活泼的卤原子（自由基）。在 IGCSE 阶段不需要详细描述反应机理，但应知道反应条件（紫外光）以及这是取代反应而非加成反应。

8. Alkanes as Fuels and Fractional Distillation | 烷烃作为燃料与分馏

Alkanes are the main constituents of fossil fuels: natural gas is primarily methane, while petrol, kerosene, diesel and fuel oil consist mainly of alkane mixtures with varying chain lengths. Crude oil is separated into useful fractions through fractional distillation based on boiling point differences.

烷烃是化石燃料的主要成分：天然气主要是甲烷，而汽油、煤油、柴油和燃料油则主要由不同链长的烷烃混合物组成。原油通过分馏根据沸点差异被分离成有用馏分。

In the fractionating column, hot crude oil vapours rise and cool. Fractions with smaller molecules and low boiling points (e.g. petrol) condense near the top, while fractions with larger molecules and high boiling points (e.g. bitumen) collect at the bottom. Understanding this process links alkanes to the industrial world and is frequently examined.

在分馏塔中，热原油蒸汽上升并冷却。分子小、沸点低的组分（如汽油）在塔顶附近冷凝，而分子大、沸点高的组分（如沥青）在塔底收集。理解这一过程将烷烃与工业世界联系起来，也是常考内容。

9. Environmental Considerations | 环境考量

Burning alkane fuels has significant environmental impacts. Complete combustion produces carbon dioxide, a greenhouse gas that contributes to global warming. Incomplete combustion emits carbon monoxide and soot, which harm human health and the environment. Sulfur impurities in some fuels can produce sulfur dioxide, leading to acid rain, but this is more a property of the fuel source than of alkanes themselves.

燃烧烷烃燃料对环境有重大影响。完全燃烧产生二氧化碳，这是一种导致全球变暖的温室气体。不完全燃烧排放一氧化碳和碳粒，损害人类健康和环境。某些燃料中的硫杂质会产生二氧化硫，导致酸雨，但这更多是燃料来源的特性而非烷烃本身的性质。

When answering questions on fuels, always consider the balance between energy output and pollution. Modern engines and catalytic converters aim to promote more complete combustion and reduce harmful emissions. You might also connect this to the need for alternative fuels and renewable energy sources.

在回答关于燃料的问题时，始终要权衡能量产出与污染之间的关系。现代发动机和催化转换器旨在促进更完全的燃烧并减少有害排放。你还可以将此与替代燃料和可再生能源的需求联系起来。

10. Key Exam Tips and Summary | 考试技巧与总结

For IGCSE AQA Chemistry, ensure you can:

对于 IGCSE AQA 化学，请确保你能够：

State the general formula of alkanes (CₙH₂ₙ₊₂) and identify a homologous series.
说出烷烃的通式 (CₙH₂ₙ₊₂) 并识别同系列。
Name and draw the first ten straight-chain alkanes and recognise structural isomers for butane and pentane.
命名并绘制前十种直链烷烃，并识别丁烷和戊烷的结构异构体。
Describe and explain trends in boiling point, viscosity and flammability with increasing chain length.
描述并解释 随链长增加沸点、粘度和可燃性的变化趋势。
Write balanced equations for complete and incomplete combustion, and understand the hazards of CO and soot.
书写配平的完全和不完全燃烧方程式，并理解 CO 和碳粒的危害。
Recall the conditions for substitution with halogens (UV light) and write equations for the reaction.
记忆卤素取代的条件（紫外光）并书写反应方程式。
Link alkanes to crude oil and fractional distillation, naming typical fractions and their uses.
将烷烃与原油和分馏联系起来，命名典型馏分及其用途。

Being thorough with these points will give you confidence across multiple exam questions, from multiple-choice to extended response. Practice drawing displayed structures and balancing combustion equations regularly. Alkanes may appear simple, but a deep understanding of their patterns opens the door to the whole organic chemistry section.

扎实掌握这些要点将使你能够自信应对各类考题，从选择题到拓展问答。定期练习绘制显示式和配平燃烧方程式。烷烃看似简单，但对其变化规律的深入理解将为你打开整个有机化学部分的大门。

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Alkanes | IGCSE AQA 化学：烷烃 考点精讲