📚 Alkanes for GCSE Edexcel Chemistry | GCSE Edexcel 化学：烷烃 考点精讲
Alkanes are the simplest family of organic molecules and form the foundation of the GCSE Edexcel Chemistry organic chemistry topic. Understanding their structure, properties, and reactions is essential for exam success. This article covers all key points, from general formula to cracking, with clear explanations and examples.
烷烃是最简单的有机分子家族，也是 GCSE Edexcel 化学有机化学主题的基础。理解它们的结构、性质和反应对于考试成功至关重要。本文涵盖了从通式到裂解的所有考点，提供清晰的解释和示例。
1. What are Alkanes? | 什么是烷烃？
Alkanes are a family of hydrocarbons that contain only single carbon-carbon bonds. They are described as saturated hydrocarbons because each carbon atom forms four single covalent bonds. This means they contain the maximum possible number of hydrogen atoms per carbon atom.
烷烃是一类只含有碳-碳单键的碳氢化合物。它们被称为饱和烃，因为每个碳原子都形成四个单共价键。这意味着它们含有每个碳原子所能结合的最大氢原子数。
2. General Formula & Homologous Series | 通式与同系物
The alkanes form a homologous series. The general formula of an alkane is CₙH₂ₙ₊₂. As you go up the series, each successive alkane differs by a CH₂ unit from the previous one. All members have similar chemical properties and show a gradual trend in physical properties.
烷烃形成一个同系物。烷烃的通式为 CₙH₂ₙ₊₂。在这个系列中，每相邻两个烷烃相差一个 CH₂ 单元。所有成员具有相似的化学性质，并在物理性质上表现出递变规律。
3. Naming the First Four Alkanes | 前四种烷烃的命名
The names of the first four straight-chain alkanes are Methane, Ethane, Propane, and Butane. You must learn these names and their prefixes because they form the basis for naming other organic compounds.
前四种直链烷烃的名称是甲烷、乙烷、丙烷和丁烷。你必须记住这些名称及其前缀，因为它们构成了其他有机化合物命名的基础。
- Methane (CH₄) – 1 carbon / 甲烷 (CH₄)
- Ethane (C₂H₆) – 2 carbons / 乙烷 (C₂H₆)
- Propane (C₃H₈) – 3 carbons / 丙烷 (C₃H₈)
- Butane (C₄H₁₀) – 4 carbons / 丁烷 (C₄H₁₀)
4. Drawing Structural Formulas | 绘制结构式
You need to be able to draw displayed (full structural) formulas for the first four alkanes. Each carbon atom is shown, along with all hydrogen atoms and single bonds. For example, butane can be drawn as a straight chain of four carbon atoms, each bonded to the required number of hydrogen atoms.
你需要能够画出前四种烷烃的显示式（完整结构式）。要画出每个碳原子、所有的氢原子和单键。例如，丁烷可以画成四个碳原子的直链，每个碳原子与所需数量的氢原子成键。
When drawing structural formulas, always check that every carbon atom has exactly four bonds.
绘制结构式时，务必检查每个碳原子恰好形成四个键。
5. Structural Isomerism | 结构异构现象
Isomers are molecules with the same molecular formula but different structural formulas. Butane (C₄H₁₀) has two structural isomers: butane (straight-chain) and methylpropane (branched). Methylpropane is sometimes called isobutane. In the exam, you may be asked to draw the branched isomer of butane and explain that it has a lower boiling point due to weaker intermolecular forces caused by less surface contact.
异构体是指分子式相同但结构式不同的分子。丁烷 (C₄H₁₀) 有两种结构异构体：丁烷（直链）和甲基丙烷（支链）。甲基丙烷有时被称为异丁烷。考试中可能会要求你画出丁烷的支链异构体，并解释其沸点较低是因为分子间接触面减小，导致分子间力减弱。
6. Physical Properties of Alkanes | 烷烃的物理性质
As the number of carbon atoms increases, the boiling points, viscosity, and melting points of alkanes increase. This is because longer chains have stronger intermolecular forces (London dispersion forces). Short-chain alkanes are more volatile and make excellent fuels, while long-chain alkanes are thick and less flammable.
随着碳原子数增加，烷烃的沸点、粘度和熔点升高。这是因为更长的链具有更强的分子间力（伦敦分散力）。短链烷烃更易挥发，是优质燃料；长链烷烃则较粘稠，不易燃烧。
| Number of Carbons / 碳数 | State at Room Temperature / 室温状态 | Example / 示例 |
|---|---|---|
| 1 – 4 | Gas / 气体 | Methane, Butane / 甲烷、丁烷 |
| 5 – 17 | Liquid / 液体 | Petrol, Kerosene / 汽油、煤油 |
| 18+ | Solid / 固体 | Paraffin wax / 石蜡 |
7. Complete Combustion | 完全燃烧
Alkanes burn readily in a plentiful supply of oxygen to produce carbon dioxide and water vapour. This is complete combustion. The reaction is highly exothermic, which is why alkanes are so useful as fuels.
烷烃在充足氧气中容易燃烧，生成二氧化碳和水蒸气，这就是完全燃烧。反应放热剧烈，这就是烷烃作为燃料如此有用的原因。
CH₄ + 2O₂ → CO₂ + 2H₂O
Always balance your combustion equations carefully and state that a blue flame is observed when combustion is complete.
务必仔细配平燃烧方程式，并说明完全燃烧时观察到的是蓝色火焰。
8. Incomplete Combustion | 不完全燃烧
If the oxygen supply is limited, alkanes undergo incomplete combustion. This produces carbon monoxide (a toxic, colourless, odourless gas) and/or carbon (soot) along with water. Carbon monoxide reduces the blood’s ability to carry oxygen, and soot can block burners and pollute the air.
如果氧气供应有限，烷烃会发生不完全燃烧，生成一氧化碳（一种有毒、无色、无味的气体）和/或碳（炭黑）以及水。一氧化碳会降低血液的携氧能力，炭黑会堵塞燃烧器并污染空气。
2CH₄ + 3O₂ → 2CO + 4H₂O
CH₄ + O₂ → C + 2H₂O
Examiners often ask about the dangers of incomplete combustion and how to ensure complete combustion (provide adequate ventilation).
考官经常询问不完全燃烧的危害以及如何确保完全燃烧（提供足够通风）。
9. Substitution Reaction with Halogens | 与卤素的取代反应
Alkanes are generally unreactive because the C–C and C–H bonds are strong and non-polar. However, they do react with halogens such as 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.
烷烃通常不活泼，因为 C–C 和 C–H 键强且非极性。然而，在紫外光 (UV) 存在下，它们会与氯或溴等卤素发生反应。这是一种取代反应，因为一个氢原子被卤素原子取代。
CH₄ + Cl₂ → CH₃Cl + HCl
The reaction can continue, producing a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane. You must be able to describe the reaction conditions: UV light (or high temperature) and the need to write a word equation and balanced symbol equation.
该反应可以继续进行，生成一氯甲烷、二氯甲烷、三氯甲烷和四氯甲烷的混合物。你必须能够描述反应条件：紫外光（或高温），并写出文字方程式和配平的符号方程式。
10. Alkanes from Crude Oil | 来自原油的烷烃
Crude oil is a mixture of many different hydrocarbons, mostly alkanes. Fractional distillation separates crude oil into fractions containing molecules of similar chain lengths. Each fraction contains alkanes with similar boiling points. Short-chain alkanes are collected near the top of the column, while long-chain alkanes condense near the bottom.
原油是多种不同碳氢化合物的混合物，主要是烷烃。分馏将原油分离成包含相似链长分子的馏分。每个馏分包含沸点相近的烷烃。短链烷烃在分馏塔顶部收集，长链烷烃在底部冷凝。
The demand for short-chain alkanes and alkenes is much higher than for long-chain residues. This leads to the process of cracking.
对短链烷烃和烯烃的需求远高于长链残渣，这就引出了裂解过程。
11. Cracking | 裂解
Cracking is a thermal decomposition reaction used to break large, less useful alkane molecules into smaller, more useful alkanes and alkenes. This is done by heating with a catalyst (catalytic cracking) or by mixing with steam at high temperatures (steam cracking).
裂解是一种热分解反应，用于将大的、不太有用的烷烃分子分解成更小、更有用的烷烃和烯烃。这通过加热催化剂（催化裂化）或在高温下与水蒸汽混合（蒸汽裂化）来完成。
C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆
The smaller alkane molecules are more flammable and suitable for petrol, while the alkenes are used to make polymers and other chemicals. In the exam, you need to identify alkenes produced by cracking using bromine water, which turns from orange to colourless in the presence of an alkene.
较小的烷烃分子更易燃，适合用作汽油，而烯烃则用于制造聚合物和其他化学品。在考试中，你需要通过溴水来鉴别裂解产生的烯烃，溴水遇到烯烃会从橙色变为无色。
12. Summary & Exam Tips | 总结与考试技巧
To succeed with alkane questions: memorise the general formula CₙH₂ₙ₊₂, the names and structures of the first four alkanes, and be able to draw the branched isomer of butane. Practise balancing combustion and substitution equations. Always mention the need for UV light in substitution reactions and link incomplete combustion to carbon monoxide poisoning.
要应对好烷烃考题：记住通式 CₙH₂ₙ₊₂，前四种烷烃的名称和结构，并会画丁烷的支链异构体。练习配平燃烧和取代反应的方程式。取代反应一定要提到需要紫外光，并将不完全燃烧与一氧化碳中毒联系起来。
Remember that alkanes are saturated and therefore only undergo substitution or combustion – not addition reactions. Use the correct terminology: homogeneous series, volatility, viscosity, isomer, and so on.
记住烷烃是饱和的，因此只能发生取代或燃烧反应，不能发生加成反应。使用正确的术语：同系物、挥发性、粘度、异构体等。
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