📚 GCSE Chemistry: Alkanes Exam Focus | GCSE 化学：烷烃 考点精讲
Alkanes are a fundamental topic in GCSE Chemistry, forming the basis for understanding organic chemistry and fuels. This article gathers all the essential concepts, equations, and exam tips you need – from structures and naming to reactions and environmental impact.
烷烃是 GCSE 化学中的一个基础主题，构成了理解有机化学和燃料的起点。本文汇集了你需要掌握的全部核心概念、方程式和考试技巧——从结构、命名到化学反应和环境影响。
1. What are Alkanes? | 什么是烷烃？
Alkanes are saturated hydrocarbons made up of only carbon and hydrogen atoms. ‘Saturated’ means they contain only single covalent bonds between carbon atoms (C–C). The general formula for an alkane with n carbon atoms is CₙH₂ₙ₊₂. The simplest alkane is methane, CH₄, with one carbon atom. Ethane, C₂H₆, is the next member of the series.
烷烃是仅由碳原子和氢原子组成的饱和烃。“饱和”意味着它们只含有碳原子之间的单共价键（C–C）。含有 n 个碳原子的烷烃通式为 CₙH₂ₙ₊₂。最简单的烷烃是甲烷 CH₄，只有一个碳原子。乙烷 C₂H₆ 是该系列的下一个成员。
Because alkanes contain only single bonds, they are chemically relatively unreactive compared to alkenes. Their main reactions are combustion and substitution. All alkanes are non-polar molecules, so they do not mix with water but dissolve in organic solvents.
由于烷烃只含有单键，与烯烃相比，它们的化学性质相对不活泼。它们的主要反应是燃烧与取代。所有烷烃都是非极性分子，因此不与水混合，但能溶于有机溶剂。
2. Naming Straight-Chain Alkanes | 直链烷烃的命名
The first ten straight-chain alkanes must be learned by heart. Each name ends with ‘-ane’, showing the compound belongs to the alkane family. The prefix indicates the number of carbon atoms: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6), hept- (7), oct- (8), non- (9), dec- (10).
必须牢记前十个直链烷烃的名称。每个名称以“-烷”结尾，表明该化合物属于烷烃家族。前缀表示碳原子个数：甲-（1）、乙-（2）、丙-（3）、丁-（4）、戊-（5）、己-（6）、庚-（7）、辛-（8）、壬-（9）、癸-（10）。
| Carbon atoms | Name | Molecular formula |
|---|---|---|
| 1 | Methane | CH₄ |
| 2 | Ethane | C₂H₆ |
| 3 | Propane | C₃H₈ |
| 4 | Butane | C₄H₁₀ |
| 5 | Pentane | C₅H₁₂ |
| 6 | Hexane | C₆H₁₄ |
| 7 | Heptane | C₇H₁₆ |
| 8 | Octane | C₈H₁₈ |
| 9 | Nonane | C₉H₂₀ |
| 10 | Decane | C₁₀H₂₂ |
3. Structure and Bonding | 结构与成键
Each carbon atom in an alkane forms four single covalent bonds. The bonding pairs of electrons repel each other equally, giving a tetrahedral shape around every carbon atom. The H–C–H bond angle is approximately 109.5°. All bonds are sigma bonds, and electron clouds are symmetrical, resulting in non-polar molecules.
烷烃中每个碳原子形成四个单共价键。成键电子对彼此等量排斥，使每个碳原子周围呈四面体形状。H–C–H 键角约为 109.5°。所有键均为 σ 键，电子云对称分布，因此分子是非极性的。
The structural formula can be displayed as a chain of carbon atoms bonded to hydrogen atoms. For example, ethane is CH₃–CH₃. In longer chains, the carbon backbone is drawn in a zig-zag pattern to represent the tetrahedral arrangement in three dimensions.
结构式可表示为与氢原子相连的碳原子链。例如乙烷可以写成 CH₃–CH₃。在较长的碳链中，碳骨架画成锯齿形，以表现其在三维空间中的四面体排列。
4. Physical Properties and Trends | 物理性质及其变化趋势
As the carbon chain length increases, the boiling point of alkanes rises steadily. Longer molecules have a larger surface area, leading to stronger London dispersion forces (intermolecular forces) that require more energy to overcome. This explains why methane, ethane, propane and butane are gases at room temperature, while pentane and heavier alkanes are liquids, and very long chains become waxy solids.
随着碳链长度增加，烷烃的沸点稳定升高。更长的分子具有更大的表面积，导致更强的伦敦色散力（分子间作用力），需要更多能量来克服。这解释了为何甲烷、乙烷、丙烷和丁烷在室温下是气体，而戊烷和更重的烷烃是液体，极长的碳链则成为蜡状固体。
Viscosity (thickness) also increases with chain length because longer molecules tangle more easily. Flammability tends to decrease as molecular size increases – shorter alkanes ignite more readily. All alkanes are insoluble in water (they are non-polar) and are less dense than water, so they float.
粘度（稠度）也随链长增加而增大，因为较长的分子更容易缠结。可燃性通常随分子尺寸增大而降低——短链烷烃更容易点燃。所有烷烃都不溶于水（非极性），且密度小于水，所以会浮在水上。
5. Combustion Reactions | 燃烧反应
Alkanes burn in plenty of oxygen to produce carbon dioxide and water. This is called complete combustion. The reaction is highly exothermic, which is why alkanes are used as fuels. For methane:
烷烃在充足的氧气中燃烧生成二氧化碳和水，这叫作完全燃烧。反应高度放热，因此烷烃被用作燃料。以甲烷为例：
CH₄ + 2O₂ → CO₂ + 2H₂O
When the oxygen supply is limited, incomplete combustion occurs. This produces carbon monoxide (a toxic, colourless, odourless gas) or carbon (soot) along with water. For example, incomplete combustion of methane may produce CO and H₂O:
当氧气供应不足时，会发生不完全燃烧。产物包括一氧化碳（一种有毒、无色无味的气体）或碳（烟灰）以及水。例如甲烷的不完全燃烧可能生成 CO 和 H₂O：
2CH₄ + 3O₂ → 2CO + 4H₂O
Exam tip: you must be able to test the products of combustion. Carbon dioxide turns limewater milky; water vapour turns blue cobalt chloride paper pink.
考试技巧：你必须掌握燃烧产物的检验方法。二氧化碳使石灰水变浑浊；水蒸气使蓝色氯化钴试纸变粉红色。
6. Substitution with Halogens | 与卤素的取代反应
Alkanes undergo a substitution reaction with halogens (chlorine or bromine) in the presence of ultraviolet (UV) light. A hydrogen atom on the alkane is replaced by a halogen atom, producing a haloalkane and a hydrogen halide. The general word equation is:
烷烃在紫外光（UV）存在下与卤素（氯或溴）发生取代反应。烷烃上的一个氢原子被卤素原子取代，生成卤代烷和卤化氢。通用的文字方程式为：
Alkane + Halogen → Haloalkane + Hydrogen halide
For methane and chlorine:
以甲烷和氯气为例：
CH₄ + Cl₂ → CH₃Cl + HCl
This reaction only occurs when UV light provides energy to break the halogen molecule into reactive atoms. Further substitution can replace more hydrogen atoms, forming a mixture of products such as dichloromethane, trichloromethane, and tetrachloromethane.
该反应仅在紫外光提供能量将卤素分子分解为活性原子时发生。进一步取代可替换更多氢原子，生成二氯甲烷、三氯甲烷和四氯甲烷等混合物。
7. Isomers of Alkanes | 烷烃的同分异构体
Isomers are molecules that have the same molecular formula but different structural formulas – the atoms are arranged differently. Straight-chain alkanes with four or more carbon atoms can form branched-chain isomers. Butane (C₄H₁₀) has two isomers: n-butane (a straight chain) and methylpropane (commonly called isobutane), which has a branched structure.
同分异构体是具有相同分子式但结构式不同的分子——原子的排列方式不同。含有四个或更多碳原子的直链烷烃可以形成支链异构体。丁烷（C₄H₁₀）有两种异构体：正丁烷（直链）和甲基丙烷（常称异丁烷），后者具有支链结构。
Pentane (C₅H₁₂) has three isomers: n-pentane, 2-methylbutane, and 2,2-dimethylpropane. Branched isomers usually have lower boiling points than their straight-chain counterparts because the more spherical shape reduces the surface area for intermolecular forces.
戊烷（C₅H₁₂）有三种异构体：正戊烷、2-甲基丁烷和 2,2-二甲基丙烷。支链异构体通常比相应的直链异构体沸点更低，因为更接近球形的形状减小了分子间力作用的表面积。
8. Sources: Crude Oil and Fractional Distillation | 来源：原油与分馏
Alkanes are primarily obtained from crude oil, a finite fossil fuel. Crude oil is a mixture of many hydrocarbons, which is separated into useful fractions by fractional distillation. The process uses a fractionating column that is hot at the bottom and cooler at the top.
烷烃主要来自原油，一种有限的化石燃料。原油是多种碳氢化合物的混合物，通过分馏分离为有用的馏分。该过程使用分馏塔，塔底温度最高，越往上温度越低。
| Fraction | Approx. chain length | Uses |
|---|---|---|
| Refinery gases | C₁ – C₄ | Bottled gas, heating |
| Gasoline (petrol) | C₅ – C₁₀ | Car fuel |
| Kerosene | C₁₀ – C₁₆ | Jet fuel, heating |
| Diesel | C₁₄ – C₂₀ | Fuel for diesel engines |
| Fuel oil | C₂₀ – C₅₀ | Ships, power stations |
| Bitumen | > C₅₀ | Roofing, road surfacing |
Fractions with smaller, lighter molecules have lower boiling points and condense near the top of the column. Heavier fractions with long-chain alkanes condense lower down.
含有较小、较轻分子的馏分沸点较低，在塔的顶部冷凝。长链烷烃较重的馏分则在较低处冷凝。
9. Environmental Impact of Using Alkanes | 使用烷烃的环境影响
Burning alkane fuels releases substances that harm the environment. Complete combustion produces carbon dioxide, a greenhouse gas that contributes to global warming. Incomplete combustion releases carbon monoxide, which is toxic – it binds to haemoglobin in blood and reduces oxygen transport – and soot particles (particulates) that can cause lung diseases.
燃烧烷烃燃料会释放危害环境的物质。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。不完全燃烧释放有毒的一氧化碳——它与血液中的血红蛋白结合，降低氧气输送——以及烟灰颗粒（颗粒物），可能导致肺部疾病。
Many crude oil sources contain sulfur impurities. During combustion, sulfur reacts with oxygen to form sulfur dioxide (SO₂), which dissolves in rainwater to form acid rain. Acid rain damages buildings, kills aquatic life, and harms forests. Catalytic converters in car exhausts reduce CO and NOₓ emissions, but CO₂ remains a long-term challenge.
许多原油中含有硫杂质。燃烧时，硫与氧气反应生成二氧化硫（SO₂），它溶解在雨水中形成酸雨。酸雨损坏建筑物，杀死水生生物，危害森林。汽车排气系统中的催化转化器可减少 CO 和 NOₓ 排放，但 CO₂ 仍是一个长期的挑战。
To reduce these impacts, chemists are developing alternative fuels such as hydrogen (which burns to produce only water) and biofuels from plants. Exam questions may ask you to evaluate the pros and cons of different fuels.
为了减少这些影响，化学家正在开发替代燃料，如氢气（燃烧只产生水）和来自植物的生物燃料。考试题目可能会要求你评估不同燃料的优缺点。
10. Summary and Exam Tips | 总结与考试技巧
Focus on these key points: general formula CₙH₂ₙ₊₂; names and formulas of the first ten alkanes; tetrahedral structure and non-polar nature; trends in boiling point, viscosity and flammability; equations for complete and incomplete combustion; testing for CO₂ and H₂O; substitution reaction conditions (UV light) and example equation; ability to recognise isomers of butane and pentane; fractional distillation of crude oil and uses of fractions; environmental problems caused by burning alkanes.
请重点掌握以下内容：通式 CₙH₂ₙ₊₂；前十种烷烃的名称和分子式；四面体结构和非极性特征；沸点、粘度和可燃性的变化趋势；完全燃烧与不完全燃烧的方程式；CO₂ 和 H₂O 的检验；取代反应的条件（紫外光）及反应实例；识别丁烷和戊烷的同分异构体；原油的分馏及馏分的用途；燃烧烷烃引起的环境问题。
When balancing combustion equations, first balance carbon, then hydrogen, and finally oxygen. Always state that substitution requires UV light. Use correct terminology: saturated, hydrocarbon, substitution, isomers. Practise drawing displayed structural formulas clearly.
配平燃烧方程式时，先配平碳，再配平氢，最后配平氧。始终要说明取代反应需要紫外光。使用正确的术语：饱和、烃、取代、同分异构体。练习清楚地画出显示结构式。
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