📚 GCSE CIE Chemistry: Alkanes – Key Points | GCSE CIE 化学：烷烃 考点精讲
Alkanes are a fundamental family of hydrocarbons that form the backbone of organic chemistry in the CIE IGCSE syllabus. Mastering their structure, naming, properties, and reactions is essential for exam success, as questions regularly test combustion, substitution, and patterns in the homologous series. This guide breaks down every key area in syllabus order, giving you clear explanations to build confidence.
烷烃是 CIE IGCSE 课程中有机化学的基础烃类家族。掌握它们的结构、命名、性质和反应对考试成功至关重要，因为燃烧、取代反应和同系物规律都是常考内容。这份考点精讲将按照考纲顺序，逐一解析每个关键领域，帮助你建立扎实的理解。
1. What Are Alkanes? | 什么是烷烃？
Alkanes are saturated hydrocarbons, meaning they contain only carbon and hydrogen atoms joined exclusively by single covalent bonds. The term ‘saturated’ indicates that no more hydrogen atoms can be added to the molecule because all carbon–carbon bonds are single. Alkanes occur naturally in crude oil and natural gas, and they are the simplest type of organic compounds studied at IGCSE level.
烷烃是饱和烃，意味着它们仅由碳和氢原子通过单共价键连接而成。“饱和”一词表示分子中不能再加入更多的氢原子，因为所有的碳–碳键都是单键。烷烃天然存在于原油和天然气中，是 IGCSE 阶段研究的最简单的有机化合物。
Because all carbon atoms in alkanes form four single bonds, the molecules have a tetrahedral shape around each carbon, with bond angles of approximately 109.5°. This regular structure influences their physical and chemical behaviour.
由于烷烃中每个碳原子形成四个单键，分子中围绕每个碳原子呈四面体形状，键角约为 109.5°。这种规整的结构影响着它们的物理和化学性质。
2. General Formula and Homologous Series | 通式与同系物
Alkanes form a homologous series with the general formula CnH2n+2, where n is the number of carbon atoms. A homologous series is a family of organic compounds that share the same functional group (in this case, only C–C and C–H single bonds), similar chemical properties, and a gradual change in physical properties as the chain length increases. Each successive member differs by a –CH2– unit.
烷烃形成一个通式为 CnH2n+2 的同系物，其中 n 代表碳原子数。同系物是指具有相同官能团（此处仅含 C–C 和 C–H 单键）、相似化学性质且随链长增长物理性质呈递变规律的一类有机化合物。每个相邻成员相差一个 –CH2– 单元。
For the first four alkanes, n = 1 gives CH4 (methane), n = 2 gives C2H6 (ethane), n = 3 gives C3H8 (propane), and n = 4 gives C4H10 (butane). The general formula makes it easy to predict the molecular formula of any straight‑chain alkane.
对于前四个烷烃，n=1 得到 CH4（甲烷），n=2 得到 C2H6（乙烷），n=3 得到 C3H8（丙烷），n=4 得到 C4H10（丁烷）。通过通式可以轻松推出任何直链烷烃的分子式。
3. Naming Alkanes | 烷烃的命名
Straight‑chain alkanes are named using a prefix that indicates the number of carbon atoms, followed by the suffix ‘‑ane’. The first ten prefixes are: meth‑ (1), eth‑ (2), prop‑ (3), but‑ (4), pent‑ (5), hex‑ (6), hept‑ (7), oct‑ (8), non‑ (9), dec‑ (10). For example, a five‑carbon alkane is pentane, C5H12.
直链烷烃的命名采用表示碳原子数的前缀，再加上后缀“‑ane”。前十位前缀依次为：meth‑ (1)、eth‑ (2)、prop‑ (3)、but‑ (4)、pent‑ (5)、hex‑ (6)、hept‑ (7)、oct‑ (8)、non‑ (9)、dec‑ (10)。例如，含有五个碳原子的烷烃命名为 pentane（戊烷），分子式为 C5H12。
Branched alkanes are introduced briefly at IGCSE, but the focus is on recognising that an alkyl side chain (e.g. methyl –CH3) is named as a substituent. The longest continuous carbon chain gives the base name, and the position of the branch is indicated by the lowest possible number. In exam questions, you may be asked to draw or name a simple branched alkane like methylpropane.
IGCSE 阶段会简要介绍支链烷烃，但重点在于识别烷基侧链（如甲基 –CH3）作为取代基命名。最长的连续碳链提供主链名称，支链的位置用尽可能小的数字标示。考试中可能会要求你画出或命名如 methylpropane（甲基丙烷）这样的简单支链烷烃。
4. Physical Properties of Alkanes | 烷烃的物理性质
At room temperature, the first four alkanes (methane to butane) are colourless gases, pentane to heptadecane are colourless liquids, and longer chains are waxy solids. All alkanes are non‑polar molecules because the electronegativity difference between carbon and hydrogen is small, and the molecules are symmetrical. As a result, they are insoluble in water but dissolve in non‑polar solvents such as cyclohexane.
在室温下，前四种烷烃（甲烷至丁烷）为无色气体，戊烷至十七烷为无色液体，更长的碳链则为蜡状固体。所有烷烃都是非极性分子，因为碳和氢之间的电负性差很小，且分子结构对称。因此，它们不溶于水，但可溶于环己烷等非极性溶剂。
As the molecular size increases, the strength of the intermolecular forces (London dispersion forces) increases because there are more electrons. This leads to higher boiling points and higher viscosity. The trend in boiling points is a classic exam question: you must be able to explain that larger alkanes have stronger intermolecular forces requiring more energy to overcome.
随着分子尺寸增大，由于电子数目增多，分子间作用力（伦敦色散力）增强，导致沸点升高、黏度增大。沸点的递变规律是经典考题：你必须能够解释，较大的烷烃分子间作用力更强，需要更多能量才能克服。
5. Chemical Properties: Combustion | 化学性质：燃烧
Alkanes readily burn in a plentiful supply of oxygen to produce carbon dioxide and water vapour. This is complete combustion, and it is highly exothermic, which is why alkanes are widely used as fuels. The general equation for complete combustion is:
烷烃在充足的氧气中容易燃烧，生成二氧化碳和水蒸气。这就是完全燃烧，该反应高度放热，因此烷烃被广泛用作燃料。完全燃烧的通式为：
CnH2n+2 + (1.5n+0.5) O2 → n CO2 + (n+1) H2O
For example, the complete combustion of propane: C3H8 + 5 O2 → 3 CO2 + 4 H2O. In a limited supply of oxygen, incomplete combustion occurs, producing carbon monoxide (a toxic gas) or carbon (soot) alongside water. CIE questions often ask you to write balanced equations for complete combustion and to describe the dangers of incomplete combustion.
例如，丙烷的完全燃烧：C3H8 + 5 O2 → 3 CO2 + 4 H2O。在氧气不足时，发生不完全燃烧，除水外，还会生成一氧化碳（有毒气体）或碳（烟灰）。CIE 考题经常要求你书写完全燃烧的配平方程式，并描述不完全燃烧的危害。
6. Chemical Properties: Substitution Reaction with Halogens | 化学性质：与卤素的取代反应
Alkanes are generally unreactive because the C–C and C–H single bonds are strong and non‑polar. However, they do undergo a photochemical substitution reaction with halogens (chlorine or bromine) in the presence of ultraviolet (UV) light. In this reaction, a hydrogen atom is replaced by a halogen atom, producing a haloalkane and a hydrogen halide.
烷烃通常反应活性较低，因为 C–C 和 C–H 单键牢固且非极性。然而，在紫外光 (UV) 存在下，它们能与卤素（氯或溴）发生光化学取代反应。反应中，一个氢原子被卤素原子取代，生成卤代烷和卤化氢。
For methane and chlorine, the overall equation is: CH4 + Cl2 → CH3Cl + HCl. This reaction continues stepwise, so further substitution can occur, giving a mixture of products such as CH2Cl2, CHCl3, and CCl4. The key condition to remember is UV light, which provides the energy to break the Cl–Cl bond and initiate the reaction.
以甲烷和氯为例，总反应方程式为：CH4 + Cl2 → CH3Cl + HCl。该反应会逐步进行，因此可能发生进一步取代，得到 CH2Cl2、CHCl3 和 CCl4 等混合物。需要记住的关键条件是紫外光，它提供能量断裂 Cl–Cl 键并启动反应。
7. Alkanes as Fuels | 烷烃作为燃料
The main use of alkanes is as fuels. Short‑chain alkanes such as methane (natural gas), propane and butane (LPG) are excellent domestic and industrial fuels. Petrol, diesel, and kerosene are mixtures of longer‑chain alkanes obtained from crude oil. Their high exothermic combustion makes them efficient energy sources for heating, cooking, transport, and electricity generation.
烷烃的主要用途是作为燃料。甲烷（天然气）、丙烷和丁烷（液化石油气）等短链烷烃是优良的家用及工业燃料。汽油、柴油和煤油则是从原油中获得的较长的烷烃混合物。它们燃烧时高度放热，使其成为供暖、烹饪、交通和发电的高效能源。
Exam questions may ask why alkanes are good fuels: they release a large amount of energy per gram, they are relatively easy to transport, and they burn cleanly if complete combustion occurs. However, you should also be aware of the environmental impact of burning fossil fuels, such as the release of CO2 (a greenhouse gas) and CO and particulates from incomplete combustion.
考题可能会问烷烃为何是优良燃料：它们每克释放大量能量，相对容易运输，若发生完全燃烧则燃烧清洁。不过，你也应了解燃烧化石燃料对环境的影响，例如释放 CO2（温室气体）以及不完全燃烧产生 CO 和颗粒物。
8. Fractional Distillation of Crude Oil | 原油的分馏
Crude oil is a complex mixture of many different alkanes and other hydrocarbons. It is separated into useful fractions by fractional distillation in a tall column. The crude oil is heated to around 350 °C, vaporising most of the components. The vapour rises up the column, which has a temperature gradient – hot at the bottom, cool at the top.
原油是多种不同烷烃及其他烃类的复杂混合物。通过分馏塔中的分馏过程，可将它分离成有用的馏分。原油被加热至约 350 °C，大部分组分汽化。蒸气沿塔上升，塔内存在温度梯度——底部热、顶部冷。
Larger alkane molecules with high boiling points condense near the bottom and are collected as bitumen, fuel oil, and diesel. Smaller alkanes with low boiling points rise higher and condense as kerosene, petrol (gasoline), and finally refinery gas at the top. Each fraction is a mixture with a specific range of boiling points and a corresponding range of carbon chain lengths.
沸点高的大分子烷烃在塔底附近冷凝，收集得到沥青、燃料油和柴油。沸点低的小分子烷烃升至较高处，依次冷凝为煤油、汽油（汽油），最顶端的炼厂气。每个馏分是具有特定沸点范围和相应碳链长度范围的混合物。
You must be able to link the boiling point of an alkane to its chain length and to describe how fractional distillation works, including the role of the temperature gradient and the uses of the main fractions.
你必须能够将烷烃的沸点与其链长联系起来，并描述分馏的工作原理，包括温度梯度的作用以及主要馏分的用途。
9. Trends in Physical Properties Along the Series | 同系物物理性质的递变规律
Understanding trends is key to answering data‑based questions. As the carbon chain length increases in alkanes:
- Boiling point and melting point increase (stronger intermolecular forces).
- Viscosity increases (more entanglement of longer chains).
- Flammability decreases (larger molecules vaporise less easily).
- Volatility decreases (lower tendency to evaporate).
These trends are a direct consequence of increasing molecular size and the strengthening of London dispersion forces.
理解递变规律是解答数据分析题的关键。随着烷烃碳链长度增加：
- 沸点和熔点升高（分子间作用力增强）。
- 黏度增大（长链分子缠绕增多）。
- 可燃性降低（较大分子不易汽化）。
- 挥发性降低（蒸发倾向减弱）。
这些规律直接源于分子尺寸增大和伦敦色散力增强。
CIE exams often provide a table of data for a homologous series and ask you to predict a value or explain the trend. For instance, you might be given the boiling points of methane, ethane, and propane and asked to estimate that of butane. Practice using the general trend to fill gaps confidently.
CIE 考试常提供同系物数据表格，要求你预测某一数值或解释递变规律。例如，给出甲烷、乙烷和丙烷的沸点，让你估算丁烷的沸点。通过练习运用总体递变规律，能够从容地填补数据空缺。
10. Environmental and Safety Issues | 环境与安全问题
While alkanes are invaluable fuels, their combustion contributes to several environmental problems. Complete combustion releases CO2, the main greenhouse gas causing global warming. Incomplete combustion produces CO, which is poisonous because it binds irreversibly to haemoglobin, reducing the blood’s oxygen‑carrying capacity. Soot (carbon particulates) can worsen respiratory diseases and deposit on buildings.
尽管烷烃是极为重要的燃料，但它们的燃烧会引发多种环境问题。完全燃烧释放 CO2，这是导致全球变暖的主要温室气体。不完全燃烧产生 CO，因其与血红蛋白不可逆结合、降低血液携氧能力而具有毒性。烟灰（碳颗粒）会加重呼吸道疾病并附着在建筑物表面。
In addition, spillage of alkanes can cause water and soil pollution, and the release of unburned hydrocarbons contributes to photochemical smog. For balanced answers, also mention that natural gas (mainly methane) is the cleanest burning fossil fuel, producing less CO2 per unit energy than coal or oil.
此外，烷烃泄漏会造成水体和土壤污染，未燃烧的碳氢化合物释放会加剧光化学烟雾。为给出全面的答案，还应提及天然气（主要成分为甲烷）是最清洁的化石燃料，单位能量产生的 CO2 比煤和石油都少。
11. Cracking – Links to Alkanes | 裂化——与烷烃的关联
Although alkanes themselves are saturated, they can be converted into more useful products by cracking. In thermal or catalytic cracking, long‑chain alkane molecules are broken down into a mixture of shorter alkanes and alkenes. Alkenes are unsaturated hydrocarbons (containing a C=C double bond) that serve as essential starting materials for polymers and chemicals.
尽管烷烃本身是饱和的，它们可通过裂化转化为更有用的产品。在热裂化或催化裂化中，长链烷烃分子被分解成短链烷烃和烯烃的混合物。烯烃是不饱和烃（含有 C=C 双键），是生产聚合物和化学品的重要原料。
Equations for cracking typically show a large alkane producing a smaller alkane and an alkene, e.g. C10H22 → C8H18 + C2H4. This topic connects to the demand for shorter‑chain alkanes as petrol and the need for alkenes in the chemical industry. Expect questions that ask for a balanced equation or the conditions required for catalytic cracking (high temperature, a zeolite catalyst).
裂化方程式通常表示一个大烷烃生成一个小烷烃和一个烯烃，例如：C10H22 → C8H18 + C2H4。这一知识点将短链烷烃作为汽油的需求与化工行业对烯烃的需求联系起来。预计考题会要求书写配平方程式或说明催化裂化的条件（高温、沸石催化剂）。
12. Key Exam Tips and Common Mistakes | 关键考试技巧与常见错误
When writing balanced equations for combustion, always count atoms carefully – a common mistake is forgetting to multiply the water molecules correctly. Remember that the general formula refers to straight‑chain alkanes only; cycloalkanes have a different formula (CnH2n). Do not confuse substitution with addition: alkanes undergo substitution (one atom replaced), not addition, because they have no carbon‑carbon double bond.
书写燃烧配平方程式时，务必仔细数原子——常见错误是未能正确乘以水分子数。记住通式只适用于直链烷烃；环烷烃有不同的通式 (CnH2n)。不要混淆取代与加成：烷烃发生的是取代（一个原子被替换），而非加成，因为它们没有碳碳双键。
In data analysis, use the correct terminology: ‘molecules’ and ‘intermolecular forces’, not ‘atoms’ or ‘bonds breaking’. Boiling involves overcoming forces between molecules, not breaking covalent bonds. Finally, always specify the condition – UV light – when describing the substitution reaction with chlorine or bromine.
在数据分析题中，使用正确的术语：“分子”和“分子间作用力”，而非“原子”或“键断裂”。沸腾是克服分子间的作用力，而不是断裂共价键。最后，描述与氯或溴的取代反应时，务必注明条件——紫外光。
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