📚 GCSE Chemistry Alkanes, Alkenes, Cracking and Polymerisation: Key Exam Points | GCSE化学 烷烃、烯烃、裂解与聚合：考点突破
Alkanes, alkenes, cracking and polymerisation form one of the most heavily examined blocks in GCSE and IGCSE Chemistry. The ideas link crude oil, fuels, addition reactions and plastics into a single story: long hydrocarbon chains are separated by fractional distillation, short chains are made by cracking, and the alkenes produced are turned into polymers. Once you understand that chain of reasoning, the exam questions become predictable.
烷烃、烯烃、裂解与聚合是 GCSE 与 IGCSE 化学中考查频率最高的模块之一。它把原油、燃料、加成反应和高分子材料串成一条主线：长链烃先经分馏分离，短链烃通过裂解获得，生成的烯烃再聚合成塑料。只要理解这条逻辑链，考题就会变得高度可预测。
1. Topic Map and Exam Weighting | 考点地图与考纲定位
Organic chemistry typically contributes around 15 to 25 per cent of the marks in a GCSE Chemistry paper. The four sub-topics in this article are almost always assessed together, because cracking is the reason alkenes exist in industry and polymerisation is the reason alkenes are valuable. Exam boards such as AQA, Edexcel, OCR and Cambridge IGCSE all test the same core ideas, with only small differences in the depth of structural formulae required.
有机化学在 GCSE 化学试卷中通常占 15% 到 25% 的分值。本文的四个子主题几乎总是联合考查：因为裂解是工业上获得烯烃的原因，而聚合是烯烃具有价值的原因。AQA、Edexcel、OCR 与剑桥 IGCSE 等考试局的考查核心完全一致，差别只在于对结构式书写要求的深度。
- Definitions you must be able to write exactly: hydrocarbon, saturated, unsaturated, alkane, alkene, cracking, polymer, monomer, addition polymerisation.
- 必须能准确默写的定义：烃、饱和、不饱和、烷烃、烯烃、裂解、聚合物、单体、加聚反应。
- Skills you must be able to perform: draw displayed formulae, balance combustion and cracking equations, draw a repeat unit, interpret fraction data.
- 必须掌握的技能：书写结构式、配平燃烧与裂解方程式、画出重复单元、解读馏分数据。
- Explanation questions worth 3 to 6 marks: why cracking is needed, why boiling points increase, why polymers cause waste problems.
- 占 3 至 6 分的解释题：为什么需要裂解、为什么沸点升高、为什么聚合物造成垃圾问题。
2. Crude Oil and Fractional Distillation | 原油与分馏
Crude oil is a finite resource formed from the remains of marine organisms buried and heated over millions of years. It is a mixture of many different hydrocarbons, which are compounds containing hydrogen and carbon only. Because the components have different boiling points, they can be separated by fractional distillation without any chemical reaction taking place.
原油是由海洋生物遗骸经数百万年埋藏与受热形成的不可再生资源，是多种不同烃的混合物；烃是只含碳、氢两种元素的化合物。由于各组分沸点不同，可以通过分馏将它们分离，过程中不发生化学反应。
In the fractionating column, crude oil is vaporised and fed in near the bottom. Vapours rise and cool. A fraction condenses when the temperature falls below its boiling point, so long-chain hydrocarbons, which have high boiling points, condense near the bottom, while short-chain hydrocarbons rise to the top as gases.
在分馏塔中，原油先被汽化并从靠近塔底处送入。蒸气上升并冷却，当温度降到某馏分的沸点以下时，该馏分就液化。因此沸点高的长链烃在塔底附近冷凝，沸点低的短链烃则上升到塔顶以气体形式收集。
| Fraction 馏分 | Approximate chain length 碳链长度 | Main use 主要用途 |
| Refinery gas 炼厂气 | C₁ to C₄ | Bottled gas, cooking fuel 瓶装燃气、燃料 |
| Petrol / gasoline 汽油 | C₅ to C₁₀ | Car fuel 汽车燃料 |
| Kerosene 煤油 | C₁₀ to C₁₆ | Aircraft fuel 航空燃料 |
| Diesel oil 柴油 | C₁₄ to C₂₀ | Lorries, buses 卡车、巴士 |
| Fuel oil 燃料油 | C₂₀ to C₃₀ | Ships, power stations 船舶、发电厂 |
| Bitumen 沥青 | C₃₀ and above | Road surfacing 道路铺设 |
A common exam trap is to say that fractional distillation separates "pure hydrocarbons". It does not. Each fraction is still a mixture of hydrocarbons with similar boiling points.
常见失分点是把分馏说成得到了"纯净的烃"。这是错误的：每个馏分仍是沸点相近的烃的混合物。
3. Alkanes: General Formula and Structure | 烷烃：通式与结构
Alkanes are the simplest homologous series of hydrocarbons. Their general formula is CₙH₂ₙ₊₂, and every carbon atom is joined to four other atoms by single covalent bonds only. Because each carbon is bonded to the maximum possible number of atoms, alkanes are described as saturated compounds.
烷烃是最简单的同系物系列。其通式为 CₙH₂ₙ₊₂，每个碳原子都只通过单共价键与其他四个原子相连。由于每个碳原子已达到最大成键数目，烷烃被称为饱和化合物。
CₙH₂ₙ₊₂ (n = 1, 2, 3, 4 ...)
The first four members are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. Members of a homologous series have the same general formula, differ by one CH₂ unit, show a gradual change in physical properties, and have similar chemical properties. Alkanes are also called saturated hydrocarbons because they contain no carbon-carbon double bond.
前四个成员依次为甲烷 CH₄、乙烷 C₂H₆、丙烷 C₃H₈、丁烷 C₄H₁₀。同系物具有相同的通式、彼此相差一个 CH₂ 单元、物理性质递变、化学性质相似。烷烃因不含碳碳双键而被称为饱和烃。
- Physical state at room temperature: C₁ to C₄ are gases, C₅ to C₁₇ are liquids, C₁₈ and above are solids.
- 常温状态：C₁ 至 C₄ 为气体，C₅ 至 C₁₇ 为液体，C₁₈ 以上为固体。
- Boiling point increases as the chain gets longer, because the intermolecular forces between larger molecules are stronger.
- 碳链越长沸点越高，因为分子间作用力随分子增大而增强。
- Viscosity increases and flammability decreases as the chain gets longer, which is why bitumen is used on roads rather than in engines.
- 碳链越长黏度越大、可燃性越低，这正是沥青用于铺路而不用于发动机的原因。
4. Combustion of Alkanes | 烷烃的燃烧
Alkanes are used as fuels because their combustion is highly exothermic. In an excess supply of oxygen, alkanes burn completely to produce carbon dioxide and water vapour, and the flame is clean and blue. This is complete combustion.
烷烃被用作燃料，因为其燃烧强烈放热。氧气充足时，烷烃完全燃烧生成二氧化碳和水蒸气，火焰呈清洁的蓝色，这就是完全燃烧。
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
When the oxygen supply is limited, incomplete combustion occurs. The products are carbon monoxide and water, or carbon and water. Carbon monoxide is a colourless, odourless and highly toxic gas that binds to haemoglobin and prevents oxygen transport, so it is a serious indoor hazard.
当氧气不足时发生不完全燃烧，产物为一氧化碳和水，或碳和水。一氧化碳是无色无味的高毒气体，会与血红蛋白结合、阻碍氧的运输，因此是严重的室内安全隐患。
2CH₄ + 3O₂ → 2CO + 4H₂O
Other pollutants from burning fossil fuels include sulfur dioxide, which forms acid rain, nitrogen oxides produced when nitrogen and oxygen react at the high temperatures inside an engine, and solid particulates that contribute to smog and respiratory disease.
化石燃料燃烧产生的其他污染物还包括：导致酸雨的二氧化硫；在发动机高温下由氮气和氧气反应生成的氮氧化物；以及造成雾霾和呼吸道疾病的固体颗粒物。
5. Alkenes: General Formula and the C=C Bond | 烯烃：通式与碳碳双键
llows each molecule to add more atoms, alkenes are described as unsaturated. The simplest alkene is ethene, C₂H₄; there is no compound called "methene" because a double bond needs two carbon atoms.
烯烃的通式为 CₙH₂ₙ，其中 n ≥ 2。其本质特征是碳碳双键 C=C。由于双键使分子还能继续结合其他原子，烯烃被称为不饱和化合物。最简单的烯烃是乙烯 C₂H₄；不存在"甲烯"，因为形成双键至少需要两个碳原子。
| Feature 比较项 | Alkanes 烷烃 | Alkenes 烯烃 |
| General formula 通式 | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Bonding 键型 | Single C–C bonds 单键 | One C=C double bond 一个双键 |
| Saturation 饱和度 | Saturated 饱和 | Unsaturated 不饱和 |
| Bromine water 溴水 | Stays orange 保持橙色 | Decolourised 褪色 |
| Typical reaction 典型反应 | Combustion, substitution 燃烧、取代 | Addition, polymerisation 加成、聚合 |
Because the double bond is a region of high electron density, alkenes are more reactive than alkanes and are attacked by electron-seeking species. For GCSE purposes, the practical consequence is simply that alkenes undergo addition reactions in which the double bond opens up and two new single bonds form.
由于双键区域电子密度高，烯烃比烷烃活泼，容易被缺电子试剂进攻。在 GCSE 层面，其实际结果就是：烯烃发生加成反应，双键打开并形成两个新的单键。
6. The Bromine Water Test | 溴水检验
The standard test to distinguish an alkane from an alkene is to add bromine water, which is an orange-brown solution of bromine in water, and shake the tube at room temperature. An alkene decolourises the bromine water; an alkane does not, so the solution remains orange.
区分烷烃与烯烃的标准方法是加入溴水（溴的水溶液，呈橙棕色），在室温下振荡试管。烯烃会使溴水褪色；烷烃不能，溶液仍为橙色。
C₂H₄ + Br₂ → C₂H₄Br₂
The colour change is from orange-brown to colourless, and it works because the bromine molecule adds across the double bond to give a colourless dibromo compound. Note two exam details: the product is 1,2-dibromoethane, and the reaction is an addition, not a substitution. Alkanes can only react with bromine in the presence of ultraviolet light, by a substitution reaction that also releases hydrogen bromide.
颜色由橙棕色变为无色，原因是溴分子加成到双键两侧，生成无色的二溴化合物。两个易考细节：产物是 1,2-二溴乙烷；该反应属于加成而非取代。烷烃只有在紫外光照射下才能与溴发生取代反应，同时放出溴化氢。
- Write "decolourised" or "turns colourless", not "turns clear", to gain the mark.
- 答题应写"褪色/变为无色"，不要写"变清澈"，否则可能失分。
- State that the test is carried out at room temperature and with shaking for a valid comparison.
- 说明该检验在室温下进行并需振荡，才能构成有效对比。
- Do not confuse bromine water with bromine liquid; only the aqueous solution is used in this test.
- 不要把溴水与液态溴混淆，本检验只使用溴的水溶液。
7. Addition Reactions of Alkenes | 烯烃的加成反应
In an addition reaction, two reactant molecules combine to form a single product, and the double bond becomes a single bond. No other product is formed, so atom economy is 100 per cent. Four addition reactions are commonly examined.
加成反应中，两个反应物分子结合生成一种产物，双键变为单键，不生成其他副产物，因此原子经济性为 100%。常考的加成反应有四个。
Hydrogenation 加氢： ethene reacts with hydrogen gas over a nickel catalyst at about 150 °C to give ethane. This is the basis of hardening vegetable oils to make margarine.
加氢：乙烯在约 150 °C、镍催化下与氢气反应生成乙烷，这也是植物油加氢硬化制人造黄油的原理。
C₂H₄ + H₂ → C₂H₆ (Ni catalyst, 150 °C)
Hydration 水合： ethene reacts with steam over a phosphoric acid catalyst at about 300 °C and 60 atmospheres to produce ethanol, an important industrial route to alcohol.
水合：乙烯在约 300 °C、60 个大气压、磷酸催化下与水蒸气反应生成乙醇，这是工业制乙醇的重要路线。
C₂H₄ + H₂O → C₂H₅OH (H₃PO₄ catalyst)
Halogen addition 加卤素： ethene reacts with bromine to give 1,2-dibromoethane. This is the reaction behind the bromine water test described above.
加卤素：乙烯与溴反应生成 1,2-二溴乙烷，这正是溴水检验背后的反应。
Hydrogen halide addition 加卤化氢： ethene reacts with hydrogen chloride to give chloroethane, a compound used to make other chemicals.
加卤化氢：乙烯与氯化氢反应生成氯乙烷，氯乙烷可用于制备其他化学品。
8. Cracking: Why and How | 裂解：原因与方法
Cracking is the thermal or catalytic decomposition of long-chain hydrocarbon molecules into shorter, more useful molecules. Industry needs cracking for two reasons: the demand for short-chain fuels such as petrol is higher than the supply from fractional distillation, and alkenes cannot be obtained directly from crude oil in useful quantities.
裂解是在热或催化剂作用下，把长链烃分子分解成更短、更有用分子的过程。工业上需要裂解有两个原因：汽油等短链燃料的需求量大于分馏所能供给的量；而烯烃无法从原油中直接大量获得。
Cracking always produces at least one alkene, and the products are always a mixture rather than a single pure substance. The two industrial methods are compared below.
裂解必定至少生成一种烯烃，而且产物总是混合物而非单一纯净物。两种工业方法对比如下。
| Method 方法 | Conditions 条件 | Notes 说明 |
| Thermal cracking 热裂解 | High temperature, often 700 to 1000 °C, high pressure | Produces alkenes and hydrogen; uses a lot of energy |
| Catalytic cracking 催化裂解 | About 450 to 500 °C, slight pressure, zeolite catalyst | Faster and cheaper, produces branched and cyclic hydrocarbons for petrol |
When writing cracking equations, remember that every carbon and hydrogen atom must be accounted for, and that the total number of hydrogen atoms must be even because alkenes contain an even number of hydrogens and alkanes contain an even number as well.
书写裂解方程式时要注意：碳、氢原子数必须守恒；由于烷烃与烯烃的氢原子数都是偶数，方程式两边的氢原子总数必然是偶数。
9. Worked Cracking Calculations | 裂解方程式配平实例
A typical exam question gives one long-chain alkane and asks you to complete the products. Work through the carbon and hydrogen totals one element at a time.
典型考题给出一种长链烷烃，要求补全产物。按元素分别核对碳、氢总数即可。
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
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