Edexcel化学 烷烃 烯烃 裂解 加聚 醇 羧酸
Edexcel 的有机化学全部压在 Paper 2 上。很多学生把分馏、裂解、加聚背成三张互不相干的清单，一到综合题就串不起来。真正贯穿这一章的线索只有一条：碳链的长短怎么变，双键从哪来、又到哪去。抓住这条线，烷烃、烯烃、聚合物、醇和羧酸就是一个连续的过程，而不是五段需要死记的材料。下面按复习顺序拆成十二块，每块都给到能直接写进答题纸的说法。
Organic chemistry on the Pearson Edexcel GCSE Chemistry (1CH0) specification sits entirely on Paper 2, and the questions rarely test one topic in isolation. Candidates who learn fractional distillation, cracking and addition polymerisation as three separate lists tend to freeze when a question asks them to trace a feedstock through to a product. The chain-length story below is the thread that holds the unit together.
有机内容落在 Edexcel 的哪两个 Topic
Pearson Edexcel GCSE Chemistry (1CH0) 把有机内容分在 Topic 8 Fuels and Earth science 与 Topic 9 Separate chemistry 2 两处。前者讲原油、分馏、燃烧与裂解，后者讲聚合物、醇和羧酸。化学单科的考纲覆盖 Topic 9 的全部条目，组合科学的有机题集中在燃料与烃。这个差别决定复习深度：只考组合科学的学生可以把重复单元的画法放轻一些，化学单科的学生要练到不看书就能画对。试卷结构上，Paper 1 覆盖 Topic 1 到 5，Paper 2 覆盖 Topic 6 到 9。两张卷各一小时四十五分钟、各一百份、各占一半，有机题不会出现在 Paper 1。现行大纲自 2016 年 9 月起施教、2018 年夏季首次考试，具体条目编号请以 Pearson 官网的 specification 为准。
The two relevant sections are Topic 8, which covers crude oil, fractional distillation, combustion and cracking, and Topic 9, which adds polymers, alcohols and carboxylic acids. Combined Science candidates meet the organic content mainly through fuels and hydrocarbons, while Triple Science candidates are examined on everything in Topic 9. Both papers run for 1 hour 45 minutes and carry 100 marks, so the organic questions always appear on Paper 2; check the current Pearson specification for the exact numbered statements.
分馏只按沸点分离，不是化学反应
原油是多种烃混在一起的液体，分馏只按沸点把它们分开，过程中没有新物质生成。分馏塔底部温度高、顶部温度低，原油先被加热汽化，再从塔底往塔顶走。链越长，分子间作用力越强，沸点越高，于是在塔底附近就冷凝下来。短链分子沸点低，一直升到塔顶才冷凝。从塔顶到塔底依次是炼厂气、汽油、煤油、柴油、燃料油和沥青。链长增加还带来两个可观察的变化：黏度变大，越不容易点燃。答题时把沸点不同这四个字写清楚就够，不要写成密度不同或质量不同，这两种说法都不给分。
Fractional distillation is a physical separation rather than a reaction, and every mark comes from the boiling-point difference between chains of different length. Longer chains experience stronger intermolecular forces, condense nearer the hot base of the column, and leave as the viscous fractions used for fuel oil and bitumen; short chains travel to the cooler top and come off as refinery gas and petrol. Writing that the fractions differ in density or mass earns nothing at all.
烷烃通式与命名：CnH2n+2 怎么用
烷烃的通式是 CnH2n+2，所有碳碳键都是单键，所以称为饱和烃。甲、乙、丙、丁四个前缀分别对应一、二、三、四个碳，甲烷是 CH4、乙烷是 C2H6、丙烷是 C3H8、丁烷是 C4H10。作答要用 displayed formula 把每一根键画出来，只写分子式通常拿不到结构分。同分异构是常见的失分点，C4H10 除了直链的丁烷，还有一支带支链的结构。链长增加时沸点升高、挥发性下降，这条趋势在解释汽油为什么比柴油更容易点燃时直接能用。很多学生以为烷烃性质都一样，实际上一旦碳链变长，蒸发与点燃的难易差别就出来了。
All alkanes follow CnH2n+2, contain only single carbon-carbon bonds, and are therefore described as saturated. The prefixes meth, eth, prop and but signal one, two, three and four carbons, which lets you name any member of the series once the structure has been drawn out. Branched isomers of the same molecular formula boil at slightly different temperatures, so a fully displayed formula is often worth a mark on its own.
完全燃烧与不完全燃烧的产物差别
氧气充足时，烃完全燃烧生成二氧化碳和水，同时放出大量热，这正是烃被当作燃料的原因。氧气不足时产物换成一氧化碳、碳颗粒和水。一氧化碳无色无味，会和血红蛋白结合，让血液的运氧能力下降，中毒往往在没有察觉的情况下发生。实验现象上，火焰发黄并且带黑烟，基本可以判断为不完全燃烧。家用燃气设备要装在通风处，一氧化碳报警器的作用就在这里。写方程式时注意配平，甲烷完全燃烧的系数依次是一、二、一、二。常见的错误是漏掉水，或者把一氧化碳写成二氧化碳，一字之差整道题的分就没了。
Complete combustion needs a plentiful supply of oxygen and gives carbon dioxide and water, releasing the energy that makes hydrocarbons useful as fuels. When oxygen is limited the products shift to carbon monoxide, soot and water, and carbon monoxide is dangerous because it binds to haemoglobin and reduces the oxygen carried around the body, often without any warning. A yellow, smoky flame is the visible clue, and every symbol equation still has to be balanced with the correct state symbols.
裂解：把长链切短来匹配需求
分馏只能按沸点分配，不能改变各组分之间的比例，而市场需要的小分子燃料和化工原料远比原油里天然含有的多。裂解就是把这个比例调过来：把长链烃加热到高温，或者在高温下让它通过催化剂，使碳链断开。热裂解靠高温和高压，催化裂解用蒸汽配硅铝类催化剂，两种说法在试卷上都出现过。典型产物是一个较短的烷烃加一个烯烃，例如 C10H22 裂成 C8H18 与 C2H4。裂解出来的烯烃不是废料，它们是制造塑料的原料。这一步最容易混的是把产物写成两个烷烃，只有产物里出现双键，那才是烯烃。
Cracking exists because fractional distillation cannot change the ratio of fractions that crude oil supplies, while demand for petrol and for chemical feedstock is far higher than the natural yield. Heating long chains, either thermally at high temperature and pressure or over a silica-alumina catalyst with steam, breaks them into shorter alkanes plus alkenes. The alkene is the valuable half of the product because it feeds polymer manufacture, so an equation that produces two alkanes has almost certainly missed the point.
烯烃与溴水检验：现象和结论要分开写
烯烃的通式是 CnH2n，含有一个碳碳双键，所以称为不饱和烃，乙烯 C2H4 是最简单的成员。双键让烯烃比烷烃活泼得多，加成反应是它们的标志性反应。检验方法很固定：把气体通入橙色的溴水，褪成无色说明存在双键。烷烃在暗处不与溴水反应，颜色保持不变。这里有一个容易忽略的细节，烷烃在强光下会和溴发生取代，同样让颜色变浅，所以作答要写明在暗处或室温下进行。实验题里现象与结论是两栏分，不要把它们合成一句话。
Alkenes share the general formula CnH2n and owe their reactivity to the carbon-carbon double bond, which can open up and add other atoms across it. Bromine water turns from orange to colourless when shaken with an alkene, while the same test leaves an alkane unchanged in the dark. Mentioning the dark matters, because alkanes will substitute with bromine in strong light and fade the colour for a completely different reason.
加聚反应：重复单元的画法决定得分
加聚反应里，含双键的单体一个接一个连起来，双键打开变成长链上的单键，过程中不脱去任何小分子。聚乙烯的重复单元是两个碳、四个氢、两根单键，方括号外面写 n。画重复单元时方括号要穿过碳碳单键，不要保留双键，也不要把 n 写进括号里侧。不同单体的聚合物性质差别来自链的结构，支链少的聚乙烯排列更整齐、更硬更耐热，支链多的则更软更透明。很多学生以为写出 poly(ethene) 这个名字就够，阅卷时结构式才是给分点。
In addition polymerisation the double bond of each monomer opens and joins to the next, so the polymer contains exactly the same atoms as the monomers with nothing eliminated. A repeat unit shows two carbon atoms joined by a single bond inside square brackets with the letter n outside, and dragging that bracket across the bond is what examiners look for. Chain branching explains why one polymer is rigid enough for a bottle while another stays soft and flexible.
聚合物的处置：难降解与可回收是两件事
加聚物化学性质稳定，微生物难以分解，扔进环境里会存在很长时间。填埋占用土地，焚烧可以回收能量，但塑料里若含氯，燃烧会生成氯化氢等有害气体。回收的难点在分类，不同塑料混在一起熔融后性能下降，所以分拣环节决定了再生料的质量。现在的研究方向包括由植物原料制得的聚合物，以及被设计成可被微生物或光照分解的材料。答题时把难降解和可回收分成两条写，不要只写污染环境这四个字。
Most addition polymers resist chemical attack, which is precisely why they persist in landfill for decades instead of breaking down like paper. Burning recovers energy but releases hydrogen chloride and other toxic gases when chlorinated plastics are in the mix, and mixed recycling yields a lower-grade material than properly sorted waste. Plant-based and biodegradable polymers are being developed, although their properties and disposal routes still differ from the poly(ethene) in a carrier bag.
醇的两种制法：发酵与水合
乙醇可以用发酵法得到：葡萄糖在酵母作用下分解成乙醇和二氧化碳，条件是缺氧、温度控制在二十五到三十五摄氏度、体系中有水。一旦接触空气，微生物会继续把乙醇氧化成乙酸，酒变醋就是这个过程。工业上更常用乙烯水合法，乙烯与蒸汽在磷酸催化下、约三百摄氏度、加压反应生成乙醇。两条路线各有取舍，发酵用可再生原料，速率慢，得到的是稀溶液，必须蒸馏提纯。水合反应快、产物纯，但依赖来自化石原料的乙烯，还需要耐压设备。比较题一定要写到原料来源这一层，只写快慢拿不全分。
Ethanol comes from two routes that examiners like to compare. Fermentation uses yeast on a sugar solution at around 25 to 35 degrees Celsius with air excluded, which is slow and gives a dilute product that has to be distilled; catalytic hydration passes ethene and steam over phosphoric acid at roughly 300 degrees Celsius under pressure. The first route draws on renewable feedstock, the second on fossil-derived ethene, and that trade-off is usually worth a mark.
醇的反应：燃烧、与钠、氧化成羧酸
乙醇燃烧生成二氧化碳和水，放出热量，这就是生物乙醇可以作为燃料的原因。乙醇与钠反应放出氢气，比水与钠温和得多，气泡产生得慢，说明乙醇给出氢离子的能力比水弱。氧化反应是另一条常考路线，乙醇先被氧化成乙醛，再进一步氧化成乙酸，实验室里用酸化重铬酸钾，微生物也能完成同一件事。现象上重铬酸钾的橙色会转向绿色。乙醇还是常用溶剂，能溶解许多不溶于水的物质。这一步最容易错的是把氧化产物直接写成二氧化碳，燃烧和氧化是两个不同的过程。
Ethanol burns to carbon dioxide and water, which is why bioethanol is blended into petrol, and it reacts with sodium to release hydrogen far more gently than water does. Oxidation is the reaction most often confused with combustion: acidified potassium dichromate takes ethanol through ethanal to ethanoic acid, turning from orange to green as it goes. Ethanol also acts as a solvent for substances that water will not dissolve.
羧酸：弱酸性带来的三条反应路线
羧酸的官能团是羧基，写作 COOH，乙酸的结构简式是 CH3COOH。乙酸是食醋的主要成分，溶于水后只部分电离，所以它是弱酸，pH 比同浓度的盐酸高，与金属和碳酸盐反应也更慢。乙酸与镁反应生成盐和氢气，与碳酸钠反应生成盐、水和二氧化碳，与氢氧化钠反应生成盐和水。三类反应的现象分别是气泡较慢、气泡更快且能让石灰水变浑浊、放热但没有明显外观变化。酯化反应在 A Level 才展开，GCSE 只要求知道羧酸与醇能生成酯。常见的错误是把羧基写成 COH，或者把乙酸当成强酸来算 pH。
A carboxylic acid carries the COOH group, and ethanoic acid, the acid in vinegar, is written CH3COOH or CH3CO2H. It is a weak acid because it only partially ionises in water, so its pH is higher than that of hydrochloric acid at the same concentration and its reactions with metals and carbonates are visibly slower. Magnesium gives hydrogen, sodium carbonate gives carbon dioxide that turns limewater milky, and sodium hydroxide simply neutralises it.
六周复习安排：按碳链主线推进
复习节奏按碳链这条线走，比按课本章节顺序背要省时间。头两周把分馏、烷烃命名和燃烧方程式练熟，每个方程式都动手配平一遍。第三周专攻裂解与烯烃，把溴水检验的现象与结论各写三遍。第四周练加聚与重复单元，每天画三个不同单体的重复单元。第五周处理醇与羧酸的反应，把八条方程式默写到不看笔记。第六周只做 Paper 2 的有机真题，统计错题类型，不再刷新题。每周固定动作是默写方程式、画结构式、做十道选择题，量不大但不能中断。阅卷时最常见的丢分点是方程式没配平、重复单元保留了双键、把溴水褪色写成变清。把这三条写在错题本第一页，考前只看这一页。
A six-week plan built around chain length beats re-reading the textbook chapter by chapter. Spend the first fortnight on distillation, naming and balanced combustion equations, then take one week each for cracking and alkenes, addition polymerisation, and the alcohol and carboxylic acid reactions, leaving the final week for organic items from past Paper 2 papers and a review of error types rather than new content. Keep the weekly routine fixed: write the equations from memory, draw three repeat units and answer ten multiple-choice questions; unbalanced equations, double bonds left inside repeat units and fading bromine water described as turning clear are the marks most often thrown away.
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