Edexcel化学 GCSE 分馏 烷烃 烯烃 醇 聚合物
有机化学在 Edexcel GCSE 化学（1CH0）里不是单独一块，而是分落在 Topic 8 Fuels and Earth science 与 Topic 9 Separate chemistry 2 两处，命题一律出现在 Paper 2（1CH0/2F 与 1CH0/2H）。这张卷时长 1 小时 45 分钟、满分 100 分、占总成绩的 50%，Pearson 官方 spec 从 2016 年 9 月起施教，2018 年 5 月与 6 月首次考试。有机部分的题型相当固定：写通式、写分馏顺序、写裂解与聚合的方程式、写醇与羧酸的反应条件，几乎没有需要临场推理的长论述题。下面十二节按这个顺序展开，每一节都给出一句能直接写在答题线上的说法，末尾给出一份四周复习安排。
The organic content of Edexcel GCSE Chemistry (1CH0) is not a single unit, since it is split between Topic 8 Fuels and Earth science and Topic 9 Separate chemistry 2, and every question on it appears in Paper 2, which runs for 1 hour 45 minutes, carries 100 marks and counts for 50% of the qualification; the Pearson specification was first taught from September 2016, with first assessment in May and June 2018. Questions keep to a narrow set of shapes, such as writing a general formula, ordering the fractions, balancing a cracking or polymerisation equation, or naming a reaction condition. The twelve sections below follow that order and close with a four-week revision plan.
烃、同系物与三个通式
烃是只含碳和氢两种元素的化合物，原油里的烃绝大多数是烷烃，通式 CnH2n+2 里 n 等于 1 是甲烷 CH4、n 等于 2 是乙烷 C2H6。烯烃的通式是 CnH2n，与同碳数的烷烃相比少两个氢，少的这两个氢正是构成碳碳双键的那一对。醇的通式是 CnH2n+1OH，官能团是 -OH，甲醇、乙醇、丙醇、丁醇依次把碳链加长一个碳。同系物指通式相同、官能团相同、化学性质相近，而分子式相差一个 CH2 的一组化合物。判断同系物要看三个条件：同一个通式、同一个官能团、相邻的碳链长度，缺一个就不算。很多学生以为只要都能燃烧就是同系物，把乙烯 C2H4 和乙烷 C2H6 写进同一组，阅卷时这一空只给零分。
A hydrocarbon contains carbon and hydrogen only, and the alkanes in crude oil follow the general formula CnH2n+2, so n=1 is methane and n=2 is ethane; alkenes follow CnH2n and are two hydrogens short of the corresponding alkane, precisely the pair that forms the carbon-carbon double bond. Alcohols follow CnH2n+1OH and carry the -OH group, with methanol, ethanol, propanol and butanol differing by one carbon each time. Members of a homologous series share a general formula and a functional group while differing by a CH2 unit, which is why ethene cannot sit inside the alkane series however readily it burns.
分馏塔的温度梯度与馏分顺序
原油是混合物，直接燃烧用处有限，工业上先用分馏把它拆成若干馏分。分馏塔内部存在温度梯度，底部最热、顶部最冷，加热汽化后的原油以蒸气形式进塔。碳链越短的烃沸点越低，能升到塔的高处才冷凝；碳链越长的烃沸点越高，在较低的位置就冷凝下来被抽走。塔顶拿到的是石油气，往下依次是汽油、石脑油、煤油、柴油、重油，塔底留下的沥青在常温下是固体。常见的错误是写成碳链长的先冷凝却不写冷凝位置，而位置分在评分方案里往往单独占一分。分馏得到的是馏分而不是纯物质，每一份馏分里仍然是一群碳链长度相近的烃。
Fractional distillation separates crude oil inside a column that is hot at the bottom and cool at the top, so the vapours condense at different heights according to chain length; short chains have low boiling points and travel furthest before turning back into liquid. Refinery gas leaves the top, followed downwards by petrol, naphtha, kerosene, diesel and heavy fuel oil, while bitumen stays at the bottom as a solid at room temperature. Each fraction is a mixture of hydrocarbons of similar chain length rather than a pure substance, and the mark for the condensation height is usually awarded separately from the reason.
三条性质趋势：沸点、黏度与可燃性
碳链长度一变化，三个可测性质按同一个方向移动。沸点随碳链增长而升高，原因是分子间作用力随分子变大而增强。黏度随碳链增长而升高，所以煤油比汽油更容易挂在管壁上，倒出来也更慢。可燃性随碳链增长而下降，重馏分很难直接点燃，汽油却挥发得很快也点燃得很快。用途正是跟着这三条趋势分的：汽油短链、易挥发、易点燃，适合发动机；沥青长链、黏稠、难点燃，却适合铺路。只写碳链越长沸点越高一般只能拿到一分，把黏度与可燃性两个方向也写出来，才拿得到第三分。
Three properties move together with chain length, and the reason lies in the intermolecular forces, which grow stronger as the molecules get bigger; boiling point and viscosity both rise while flammability falls. Kerosene clings to a tube more stubbornly than petrol and ignites less readily, which is exactly why the two fractions end up doing different jobs instead of being blended into one fuel. Answers that state only the boiling-point trend usually earn one mark, since the viscosity and flammability trends are separate points on the mark scheme.
完全燃烧与不完全燃烧
烃燃烧的产物取决于氧气供给量。氧气充足时完全燃烧，碳全部变成二氧化碳，氢全部变成水，例如 CH4 + 2O2 -> CO2 + 2H2O。氧气不足时发生不完全燃烧，产物里出现一氧化碳与碳烟，其中一氧化碳无色无味却有毒性，因为血红蛋白与它的结合能力强于与氧气的结合能力。方程式的配平常被写漏，甲烷这个例子两边各有 1 个碳、4 个氢、4 个氧，缺一项就不平衡。学生最常把一氧化碳写成二氧化碳，两个词只差一个字，得分却完全不同。含硫燃料燃烧还会生成二氧化硫，遇水汽形成酸雨，这一条在环境题里单独占分。
The products of burning a hydrocarbon depend on how much oxygen is available, because complete combustion gives carbon dioxide and water only while a limited supply also produces carbon monoxide and soot. Carbon monoxide is colourless, odourless and poisonous, since haemoglobin binds it more strongly than it binds oxygen, and both carbon dioxide and water vapour act as greenhouse gases. Balancing matters as much as naming the products, as CH4 + 2O2 -> CO2 + 2H2O carries 1 carbon, 4 hydrogen and 4 oxygen atoms on each side.
裂解：把长链打短并得到烯烃
分馏拿到的馏分与市场需求并不匹配，重馏分多而汽油少，于是用裂解把长链打断。裂解是把烃蒸气通过热催化剂或与高温水蒸气接触，让长链断裂成短链烃，产物里既有烷烃也有烯烃。催化裂解常用氧化铝作催化剂，蒸汽裂解用高温水蒸气。一个可以直接写在答题线上的例子是 C10H22 -> C8H18 + C2H4，左边的碳数是 10，右边 8 加 2 也是 10。裂解只改变分子大小，不改变元素组成，所以反应前后的原子种类完全一样。阅卷时最常见的问题是产物漏写，只写一个烷烃而不写烯烃，那道小题的两个得分点只能拿到一个。
Cracking exists because fractional distillation delivers more heavy fractions than the market wants, so long chains are broken into shorter and more useful ones by passing the vapour over a hot catalyst such as aluminium oxide or by meeting steam at high temperature. A workable example is C10H22 -> C8H18 + C2H4, where the ten carbon atoms on the left reappear as eight plus two on the right. Because cracking only splits molecules, both alkanes and alkenes come out of it, and answers that name a single product usually lose the alkene mark.
烯烃与溴水检验
烯烃的通式是 CnH2n，分子里含一个碳碳双键，双键让它比烷烃活泼得多。乙烯 CH2=CH2 是最简单的烯烃，加成反应里双键打开，两个碳各接上一个新原子或原子团。检验烯烃用溴水：把气体通入橙色的溴水，颜色褪去说明有双键；换成烷烃通入同样的溴水，颜色保持不变。这一条几乎每张卷子都考，答题时要写橙色褪为无色，也要写清是溴水而不是溴化氢。很多学生以为只要颜色变浅就算阳性，其实稀释本身也会让颜色变浅，所以试管要对着白纸比较，并且和烷烃做一组对照。写反应类型时注意区分加成与聚合，溴水检验只发生加成，不生成聚合物。
Alkenes carry a carbon-carbon double bond and follow CnH2n, which makes them far more reactive than the corresponding alkanes; ethene is the simplest member, and its double bond opens in an addition reaction so that each carbon gains a new atom or group. The standard test uses bromine water, which is orange and loses its colour with an alkene while staying orange with an alkane. Answers should describe the colour fading in front of white paper with an alkane run alongside, because dilution alone lightens the orange without any reaction.
加成聚合与从聚合物反推单体
聚合物是把许多小分子连成一条长链的产物，参与连接的小分子叫单体。加成聚合发生在双键上，乙烯分子彼此连接得到聚乙烯，方程式写作 n CH2=CH2 -> (-CH2-CH2-)n，方括号里的重复单元与单体只差一个双键。聚氯乙烯的单体是氯乙烯 CH2=CHCl，聚合物名称去掉聚字通常就是单体名称，这一条最容易拿分也最容易记混。从聚合物反推单体时先看书括号里的重复单元，把主链上的单键改回双键；重复单元里两个碳上各带一个取代基时，单体仍然是同一个烯烃。答题卡上要求写出方括号与下标 n，漏掉 n 就算结构表达不完整。常见的错误是把重复单元写成单体，结果多写出一个双键。
Addition polymerisation opens the double bond so that many identical monomers join into one long chain, and the repeating unit inside the brackets differs from the monomer only by that lost double bond. Poly(chloroethene), better known as PVC, comes from chloroethene, and in most cases stripping the poly prefix and the brackets recovers the monomer name. Working backwards from a drawn polymer means restoring a double bond along the backbone, and omitting the subscript n is treated as an incomplete structure rather than a neat answer.
醇与乙醇的两条工业路线
醇的官能团是 -OH，甲醇、乙醇、丙醇、丁醇构成一个同系物系列。醇能燃烧放热，能与钠反应放出氢气，也能被氧化成羧酸。乙醇的工业路线有两条：发酵法用酵母把葡萄糖分解成乙醇和二氧化碳，条件是厌氧并且温度保持在 25 到 35 摄氏度；水合法用高温高压的水蒸气与乙烯加成，得到不含水的乙醇，纯度更高。发酵液里的乙醇浓度有限，要得到较高浓度还得靠分馏，乙醇沸点 78 摄氏度、水 100 摄氏度，这个差值就是分离的依据。把两条路线的条件写反是这一节最常见的失分方式。
Alcohols carry the -OH group and follow CnH2n+1OH, with methanol, ethanol, propanol and butanol sharing the same reactions such as burning, reacting with sodium and being oxidised to a carboxylic acid. Ethanol is produced either by fermenting glucose with yeast enzymes under anaerobic conditions at 25 to 35 degrees Celsius, or by adding steam to ethene at high temperature and pressure for a purer product. Fermentation falls well short of a concentrated solution, so the ethanol is separated later by distillation on the strength of boiling points of 78 and 100 degrees Celsius.
羧酸、酯化与酯
羧酸的通式写作 CnH2n+1COOH，官能团是 -COOH，乙酸就是食醋里提供酸味的那一个。羧酸具备酸的通性：与活泼金属反应放出氢气，与碱中和生成盐和水，与碳酸盐反应放出二氧化碳。羧酸与醇在浓硫酸催化下生成酯和水，甲醇与乙酸反应得到乙酸甲酯，产物有水果气味。这个反应是可逆的，生成的水会压低正反应的程度，所以浓硫酸同时起催化与吸水两个作用。写产物时常见的错误是把水漏掉，每生成一个酯分子就同时生成一个水分子。判断题目问的是酯化还是聚合，先看单体里有没有 -COOH 与 -OH 这对组合。
Carboxylic acids contain the -COOH group and behave like typical acids, releasing hydrogen with reactive metals, neutralising bases and giving off carbon dioxide from carbonates. Reacting an alcohol with a carboxylic acid in the presence of concentrated sulfuric acid gives an ester and water, so methanol and ethanoic acid produce methyl ethanoate with its characteristic fruity smell. The reaction is reversible and the water it forms opposes the forward step, which is why concentrated sulfuric acid acts as both catalyst and drying agent, and leaving the water out of the products costs a mark.
缩聚：聚酯与水分子
缩聚与加聚的差别在产物数量：加聚只生成聚合物，缩聚每连一个键就同时放出一个小分子，通常是水。做聚酯需要两种单体，一种是含两个羧基的二羧酸，另一种是含两个羟基的二醇，官能团分别位于分子两端，链才能不断延伸。每形成一条酯键就放出 1 个水分子，这是缩聚题的固定得分点，写聚合方程式时水不能省。这一部分在 1CH0 里属于 Higher 内容，Foundation 卷不要求掌握。判断题目考的是加聚还是缩聚，先看单体是不是同一个烯烃，再看产物一侧有没有水。
Condensation polymerisation differs from addition polymerisation in how many products appear, since every new bond releases a small molecule, normally water. A polyester needs two monomers, a dicarboxylic acid carrying two -COOH groups and a diol carrying two -OH groups, so that each molecule reacts at both ends and the chain keeps growing. One water molecule is released per ester link formed, which is the fixed mark-scheme point on these questions, and the topic sits in the Higher tier content of 1CH0 rather than on the Foundation paper.
失分最集中的五个位置
有机化学的失分集中在几个固定位置。题面要求写结构式时写成分子式很常见，例如把 C2H4 直接写在需要显示双键的横线上，这个空只给零分。聚合方程式里的两个 n 也常漏，单体前的系数 n 与重复单元后的下标 n 缺一个就扣结构分。判断同系物时只看能不能燃烧而不看官能团，会把烯烃放进烷烃一组而丢掉全部分数。反应条件写错是另一类集中失分：发酵要厌氧、酯化要浓硫酸催化、裂解要催化剂或高温水蒸气，条件写错时结论分一般保留、条件分全部失掉。中英文名称互转也需要练，英文题干里把 alkane 认成 alkene，整道结构题都会做错。
Organic marks are lost in a few predictable places, and the first is structure; writing a molecular formula such as C2H4 where the question asks for a displayed structure scores nothing. Polymerisation equations punish a missing coefficient or subscript n, and homologous series questions punish candidates who test only for burning instead of checking the functional group. Conditions form a separate cluster, since fermentation must be anaerobic, esterification needs concentrated sulfuric acid and cracking needs a catalyst or steam, while misreading alkane as alkene in an English paper can wreck a whole structure question.
四周复习安排
把 Topic 8 与 Topic 9 的有机内容摊到四周比较稳。开头一周只处理通式与命名，把甲烷到丁烷、乙烯到丁烯、甲醇到丁醇三组名称与分子式各默写三遍，直到不看笔记也能写出 CnH2n+2、CnH2n、CnH2n+1OH。中间两周做分馏、裂解与聚合物，把馏分顺序、三条性质趋势、加聚与缩聚的方程式各整理成一页图，酯化的条件与产物写成一个可以照抄的模板句。最后一周做限时真题，Paper 2 时长 1 小时 45 分钟、满分 100 分，有机部分通常占 8 到 12 分，做完按条件错、名称错、方程式错三类归档错题。这样安排的好处是每种题型至少过两遍，弱项在最后一周还有补救的余地。
Spreading the organic content of Topics 8 and 9 over four weeks keeps the load even, so the first week goes to formulae and naming until CnH2n+2, CnH2n and CnH2n+1OH come out of memory. The middle fortnight belongs to distillation, cracking and polymers, with one page each for the fraction order, the three property trends and worked addition and condensation equations. Finish with timed past papers, since Paper 2 runs for 1 hour 45 minutes and the organic items usually carry 8 to 12 marks, then file every mistake under conditions, names or equations.
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