📚 Carboxylic Acids Explained for GCSE WJEC Chemistry | GCSE WJEC 化学：羧酸 考点精讲
Carboxylic acids are a vital family of organic compounds that show up in everything from vinegar to aspirin. For your GCSE WJEC Chemistry course, you need to understand their functional group, naming rules, physical properties, and – most importantly – their reactions with metals, bases, carbonates, and alcohols. This article breaks down every key concept, using simple language and clear examples to help you revise effectively and score top marks in the exam.
羧酸是一类重要的有机化合物，从食醋到阿司匹林都有它们的身影。在 GCSE WJEC 化学课程中，你需要掌握羧酸的官能团、命名规则、物理性质，以及最重要的——它们与金属、碱、碳酸盐和醇类的反应。本文用简明的语言和清晰的例子，把每一个核心概念拆解透彻，助你高效复习，在考试中拿下高分。
1. What Are Carboxylic Acids? | 什么是羧酸？
Carboxylic acids are a homologous series of organic compounds that contain the carboxyl functional group, –COOH. They are weak acids, meaning they partially ionise in water to release hydrogen ions (H⁺). The simplest carboxylic acid is methanoic acid (HCOOH), followed by ethanoic acid (CH₃COOH), which is commonly known as acetic acid and gives vinegar its sour taste and pungent smell.
羧酸是一类同系物，含有羧基官能团（–COOH）。它们是弱酸，在水中部分电离，释放出氢离子（H⁺）。最简单的羧酸是甲酸（HCOOH），其次是乙(CH₃COOH)，也就是我们熟悉的醋酸，它是食醋酸味和刺激性气味的来源。
All carboxylic acids share the same general formula CₙH₂ₙ₊₁COOH (for monocarboxylic acids with one –COOH group). As a homologous series, they show a gradual trend in physical properties, such as increasing boiling points with longer carbon chains. In the laboratory, they appear as colourless liquids or solids with sharp, often unpleasant smells.
所有一元羧酸都符合通式 CₙH₂ₙ₊₁COOH（含一个 –COOH 基团）。作为同系物，它们的物理性质呈现规律性变化，比如沸点随碳链增长而升高。实验室中，羧酸通常为无色液体或固体，具有尖锐、有时难闻的气味。
2. Functional Group and General Formula | 官能团与通式
The functional group of carboxylic acids is the carboxyl group, written as –COOH. It is made up of a carbonyl group (C=O) and a hydroxyl group (– e same carbon atom. This arrangement gives the molecule its acidic character because the O–H bond can break, releasing a proton (H⁺).
羧酸的官能团是羧基，写作 –COOH。它由一个羰基（C=O）和一个羟基（–OH）连接在同一个碳原子上构成。这种结构赋予分子酸性，因为 O–H 键可以断裂，释放出质子（H⁺）。
The general formula for a saturated monocarboxylic acid is CₙH₂ₙ₊₁COOH, where n starts from 0 (methanoic acid). In displayed formula diagrams, you must show the double bond between carbon and oxygen, and the single bond between carbon and the –OH group. The WJEC exam often asks learners to draw the first four members: methanoic, ethanoic, propanoic, and butanoic acids.
饱和一元羧酸的通式为 CₙH₂ₙ₊₁COOH，其中 n 从 0 开始（甲酸）。在结构简图中，必须画出碳氧双键以及碳与 –OH 基团的单键。WJEC 考试常要求考生画出前四个成员的结构：甲酸、乙、丙酸和丁酸。
For ethanoic acid, the molecular formula is CH₃COOH, and its structural formula shows a methyl group (–CH₃) bonded to the carboxyl carbon. This compact structure is easily recognised and is the most frequently tested carboxylic acid at GCSE level.
乙酸的分子式为 CH₃COOH，其结构式显示一个甲基（–CH₃）连接在羧基碳上。这种紧凑的结构容易识别，也是 GCSE 阶段最常考的羧酸。
3. Naming Carboxylic Acids | 羧酸的命名
Naming follows the standard IUPAC rules. Pick the longest carbon chain that includes the carboxyl carbon, and replace the final ‘e’ of the corresponding alkane with ‘oic acid’. The carbon of the –COOH group is always counted as carbon number 1. For example, a chain of two carbon atoms gives ethanoic acid; a chain of three gives propanoic acid.
命名遵循标准 IUPAC 规则。选择包含羧基碳的最长碳链，将对应烷烃名称词尾的“e”替换为“oic acid”（中文称“酸”）。–COOH 基团的碳原子总是编号为 1。例如，两个碳原子的链为乙酸；三个碳原子的链为丙酸。
Substituents are indicated by numbers. If there is a methyl group on the second carbon of propanoic acid, the name becomes 2-methylpropanoic acid. At GCSE, WJEC expects you to name and draw the first four straight-chain acids, and to interpret simple branched names like 2-methylbutanoic acid.
取代基用数字标明位置。如果丙酸的第二个碳上有一个甲基，就命名为 2-甲基丙酸。在 GCSE 阶段，WJEC 要求考生能命名并画出前四种直链酸，并能解释简单的支链名称，如 2-甲基丁酸。
| Number of Carbon Atoms | Systematic Name | Molecular Formula |
|---|---|---|
| 1 | Methanoic acid | HCOOH |
| 2 | Ethanoic acid | CH₃COOH |
| 3 | Propanoic acid | C₂H₅COOH |
| 4 | Butanoic acid | C₃H₇COOH |
4. Physical Properties and Trends | 物理性质及变化趋势
Shorter-chain carboxylic acids (up to four carbons) are colourless liquids at room temperature, fully miscible with water. They have distinctly sharp, sour smells – ethanoic acid being the familiar vinegar odour. As the carbon chain length increases, solubility in water decreases, and boiling points rise due to stronger intermolecular forces (London forces and hydrogen bonding).
短链羧酸（不超过四个碳）在室温下为无色液体，与水完全互溶。它们具有明显尖锐的酸味——乙酸就是我们熟悉的醋味。随着碳链增长，水溶性下降，沸点因分子间作用力（伦敦力和氢键）增强而升高。
Hydrogen bonding plays a crucial role. Carboxylic acid molecules can form two hydrogen bonds with each other, creating dimers. This dimerisation significantly raises boiling points compared to alcohols of similar molecular mass. For instance, ethanoic acid boils at 118 °C, while ethanol boils at only 78 °C.
氢键起着关键作用。羧酸分子彼此之间可以形成两个氢键，产生二聚体。这种二聚化显著提高了沸点，与相对分子质量相近的醇相比更高。例如，乙酸沸点为 118 °C，而乙醇仅 78 °C。
In terms of appearance, butanoic acid is an oily liquid with a rancid butter smell, and higher members are waxy solids. You should be able to describe these trends in a WJEC exam and relate them to the size and bonding capability of the molecules.
在外观上，丁酸是一种带有腐败黄油味的油状液体，更高级的成员则是蜡状固体。你应该能够在 WJEC 考试中描述这些变化规律，并将其与分子的大小和成键能力联系起来。
5. Acidity and Ionisation | 酸性与电离
Carboxylic acids are weak acids because they only partially ionise in aqueous solution. When dissolved in water, ethanoic acid establishes an equilibrium mixture of molecules, ethanoate ions, and hydrogen ions. The reversible reaction is represented as:
羧酸是弱酸，因为它们在水溶液中仅部分电离。乙酸溶于水时，会形成一个由分子、乙酸根离子和氢离子组成的平衡混合物。可逆反应表示为：
CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
The position of equilibrium lies far to the left, meaning only a small fraction of acid molecules donate a proton. This explains why carboxylic acids have a higher pH than strong acids like hydrochloric acid at the same concentration. At GCSE, a typical pH for 0.1 mol dm⁻³ ethanoic acid is around 2.9, while hydrochloric acid would be about 1.0.
平衡位置偏向左方，意味着只有一小部分酸分子给出质子。这就解释了为什么在相同浓度下，羧酸的 pH 值高于盐酸等强酸。在 GCSE 中，0.1 mol dm⁻³ 乙酸的典型 pH 值约为 2.9，而盐酸约为 1.0。
Despite being weak acids, carboxylic acids still undergo all the classic acid reactions: they neutralise bases, liberate carbon dioxide from carbonates, and react with reactive metals to produce hydrogen gas. Understanding the weak acid concept is crucial for explaining observations in titration and rate of reaction experiments.
虽然是弱酸，羧酸仍然能发生所有经典的酸反应：中和碱、从碳酸盐中释放二氧化碳，与活泼金属反应生成氢气。理解弱酸概念对于解释滴定和反应速率实验中的现象至关重要。
6. Reactions with Reactive Metals | 与活泼金属的反应
Carboxylic acids react with metals above hydrogen in the reactivity series, such as magnesium, zinc, and iron, to produce a salt and hydrogen gas. For example, ethanoic acid and magnesium ribbon fizz gently, forming magnesium ethanoate and hydrogen. The reaction is slower than with strong acids, reflecting the lower concentration of H⁺ ions.
羧酸能与金属活动性顺序中排在氢前面的金属（如镁、锌、铁）反应，生成盐和氢气。例如，乙酸和镁条发生温和的冒泡反应，生成乙酸镁和氢气。该反应比与强酸反应更慢，反映了 H⁺ 离子浓度较低的特点。
The general word equation is: Carboxylic acid + reactive metal → Salt + Hydrogen
通用的文字方程式为：羧酸 + 活泼金属 → 盐 + 氢气
A typical balanced symbol equation for this reaction is:
2CH₃COOH(aq) + Mg(s) → (CH₃COO)₂Mg(aq) + H₂(g)
You may be asked to name the salt formed. For ethanoic acid, the salts are called ethanoates; for propanoic acid, they are propanoates. The metal replaces the hydrogen of the carboxyl group, and because the acid molecule has one ionisable hydrogen, two acid molecules are needed for each magnesium ion.
考试可能会问你生成的盐的名称。对于乙酸，盐称为乙酸盐；对于丙酸，则称为丙酸盐。金属取代了羧基中的氢，因为每个酸分子有一个可电离的氢，所以每个镁离子需要两个酸分子。
7. Neutralisation with Bases | 与碱的中和反应
Carboxylic acids react with alkalis and bases to produce a salt and water only. This is a straightforward neutralisation. With sodium hydroxide, ethanoic acid forms sodium ethanoate and water. The ionic equation shows that the H⁺ from the acid combines with OH⁻ from the alkali.
羧酸与碱和金属氧化物反应，只生成盐和水。这是简单的中和反应。与氢氧化钠反应时，乙酸生成乙酸钠和水。离子方程式显示，酸中的 H⁺ 与碱中的 OH⁻ 结合。
Word equation: Ethanoic acid + Sodium hydroxide → Sodium ethanoate + Water
文字方程式：乙酸 + 氢氧化钠 → 乙酸钠 + 水
CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)
Metal oxides such as copper(II) oxide also act as bases. When warmed with ethanoic acid, the black copper(II) oxide dissolves to form a blue solution of copper(II) ethanoate. This reaction is often used in the preparation of carboxylic acid salts in the lab.
金属氧化物（如氧化铜）同样可作为碱。与乙酸一起温热时，黑色的氧化铜溶解，形成蓝色的乙酸铜溶液。该反应常用于实验室制备羧酸盐。
For WJEC exams, you must be able to write balanced equations for these neutralisations and identify the salt produced. The naming follows the pattern: [metal] [alkanoate].
在 WJEC 考试中，你必须能写出这些中和反应的配平方程式，并指出生成的盐。命名遵循 [金属] [烷酸根] 的模式。
8. Reaction with Carbonates and Test | 与碳酸盐的反应及检验
Carboxylic acids, like all acids, react with metal carbonates and hydrogencarbonates to produce a salt, water, and carbon dioxide gas. This provides a classic test: bubbling CO₂ turns limewater cloudy. Ethanoic acid and sodium carbonate fizz vigorously, but more gently than with a strong acid.
与所有酸一样，羧酸能与金属碳酸盐和碳酸氢盐反应，生成盐、水和二氧化碳气体。这提供了一个经典的检验方法：冒出的 CO₂ 能使石灰水变浑浊。乙酸与碳酸钠反应会剧烈冒泡，但比强酸温和。
Word equation: Ethanoic acid + Sodium carbonate → Sodium ethanoate + Water + Carbon dioxide
文字方程式：乙酸 + 碳酸钠 → 乙酸钠 + 水 + 二氧化碳
2CH₃COOH(aq) + Na₂CO₃(s) → 2CH₃COONa(aq) + H₂O(l) + CO₂(g)
With hydrogencarbonates, the reaction is similar but releases CO₂ with less consumption of acid per mole of carbonate. For sodium hydrogencarbonate:
与碳酸氢盐反应类似，但每摩尔碳酸盐消耗的酸更少。以碳酸氢钠为例：
CH₃COOH(aq) + NaHCO₃(s) → CH₃COONa(aq) + H₂O(l) + CO₂(g)
This reaction is often used to distinguish a carboxylic acid from an alcohol. Alcohols do not react with carbonates, so the effervescence confirms the presence of the –COOH group. In a typical exam question, you might be given unknown organic liquids and asked to describe a chemical test to identify the carboxylic acid.
该反应常用来区分羧酸和醇。醇不与碳酸盐反应，所以冒泡现象能证实 –COOH 基团的存在。典型考题可能会给出未知有机液体，要求描述一个化学检验方法来确定哪个是羧酸。
9. Esterification Reaction | 酯化反应
One of the most characteristic reactions of carboxylic acids is esterification, where a carboxylic acid reacts with an alcohol in the presence of an acid catalyst (usually concentrated sulfuric acid) to form an ester and water. This is an equilibrium reaction, and it is widely used to produce fragrant compounds found in perfumes and flavourings.
羧酸最具代表性的反应之一是酯化反应：羧酸与醇在酸催化剂（通常是浓硫酸）存在下反应，生成酯和水。这是一个可逆反应，广泛用于生产香水、调味剂中的芳香化合物。
For ethanoic acid and ethanol, the reaction produces ethyl ethanoate, a sweet-smelling ester. The equation is:
乙酸和乙醇反应生成乙酸乙酯，一种芳香味的酯。反应方程式：
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
Concentrated sulfuric acid acts as a catalyst and a dehydrating agent, helping to shift the equilibrium to the right by removing water. In the lab, the mixture is usually heated gently under reflux, and the ester is separated by distillation. You should know how to name esters: the alkyl group from the alcohol comes first, then the alkanoate part from the acid.
浓硫酸既是催化剂，又是脱水剂，通过除去水帮助平衡向右移动。实验室中，通常将混合物在回流下温和加热，酯通过蒸馏分离。你需要知道酯的命名规则：来自醇的那部分烷基写在前面，然后是来自酸的烷酸根。
WJEC requires you to write balanced equations for esterification and recognise that esters have the functional group –COO–. Esters are volatile compounds with pleasant fruity smells, in sharp contrast to the irritating odours of the parent carboxylic acids.
WJEC 要求你写出酯化反应的配平方程式，并认识到酯的官能团为 –COO–。酯是挥发性化合物，带有宜人的水果香味，与母体羧酸的刺激性气味形成鲜明对比。
10. Daily Life and Industrial Uses | 日常与工业用途
Carboxylic acids and their derivatives have an enormous range of applications. Ethanoic acid is the main component of vinegar (typically a 5 % solution) used in food preservation and flavouring. Methanoic acid occurs naturally in ant stings and nettle leaves and is used in rubber processing and leather tanning.
羧酸及其衍生物有着极其广泛的用途。乙酸是食醋的主要成分（通常为 5 % 溶液），用于食品防腐和调味。甲酸天然存在于蚂蚁叮咬和荨麻叶中，用于橡胶加工和皮革鞣制。
Longer-chain carboxylic acids, such as stearic acid and oleic acid, are found in fats and oils and are the starting materials for soap production via saponification. Citric acid, a polycarboxylic acid, is widely used as a food acidulant and cleaning agent. Salicylic acid leads to the synthesis of aspirin, highlighting the medical importance of this functional group.
长链羧酸如硬脂酸和油酸，存在于油脂中，是通过皂化反应生产肥皂的起始原料。柠檬酸是一种多元羧酸，广泛用作食品酸味剂和清洁剂。水杨酸可用于合成阿司匹林，凸显了这一官能团在医学上的重要性。
Esters derived from carboxylic acids are used as solvents, plasticisers, and – most famously – as synthetic fruit flavours in sweets and drinks. Understanding the link between structure, functional group, and real-world use is a common feature of WJEC application questions.
由羧酸合成的酯被用作溶剂、增塑剂，以及最著名的——糖果和饮料中的人造水果味剂。理解结构、官能团与实际应用之间的联系，是 WJEC 应用题常见的考查内容。
11. Summary of Reactions and Quick Comparison | 反应小结与对比
Below is a quick-reference table summarising the key reactions of carboxylic acids, which you can use for last-minute revision. Each reaction highlights the reactants, products, and what you would observe.
下面是一张速查表，总结了羧酸的关键反应，可供考前快速复习。每个反应都列出了反应物、生成物及现象。
| Reaction | Reactant | Main Products | Observations |
|---|---|---|---|
| With metal | Mg, Zn, Fe | Salt + H₂ | Bubbles, metal dissolves |
| With alkali | NaOH, KOH | Salt + H₂O | Heat released, no visible fizz |
| With metal oxide | CuO, ZnO | Salt + H₂O | Solid dissolves, colour change |
| With carbonate | Na₂CO₃, CaCO₃ | Salt + H₂O + CO₂ | Fizzing, limewater cloudy |
| With alcohol | Ethanol, etc. + H₂SO₄ | Ester + H₂O | Sweet fruity smell |
Remember that all these reactions ultimately depend on the acidic O–H bond in –COOH. The more you practise writing balanced equations and naming the salts, the more confident you will be in the WJEC exam.
请记住，所有这些反应归根结底都依赖于 –COOH 中酸性的 O–H 键。你越是练习书写配平方程式和命名盐类，考试时就越有信心。
12. Exam Tips and Common Mistakes | 考试技巧与常见错误
Many students lose marks by forgetting that carboxylic acids are weak acids. Do not write a full forward arrow in ionisation; always use the equilibrium arrow (⇌). When drawing the structure, make sure the –COOH group is clearly shown with a double bond and a hydroxyl group, and never write COOH as two separate atoms disconnected from the carbon chain.
许多考生因忘记羧酸是弱酸而失分。电离时不要用单向箭头，一定要用可逆符号（⇌）。画结构时，确保 –COOH 基团清晰地显示双键和羟基，绝不要把 COOH 写成与碳链脱节的两个独立原子。
the acid. For example, methanol + ethanoic acid → methyl ethanoate, not ethyl methanoate.
另一个常见误区是搞混酯的命名。来自醇的烷基写在前，来自酸的烷酸根在后。例如，甲醇 + 乙酸 → 乙酸甲酯，而不是甲酸乙酯。
When describing a test to distinguish a carboxylic acid from an alcohol, always say “add sodium carbonate solution; if the substance is a carboxylic acid, effervescence occurs and the gas turns limewater milky”. Simply writing “add carbonate” is not specific enough for full marks.
在描述区分羧酸和醇的检验时，一定要写“加入碳酸钠溶液；如果该物质是羧酸，就会冒泡，气体使石灰水变浑浊”。仅写“加入碳酸盐”不够具体，拿不到满分。
Finally, remember that all salts formed from ethanoic acid end in “ethanoate”, not “acetate” unless the question specifies the traditional name – WJEC scientific terminology favours systematic names.
最后，记住由乙酸生成的盐，词尾都是“ethanoate”（中文称“乙酸某”），除非题目特意要求俗名——WJEC 的科学术语倾向使用系统命名。
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