📚 IGCSE Chemistry: Carboxylic Acids | IGCSE化学：羧酸考点精讲
Carboxylic acids are a homologous series of organic compounds containing the –COOH functional group. In IGCSE Chemistry, you need to understand their structure, naming, weak acidic nature, and key reactions such as esterification and reactions with carbonates. This guide covers every essential point you must master for your exam.
羧酸是一类含有–COOH官能团的同系有机物。在IGCSE化学中，你需要掌握它们的结构、命名、弱酸性以及与碳酸盐反应、酯化等关键反应。本指南涵盖你必须掌握的每一个考点。
1. What Are Carboxylic Acids? | 什么是羧酸？
Carboxylic acids are organic compounds that contain at least one carboxyl group, –COOH. This functional group consists of a carbonyl (C=O) and a hydroxyl (O–H) attached to the same carbon atom. The general formula for a monocarboxylic acid is CnH2n+1COOH, where n starts from 0 for methanoic acid. They form a homologous series with a gradual change in physical properties.
羧酸是至少含有一个羧基（–COOH）的有机化合物。该官能团由连接在同一个碳原子上的羰基（C=O）和羟基（O–H）组成。一元羧酸的通式为CnH2n+1COOH，甲酸的 n = 0。它们组成一个物理性质渐变同系物系列。
2. Functional Group: –COOH | 官能团：羧基
The carboxyl group is the reactive site of carboxylic acids. It is polar because of the electronegative oxygen atoms, which makes small carboxylic acids soluble in water. The hydrogen atom attached to oxygen can be released as H⁺, giving these compounds their acidic character. In displayed formulas, you must show all bonds: O=C–O–H.
羧基是羧酸的反应位点。由于氧原子的电负性，羧基具有极性，使得小分子羧酸可溶于水。与氧相连的氢原子可释放出 H⁺，赋予这类化合物酸性。在结构式中，必须画出所有键：O=C–O–H。
3. Naming Carboxylic Acids | 羧酸的命名
In IGCSE, you only need to name and draw the first four unbranched members. The name ends in ‘-oic acid’. The carbon of the –COOH group is always carbon number 1. The alkyl chain is named by counting the total number of carbon atoms, including the carboxyl carbon.
在IGCSE中，你只需命名前四个直链羧酸。名称以“-酸”结尾。–COOH 基团的碳总是编号为 1。烷基链按碳原子总数（包括羧基碳）来命名。
| Carbon atoms | IUPAC name (English) | 中文名 | Formula |
|---|---|---|---|
| 1 | Methanoic acid | 甲酸 | HCOOH |
| 2 | Ethanoic acid | 乙酸 | CH3COOH |
| 3 | Propanoic acid | 丙酸 | C2H5COOH |
| 4 | Butanoic acid | 丁酸 | C3H7COOH |
4. Physical Properties | 物理性质
The first two members (methanoic and ethanoic acids) are colourless liquids with sharp, pungent smells. They have relatively high boiling points compared to alkanes of similar molecular mass because carboxylic acid molecules form hydrogen bonds with one another, existing as dimers in the pure liquid state. Small carboxylic acids are miscible with water due to hydrogen bonding with water molecules. As the hydrocarbon chain lengthens, solubility in water decreases while solubility in organic solvents increases.
前两种羧酸（甲酸和乙酸）是具有刺激性气味的无色液体。与相对分子质量相近的烷烃相比，羧酸的沸点较高，因为羧酸分子间能形成氢键，在纯液态时以二聚体形式存在。小分子羧酸由于能与水分子形成氢键而可与水混溶。随着烃链增长，水溶性降低，在有机溶剂中的溶解度增加。
5. Weak Acids: Partial Dissociation | 弱酸性：部分解离
Carboxylic acids are weak acids. In water, they only partially ionise, setting up a dynamic equilibrium. For example, ethanoic acid dissociates to a small extent:
羧酸是弱酸。在水中，它们只发生部分电离，并建立动态平衡。例如，乙酸只少量解离：
CH3COOH(aq) ⇌ CH3COO⁻(aq) + H⁺(aq)
The equilibrium lies far to the left, meaning that the concentration of hydrogen ions is much lower than that of the acid itself. This explains why a 1 mol/dm³ solution of ethanoic acid has a higher pH than a 1 mol/dm³ solution of a strong acid like hydrochloric acid.
平衡强烈偏向左边，意味着氢离子浓度远低于酸本身浓度。这解释了为什么 1 mol/dm³ 乙酸的 pH 比同浓度的强酸（如盐酸）要高。
6. Reaction with Reactive Metals | 与活泼金属的反应
Like any acid, carboxylic acids react with metals above hydrogen in the reactivity series to produce a salt and hydrogen gas. The reaction is slower than with strong acids because of the low H⁺ concentration.
与任何酸一样，羧酸可以与金属活动性顺序中位于氢前面的金属反应，生成盐和氢气。由于 H⁺ 浓度低，反应比强酸慢。
2CH3COOH(aq) + Mg(s) → (CH3COO)2Mg(aq) + H2(g)
The salt formed is named after the acid: ethanoic acid gives ethanoate salts (here, magnesium ethanoate). During the reaction, you will see bubbles of hydrogen gas, and the metal dissolves.
生成的盐以酸来命名：乙酸生成乙酸盐（这里为乙酸镁）。反应过程中可以看到氢气气泡，金属溶解。
7. Reaction with Bases and Alkalis | 与碱和碱性氧化物反应
Carboxylic acids undergo neutralisation reactions with metal oxides, hydroxides, and alkalis to form a salt and water. For example, ethanoic acid reacts with sodium hydroxide:
羧酸可以与金属氧化物、氢氧化物和碱发生中和反应，生成盐和水。例如，乙酸与氢氧化钠反应：
CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)
Similarly, with copper(II) oxide, the black solid dissolves to give a blue solution of copper(II) ethanoate:
类似地，与氧化铜反应，黑色固体溶解，得到蓝色的乙酸铜溶液：
2CH3COOH(aq) + CuO(s) → (CH3COO)2Cu(aq) + H2O(l)
8. Reaction with Carbonates and Hydrogencarbonates | 与碳酸盐和碳酸氢盐的反应
This is a key test for carboxylic acids. They react with metal carbonates or hydrogencarbonates to produce a salt, water, and carbon dioxide gas. With ethanoic acid and sodium carbonate:
这是检验羧酸的关键反应。羧酸与金属碳酸盐或碳酸氢盐反应生成盐、水和二氧化碳气体。乙酸与碳酸钠反应：
2CH3COOH(aq) + Na2CO3(s) → 2CH3COONa(aq) + H2O(l) + CO2(g)
Bubbles of colourless carbon dioxide are observed, which turn limewater milky. This reaction can distinguish a carboxylic acid from an alcohol or alkane, as those do not react with carbonates. It also confirms the presence of the –COOH group.
观察到无色二氧化碳气泡，可使石灰水变浑浊。该反应可以区分羧酸与醇或烷烃，因为后者不与碳酸盐反应。这也证实了–COOH 基团的存在。
9. Esterification Reaction | 酯化反应
Carboxylic acids react with alcohols in the presence of an acid catalyst (usually concentrated sulfuric acid) to form esters and water. This is called esterification and is a reversible reaction, represented by the equilibrium arrow ⇌.
羧酸在酸催化剂（通常为浓硫酸）存在下与醇反应，生成酯和水。该反应称为酯化，是可逆反应，用平衡箭头 ⇌ 表示。
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Conditions: warm the mixture of carboxylic acid and alcohol with a few drops of concentrated H2SO4. The ester, being volatile, can be distilled out. Esters have sweet, fruity smells and are used as food flavourings and solvents.
条件： 将羧酸和醇的混合物与几滴浓硫酸一起温热。生成的酯具有挥发性，可被蒸馏出来。酯具有甜的果香味，可用作食品调味剂和溶剂。
10. Naming Esters and Everyday Uses | 酯的命名与日常用途
An ester’s name has two parts: the alcohol part (as an alkyl group) comes first, followed by the acid part (with the -oic acid ending changed to -oate). For example, ethanol + ethanoic acid → ethyl ethanoate. Methanol + propanoic acid → methyl propanoate.
酯的名称由两部分组成：来自醇的烷基部分在前，然后是酸的部分（-酸改为-某酸酯）。例如，乙醇 + 乙酸 → 乙酸乙酯。甲醇 + 丙酸 → 丙酸甲酯。
| Alcohol | Carboxylic acid | Ester name | Ester formula |
|---|---|---|---|
| Methanol | Ethanoic acid | Methyl ethanoate | CH3COOCH3 |
| Ethanol | Ethanoic acid | Ethyl ethanoate | CH3COOC2H5 |
| Ethanol | Methanoic acid | Ethyl methanoate | HCOOC2H5 |
Many esters occur in fruits and are used in perfumes, food flavourings, and as industrial solvents. Polyesters, formed from dicarboxylic acids and diols, are important plastics. Making an ester in the lab is a common IGCSE practical activity.
许多酯天然存在于水果中，广泛用于香水、食品调味剂和工业溶剂。由二元羧酸和二元醇形成的聚酯是重要的塑料。在实验室制备酯是IGCSE常见的实验活动。
11. Comparing Carboxylic Acids with Alcohols and Alkanes | 羧酸与醇、烷烃的比较
To avoid confusion in exams, remember that alkanes are neutral, alcohols do not react with carbonates but do react with sodium, and carboxylic acids show acidic properties and give effervescence with carbonates. The –OH group in alcohols does not dissociate, whereas the –OH in –COOH can release H⁺. This functional group contrast is a classic assessment target.
为了避免考试混淆，记住：烷烃呈中性，醇不与碳酸盐反应但能与钠反应，而羧酸表现出酸性，与碳酸盐反应产生气泡。醇中的 –OH 不解离，而 –COOH 中的 –OH 可以释放 H⁺。这种官能团区别是典型的考查目标。
- Alkanes: no reaction with Na, NaOH, or Na2CO3.
- Alcohols: react with Na to give H2; no effervescence with carbonates.
- Carboxylic acids: react with Na, NaOH, and give CO2 with carbonates.
- 烷烃：与 Na、NaOH、Na2CO3 均不反应。
- 醇：与 Na 反应放出 H2；与碳酸盐不冒泡。
- 羧酸：与 Na、NaOH 反应，与碳酸盐反应产生 CO2。
12. Summary of Key Points | 考点总结
A quick checklist for IGCSE: the –COOH group is the functional group; names end in -oic acid; they are weak acids that partially ionise; they react with metals to give hydrogen, with bases to give salts, and with carbonates to give CO2; esterification with alcohols requires an acid catalyst and is reversible; esters have fruity smells and are named from the alcohol and acid used. Learn both word and symbol equations for these reactions.
IGCSE 快速检查清单：–COOH 是官能团；名称以“-酸”结尾；它们是弱酸，部分电离；与金属反应放氢气，与碱生成盐，与碳酸盐生成 CO2；与醇的酯化反应需要酸催化剂，是可逆的；酯有果香，由所用的醇和酸命名。务必掌握这些反应的文字和符号方程式。
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