Carboxylic Acids for GCSE CIE Chemistry | GCSE CIE 化学：羧酸考点精讲

📚 Carboxylic Acids for GCSE CIE Chemistry | GCSE CIE 化学：羧酸考点精讲

Carboxylic acids are a homologous series of organic compounds that contain the carboxyl functional group, -COOH. They are weak acids with a distinctive sour taste and are widely used in the food, pharmaceutical, and chemical industries. Understanding their structure, properties, and reactions is essential for the GCSE CIE Chemistry examination.

羧酸是一类含有羧基（-COOH）官能团的同系列有机化合物。它们是弱酸，具有独特的酸味，并广泛应用于食品、制药和化学工业。理解其结构、性质及反应对于 GCSE CIE 化学考试至关重要。

1. The Carboxyl Functional Group | 羧基官能团

The functional group of carboxylic acids is the carboxyl group, -COOH. It consists of a carbonyl group (C=O) and a hydroxyl group (-OH) attached to the same carbon atom. This combination makes the hydrogen atom on the -OH weakly acidic, as it can be released as a proton in aqueous solution.

羧酸的官能团是羧基（-COOH）。它由羰基（C=O）和羟基（-OH）连接在同一个碳原子上组成。这种组合使 -OH 上的氢原子呈现弱酸性，可以在水溶液中以质子形式释放。

The carboxyl group is always at the end of the carbon chain, meaning the carbon of -COOH is counted as carbon number 1 when naming the molecule.

羧基总是位于碳链的末端，这意味着在命名分子时，-COOH 的碳总是被编号为 1 号碳。

2. General Formula and Homologous Series | 通式与同系列

Carboxylic acids follow the general formula CₙH₂ₙ₊₁COOH (which can also be written as CₙH₂ₙO₂ when the carboxyl carbon is included in the chain). Each member differs from the next by a -CH₂- unit.

羧酸的通式为 CₙH₂ₙ₊₁COOH（当羧基碳计入主链时，也可写作 CₙH₂ₙO₂）。每相邻两个成员相差一个 -CH₂- 单元。

The first four straight-chain carboxylic acids are shown in the table below:

下表列出了前四个直链羧酸：

Name	Molecular Formula	Displayed Formula	中文名称
Methanoic acid	HCOOH	H-COOH	甲酸
Ethanoic acid	CH₃COOH	CH₃-COOH	乙酸
Propanoic acid	C₂H₅COOH	CH₃CH₂-COOH	丙酸
Butanoic acid	C₃H₇COOH	CH₃CH₂CH₂-COOH	丁酸

In GCSE, you should be able to recognise and name these acids and write their structural or displayed formulas.

在 GCSE 中，你需要能够识别并命名这些酸，并能写出它们的结构式或显示式。

3. Naming Carboxylic Acids | 羧酸的命名

Carboxylic acids are named by taking the stem of the alkane with the same number of carbon atoms and adding the suffix ‘-oic acid’. For example, a two-carbon carboxylic acid is called ethanoic acid (ethane + oic acid).

羧酸的命名是取相同碳原子数的烷烃词干，然后加上后缀“-酸”。例如，含有两个碳原子的羧酸称为乙酸（ethane → ethanoic acid）。

The carbon chain must include the -COOH carbon. Substituents are numbered from the carboxyl carbon as position 1. For instance, 2-methylpropanoic acid has a methyl branch on the second carbon.

碳链必须包含 -COOH 碳。取代基编号从羧基碳开始，定为1位。例如，2-甲基丙酸在第二号碳上有一个甲基分支。

You will not be required to name branched carboxylic acids in great detail, but knowing the basic system is helpful for understanding esterification and related organic chemistry.

考试中不会要求非常复杂地命名支链羧酸，但了解基本命名系统有助于理解酯化反应及相关有机化学。

4. Physical Properties | 物理性质

Short-chain carboxylic acids (up to four carbons) are completely miscible with water because the -COOH group can form hydrogen bonds with water molecules. As the hydrocarbon chain lengthens, solubility decreases because the non-polar part dominates.

短链羧酸（最多四个碳）与水完全互溶，因为 -COOH 基团能与水分子形成氢键。随着碳链增长，溶解度降低，因为非极性部分逐渐占主导。

Carboxylic acids have relatively high boiling points compared to alkanes and alcohols of similar molecular mass. This is due to the ability of carboxylic acid molecules to form strong dimers through two hydrogen bonds between their carboxyl groups.

Dimer example: RCOOH···HOOCR

与相似分子质量的烷烃和醇相比，羧酸的沸点相对较高。这是因为羧酸分子能通过两个羧基之间的氢键形成牢固的二聚体。

二聚体示意：RCOOH···HOOCR

Small carboxylic acids are colourless liquids with sharp, pungent odours. Ethanoic acid smells like vinegar, while butanoic acid has a rancid butter smell.

小分子羧酸是无色液体，具有尖锐、刺鼻的气味。乙酸有醋味，而丁酸有腐臭的黄油味。

5. Acidic Nature: Weak Acids | 酸性：弱酸

Carboxylic acids are weak acids, meaning they only partially ionise in water. The equilibrium position lies well to the left.

羧酸是弱酸，这意味着它们在水溶液中仅部分电离，平衡位置大大偏向左。

CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)

Because the concentration of hydrogen ions is low, carboxylic acids have a higher pH than a strong acid of the same concentration. For example, 0.1 mol dm⁻³ ethanoic acid has a pH of about 2.9, whereas 0.1 mol dm⁻³ hydrochloric acid has a pH of 1.0.

由于氢离子浓度低，相同浓度的羧酸比强酸的 pH 值高。例如，0.1 mol dm⁻³ 乙酸的 pH 约为 2.9，而 0.1 mol dm⁻³ 盐酸的 pH 为 1.0。

When describing reactions of carboxylic acids, remember that only the hydrogen atom in the -COOH group is acidic. The other hydrogen atoms bonded to carbon do not ionise.

在描述羧酸的反应时，请记住只有 -COOH 基团中的氢原子是酸性的。连接在碳原子上的其他氢原子不会电离。

6. Reactions with Metals | 与金属的反应

Carboxylic acids react with reactive metals (such as magnesium, zinc, and iron) to produce a carboxylate salt and hydrogen gas. These are redox reactions where the metal is oxidised and the H⁺ is reduced.

羧酸与活泼金属（如镁、锌和铁）反应，生成羧酸盐和氢气。这些是氧化还原反应，金属被氧化，H⁺ 被还原。

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

The observation is steady effervescence, and the hydrogen can be tested with a lit splint producing a ‘pop’ sound. The salt formed is named after the acid: magnesium ethanoate.

实验现象是持续冒泡，产生的氢气可用点燃的木条检验，发出‘噗’的爆鸣声。生成的盐根据酸来命名：乙酸镁。

Note that the reaction with sodium is very violent and is usually not demonstrated in school laboratories, but the equation follows the same pattern.

请注意，与钠的反应非常剧烈，通常不在学校实验室中演示，但反应方程式遵循相同模式。

7. Reactions with Bases | 与碱的反应

Aqueous alkalis (sodium hydroxide, potassium hydroxide) neutralise carboxylic acids in a typical acid-base reaction, forming a salt and water only.

碱溶液（氢氧化钠、氢氧化钾）与羧酸发生典型的中和反应，仅生成盐和水。

CH₃COOH + NaOH → CH₃COONa + H₂O

There is no effervescence, and the reaction mixture becomes warm. The salt sodium ethanoate is used in many applications, including as a food additive and in heating pads.

该反应无气泡产生，反应混合物会变热。生成的乙酸钠用途广泛，包括用作食品添加剂和暖手宝。

Because carboxylic acids are weak, the reaction with a strong base is slightly less exothermic than that of a strong acid with the same base, a point sometimes assessed in energetics questions.

由于羧酸是弱酸，它与强碱的反应放热略少于同浓度的强酸与同种碱的反应，这一点有时会在能量变化题目中考查。

8. Reactions with Carbonates | 与碳酸盐的反应

Carboxylic acids react with metal carbonates and hydrogencarbonates to produce a salt, water, and carbon dioxide gas. This reaction is a classic test for the carboxylic acid group.

羧酸与金属碳酸盐和碳酸氢盐反应，生成盐、水和二氧化碳气体。该反应是检验羧酸基团的经典方法。

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

Brisk effervescence is observed. The carbon dioxide turns limewater milky, confirming the presence of a carboxylic acid. This distinguishes carboxylic acids from alcohols and most other organic compounds.

能观察到剧烈冒泡。二氧化碳使石灰水变浑浊，从而确认羧酸的存在。这可将羧酸与醇及大多数其他有机化合物区分开。

In an exam, always write balanced equations showing the correct ratio of acid to carbonate—remember that a carbonate like Na₂CO₃ requires two acid molecules.

考试时一定要写出配平的方程式，体现酸与碳酸盐的正确比例——请记住碳酸钠（Na₂CO₃）需要两分子酸。

9. Esterification Reaction | 酯化反应

Carboxylic acids react with alcohols in the presence of a concentrated sulfuric acid catalyst and heat to form esters and water. This is a reversible condensation reaction.

羧酸与醇在浓硫酸催化并加热的条件下反应，生成酯和水。这是一个可逆的缩合反应。

RCOOH + R’OH ⇌ RCOOR’ + H₂O

For example, ethanoic acid reacts with ethanol to give ethyl ethanoate, a sweet-smelling ester used in pear-drop sweets.

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

例如，乙酸与乙醇反应生成乙酸乙酯，这是一种具有甜美香味的酯，用于梨味糖果。

The ester functional group is -COO-. The acid provides the -OH, and the alcohol provides the H to form water. Breaking the C-O bond in the acid and the O-H bond in the alcohol is a key mechanistic detail, but at GCSE you mainly need to recall the overall equation.

酯的官能团是 -COO-。酸提供 -OH，醇提供 H 生成水。断键发生在酸的 C-O 键和醇的 O-H 键，但 GCSE 阶段你主要需要掌握总反应方程式。

Common esters are used as flavourings and solvents. The reaction is slow at room temperature and requires heating under reflux for several hours.

常见的酯用作调味剂和溶剂。室温下反应很慢，需加热回流数小时。

10. Preparation of Ethanoic Acid | 乙酸的制备

Ethanoic acid can be prepared in the laboratory by the oxidation of ethanol using the oxidising agent acidified potassium dichromate(VI). The ethanol is heated under reflux to provide the necessary activation energy and to prevent the volatile organic compounds from escaping.

C₂H₅OH + 2[O] → CH₃COOH + H₂O

乙酸可在实验室通过乙醇的氧化制备，使用酸性重铬酸钾(VI)作为氧化剂。乙醇在回流装置中加热以提供活化能，并防止挥发性有机物逸散。

The orange dichromate(VI) ions are reduced to green chromium(III) ions, so the colour change from orange to green confirms the reaction is taking place.

橙色的重铬酸根(VI)离子被还原为绿色的铬(III)离子，因此由橙变绿的颜色变化可证实反应正在进行。

Industrially, ethanoic acid is produced by the fermentation of ethanol to vinegar using Acetobacter bacteria. This aerobic process produces a dilute solution of ethanoic acid (about 5-10%).

工业上，乙酸通过醋酸杆菌将乙醇发酵成醋而制得。这一需氧过程产生稀乙酸溶液（约5-10%）。

11. Common Carboxylic Acids and Their Uses | 常见羧酸及用途

Methanoic acid (HCOOH) – present in ant and nettle stings, causing irritation. It is used in leather tanning and as a preservative.

甲酸（HCOOH） – 存在于蚂蚁和荨麻的叮咬中，是引起刺痛的物质。用于皮革鞣制和防腐剂。

Ethanoic acid (CH₃COOH) – the main component of vinegar (typically 4–8% solution). It is used as a food preservative, in the manufacture of esters and polymers, and in descaling agents.

乙酸（CH₃COOH） – 醋的主要成分（通常为4-8%溶液）。用作食品防腐剂、酯类和聚合物的生产，以及除垢剂。

Propanoic acid and butanoic acid – used as feed preservatives and in the synthesis of esters for perfumes and fruit flavourings.

丙酸和丁酸 – 用作饲料防腐剂，并用于合成香水和果味香精所需的酯类。

Other examples, such as citric acid and lactic acid, are naturally occurring carboxylic acids with multiple functional groups, but they are not typically examined at GCSE level as part of the core homologous series.

其他例子如柠檬酸和乳酸，是含有多个官能团的天然羧酸，但通常不作为 GCSE 核心同系列的内容进行考查。

12. Exam Tips and Common Pitfalls | 考试技巧与常见易错点

Always write the carboxyl group as -COOH, not COOH- or Hooc. In displayed formulas, show all atoms and the double bond between C and O clearly.

书写羧基时务必写成 -COOH，而不是 COOH- 或 Hooc。在显示式中，明确画出所有原子以及 C 与 O 之间的双键。

When writing the formula of an ester, remember that the acid part loses -OH and the alcohol loses -H. The ester functional group is -COO-. A common mistake is to write the alcohol’s alkyl group first; the correct order is acid-derived alkyl first, then alcohol-derived alkyl.

在书写酯的分子式时，记住酸的部分失去 -OH，醇失去 -H。酯官能团为 -COO-。常见错误是先将醇的

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Carboxylic Acids for GCSE CIE Chemistry | GCSE CIE 化学：羧酸 考点精讲