IGCSE OCR Chemistry: Carboxylic Acids – Key Points Revision | IGCSE OCR 化学：羧酸考点精讲

📚 IGCSE OCR Chemistry: Carboxylic Acids – Key Points Revision | IGCSE OCR 化学：羧酸考点精讲

Carboxylic acids are a homologous series of organic compounds containing the functional group –COOH. In IGCSE OCR Chemistry, you need to know their structure, naming, acidity, esterification reactions, and how they are prepared from primary alcohols. This article breaks down every key concept with bilingual explanations, reaction equations, and exam‑focused tips to help you score full marks.

羧酸是一类含有官能团 –COOH 的同系物。在 IGCSE OCR 化学中，你需要掌握它们的结构、命名、酸性、酯化反应以及如何由伯醇制备。本文通过中英双语讲解、反应方程式和考点聚焦技巧，帮你逐一突破，轻松拿满分。

1. The Functional Group and Structure | 官能团与结构

The carboxyl group, –COOH, is made up of a carbonyl group (C=O) and a hydroxyl group (–OH) bonded to the same carbon atom. This unique arrangement makes carboxylic acids behave as weak acids, because the –OH hydrogen can be lost as a proton (H⁺).

羧基 –COOH 由羰基 (C=O) 和羟基 (–OH) 连接在同一个碳原子上构成。这种独特的排列使得羧酸表现为弱酸，因为 –OH 上的氢可以以质子 (H⁺) 形式离去。

In displayed formulas, the carboxyl group is written as –COOH or shown with the C=O and –OH drawn out. The general molecular formula for a straight‑chain carboxylic acid with n carbon atoms is CₙH₂ₙO₂, but you must be able to work out the formula from the structure.

在结构式中，羧基写作 –COOH 或画出 C=O 和 –OH。直链羧酸的通式为 CₙH₂ₙO₂，但需要你能根据结构式推导出分子式。

2. Naming Carboxylic Acids | 羧酸的命名

Carboxylic acids are named by replacing the final ‘e’ of the corresponding alkane with ‘oic acid’. The –COOH carbon is always counted as carbon number 1. No number is needed for the carboxyl group because it must be at the end of the chain.

命名羧酸时，将相应烷烃词尾的 ‘e’ 替换为 ‘oic acid’。–COOH 的碳原子始终编号为 1，无需标出数字，因为羧基必须在链端。

Methanoic acid (HCOOH) – the simplest carboxylic acid, found in ant stings.
甲酸 (HCOOH) – 最简单的羧酸，存在于蚂蚁毒液中。
Ethanoic acid (CH₃COOH) – the main acid in vinegar.
乙酸 (CH₃COOH) – 食醋中的主要酸。
Propanoic acid (C₂H₅COOH) – three‑carbon acid.
丙酸 (C₂H₅COOH) – 含有三个碳原子的羧酸。
Butanoic acid (C₃H₇COOH) – smells unpleasant, found in rancid butter.
丁酸 (C₃H₇COOH) – 气味难闻，存在于腐臭黄油中。

3. Physical Properties – Why High Boiling Points? | 物理性质 – 为何沸点高？

Carboxylic acids have significantly higher boiling points than alkanes and alcohols of similar molecular mass. Ethanoic acid boils at 118 °C, while ethanol boils at 78 °C. This is because carboxylic acid molecules form strong hydrogen bonds and exist as dimers in the liquid state, held together by two hydrogen bonds.

羧酸的沸点明显高于相对分子质量相近的烷烃和醇。乙酸沸点为 118 °C，而乙醇仅为 78 °C。这是因为羧酸分子间形成强氢键，在液态时以二聚体形式存在，两个分子通过两个氢键连接。

Carboxylic acids with up to four carbons are miscible with water in all proportions, as the –COOH group can hydrogen‑bond with water molecules. As the hydrocarbon chain grows longer, solubility decreases because the non‑polar part dominates.

含有至多四个碳原子的羧酸可与水以任意比混溶，因为 –COOH 基团能与水分子形成氢键。随着碳链增长，非极性部分占主导，溶解度下降。

4. Weak Acidity and Ionisation | 弱酸性及电离

Carboxylic acids are weak acids. In water, they only partially ionise, forming a small concentration of H⁺ ions and carboxylate ions. For ethanoic acid, the reversible reaction is:

羧酸属于弱酸。在水中它们仅发生部分电离，产生低浓度的 H⁺ 和羧酸根离子。乙酸的可逆电离方程式为：

CH₃COOH ⇌ CH₃COO⁻ + H⁺

This is why carboxylic acids have a pH of around 3–4 in typical solutions, higher (less acidic) than strong acids like HCl of the same concentration. They turn blue litmus red, but do not react as vigorously with metals as strong acids do.

这就是为何羧酸溶液的 pH 通常在 3–4 左右，比同浓度的强酸（如 HCl）酸性弱。它们能使蓝色石蕊试纸变红，但与金属的反应不如强酸剧烈。

5. Reactions with Metals, Bases, and Carbonates | 与金属、碱和碳酸盐的反应

Despite being weak, carboxylic acids undergo the typical reactions of acids. Ethanoic acid reacts with magnesium to produce hydrogen gas:

尽管酸性较弱，羧酸仍能发生酸的典型反应。乙酸与镁反应生成氢气：

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

It neutralises metal oxides and hydroxides, forming a salt and water. With sodium hydroxide, the reaction is:

它能中和金属氧化物和氢氧化物，生成盐和水。与氢氧化钠的反应为：

CH₃COOH + NaOH → CH₃COONa + H₂O

With carbonates, effervescence of carbon dioxide occurs – a classic test for an acid. Ethanoic acid with sodium carbonate:

与碳酸盐反应则产生二氧化碳气泡 – 这是检验酸性物质的经典方法。乙酸与碳酸钠的反应：

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O

Exam tip: in all these reactions, the organic product is a salt called an ethanoate (acetate). The functional group becomes –COO⁻. You must be able to name the salt, e.g., sodium ethanoate.

考试技巧：这些反应中，有机产物均为乙酸盐（醋酸根盐）。官能团变为 –COO⁻。必须会命名该盐，如乙酸钠。

6. Esterification – Making Esters | 酯化反应 – 酯的生成

One of the most important reactions in the IGCSE syllabus is esterification. Carboxylic acid reacts with an alcohol in the presence of a concentrated sulfuric acid catalyst (which also acts as a dehydrating agent) to form an ester and water. The reaction is reversible and requires heating under reflux.

IGCSE 考纲中最重要的反应之一就是酯化反应。羧酸与醇在浓硫酸催化并作为脱水剂的条件下加热，生成酯和水。该反应可逆，需要回流加热。

The general equation is:

反应通式为：

Carboxylic acid + Alcohol ⇌ Ester + Water

For ethanoic acid and ethanol:

以乙酸和乙醇为例：

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

The product is ethyl ethanoate – a sweet‑smelling ester used in solvents and flavourings. To shift the equilibrium to the right, the ester is often distilled out as it forms.

产物是乙酸乙酯 – 一种带有甜味的酯，常用作溶剂和香精。为使平衡右移，常在生成过程中将酯蒸馏分离。

7. Naming Esters Made Simple | 酯的命名简明指南

Naming an ester is straightforward: the first part comes from the alcohol, ending in ‘‑yl’, and the second part comes from the carboxylic acid, ending in ‘‑oate’. Ethanol + methanoic acid → ethyl methanoate. Methanol + propanoic acid → methyl propanoate.

酯的命名简单明了：第一部分来自醇，以 ‘‑yl’ 结尾；第二部分来自羧酸，以 ‘‑oate’ 结尾。乙醇 + 甲酸 → 甲酸乙酯。甲醇 + 丙酸 → 丙酸甲酯。

You must be able to write the structural formula of an ester given its name, or deduce the name from the formula. Look at the acyl group (from the acid) and the alkyl group (from the alcohol) split by the oxygen atom.

必须能根据名称写出酯的结构式，或由结构式推导名称。识别酰基（来自酸）和烷基（来自醇），它们由氧原子连接。

8. Hydrolysis of Esters | 酯的水解

Esters can be broken down by heating with water and either an acid or an alkali catalyst. Acidic hydrolysis is the reverse of esterification and produces the original carboxylic acid and alcohol. Alkaline hydrolysis uses a strong base like NaOH and produces the sodium salt of the carboxylic acid plus the alcohol – this is often called saponification.

酯可在酸或碱催化下与水加热分解。酸性水解是酯化的逆反应，得到原来的羧酸和醇。碱性水解使用强碱如 NaOH，生成羧酸钠盐和醇 – 常称为皂化反应。

Equation for alkaline hydrolysis of ethyl ethanoate:

乙酸乙酯碱性水解的方程式：

CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH

This reaction is irreversible because the carboxylate salt is not easily converted back to the acid. In exam questions, describe the conditions: heat with aqueous sodium hydroxide, then acidify if you need to isolate the carboxylic acid.

该反应不可逆，因为羧酸盐不易变回羧酸。作答时说明条件：与氢氧化钠水溶液加热，如需分离羧酸可再酸化。

9. Preparation of Carboxylic Acids – Oxidation of Primary Alcohols | 羧酸的制备 – 伯醇氧化

Carboxylic acids can be prepared in the lab by oxidising primary alcohols. The oxidising agent is acidified potassium dichromate(VI), K₂Cr₂O₇, with dilute sulfuric acid. The mixture is heated under reflux to ensure complete oxidation to the carboxylic acid.

实验室中可通过氧化伯醇制备羧酸。氧化剂为酸化重铬酸钾 (VI) (K₂Cr₂O₇) 加稀硫酸。混合物需回流加热，确保完全氧化为羧酸。

The alcohol is first oxidised to an aldehyde, then quickly to the carboxylic acid. Ethanol is oxidised to ethanoic acid:

醇先被氧化成醛，然后迅速被氧化成羧酸。乙醇氧化成乙酸：

C₂H₅OH + 2[O] → CH₃COOH + H₂O

Observation: the orange dichromate solution turns green as Cr³⁺ ions are formed. In an exam, remember to use [O] to represent oxygen from the oxidising agent. Distinguishing from partial oxidation to aldehyde requires reflux, not distillation.

观察现象：橙色重铬酸钾溶液变为绿色（生成 Cr³⁺ ）。考试中记得用 [O] 表示来自氧化剂的氧原子。为区别于部分氧化成醛，必须使用回流而非蒸馏。

10. Common Carboxylic Acids and Their Uses | 常见羧酸及其用途

Carboxylic Acid / 羧酸	Formula / 化学式	Common Use / 常见用途
Methanoic acid / 甲酸	HCOOH	Preservative, coagulant in rubber latex / 防腐剂、橡胶乳液的凝固剂
Ethanoic acid / 乙酸	CH₃COOH	Vinegar (4–8% solution), food additive, production of esters, plastic / 食醋 (4–8% 溶液)、食品添加剂、酯类及塑料生产
Propanoic acid / 丙酸	C₂H₅COOH	Food preservative (mould inhibitor) / 食品防腐剂（抑制霉菌）
Citric acid / 柠檬酸	C₆H₈O₇	Flavouring, cosmetics, descaling agent / 调味剂、化妆品、除垢剂

IGCSE questions often ask you to link the structure to properties. For instance, the long non‑polar chain in higher carboxylic acids explains their low solubility and use in soaps.

IGCSE 考题常要求将结构与性质关联。例如高级羧酸的长非极性链解释了其低溶解度以及在肥皂中的应用。

11. Spot the Pitfalls – Common Mistakes | 常见错误与避坑指南

Forgetting that carboxylic acids are weak acids. Do not write complete ionisation or give them a pH of 1 – they partially ionise.
忘记羧酸是弱酸。不要写出完全电离或给出 pH=1 的答案 – 它们仅部分电离。
Writing ‘ethanoate’ as the product but missing the metal ion – always write the full salt formula, e.g., sodium ethanoate CH₃COONa.
产物写成 ‘ethanoate’ 却漏掉金属离子 – 务必写出完整的盐化学式，如 CH₃COONa。
Mixing up ester naming: the alcohol part goes first (methyl, ethyl), the acid part second (methanoate, ethanoate).
酯的命名搞混：醇的部分在前 (methyl, ethyl)，酸的部分在后 (methanoate, ethanoate)。
Using distillation instead of reflux for oxidation of alcohol to carboxylic acid – distillation would collect the intermediate aldehyde.
醇氧化成羧酸时错用蒸馏而非回流 – 蒸馏会收集到中间体醛。
Omitting the concentrated H₂SO₄ catalyst in esterification or labelling it as a reactant instead of a catalyst.
酯化反应漏写浓硫酸催化剂，或将其误标为反应物而非催化剂。

12. Quick Exam Strategy: 3‑Mark Ester Question | 快速应试策略：3 分酯类题的满分框架

A typical OCR question: Name the ester formed from propanoic acid and methanol, and write the equation. To secure 3 marks: (1) Correct name – methyl propanoate. (2) Correct structural isomers or displayed formula showing the ester linkage. (3) Equation with conditions: CH₃OH + C₂H₅COOH ⇌ C₂H₅COOCH₃ + H₂O, mention concentrated H₂SO₄ and heat. Avoiding common naming errors alone often lifts a grade boundary.

典型 OCR 考题：写出丙酸与甲醇生成的酯的名称及方程式。 获得三分的框架：(1) 正确命名 – 丙酸甲酯。(2) 正确写出结构式或显示酯键的结构简式。(3) 写出带条件的方程式：CH₃OH + C₂H₅COOH ⇌ C₂H₅COOCH₃ + H₂O，提及浓 H₂SO₄ 与加热。单是避免命名错误就能提升一个等级。

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IGCSE OCR Chemistry: Carboxylic Acids – Key Points Revision | IGCSE OCR 化学：羧酸 考点精讲