📚 Carboxylic Acids for IGCSE CCEA Chemistry | IGCSE CCEA 化学：羧酸 考点精讲
Carboxylic acids are an essential homologous series in organic chemistry, characterised by the functional group –COOH. For CCEA IGCSE Chemistry, students must be able to recognise their structure, name the first few members, and recall their typical reactions, including weak acid behaviour and esterification. This article covers every key specification point, linking structure to properties and reactions, with exam-focused explanations.
羧酸是有机化学中一类重要的同系物，其官能团是 –COOH。在 CCEA IGCSE 化学中，学生需要能够识别其结构、命名前几个成员，并记住它们的典型反应，包括弱酸性和酯化反应。本文紧扣考纲要点，将结构与性质、反应联系起来，提供考试导向的讲解。
1. The Functional Group –COOH | 官能团 –COOH
The carboxyl group consists of a carbonyl group (C=O) and a hydroxyl group (–OH) attached to the same carbon atom. In shorthand, it is written as –COOH or –CO₂H. Because the hydroxyl hydrogen can be released as a proton, this group confers acidic properties on the molecule.
羧基由一个羰基 (C=O) 和一个羟基 (–OH) 连接在同一个碳原子上组成。简写为 –COOH 或 –CO₂H。由于羟基上的氢可以以质子形式释放，该官能团使分子具有酸性。
2. General Formula and Homologous Series | 通式与同系物
Carboxylic acids follow the general formula CₙH₂ₙ₊₁COOH (or CₙH₂ₙO₂ for the fully written form). As a homologous series, they share similar chemical properties and a gradual trend in physical properties. The first four members are methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (C₂H₅COOH) and butanoic acid (C₃H₇COOH).
羧酸的通式为 CₙH₂ₙ₊₁COOH（或完全形式 CₙH₂ₙO₂）。作为同系物，它们具有相似的化学性质，物理性质呈现递变规律。前四个成员为甲酸 (HCOOH)、乙酸 (CH₃COOH)、丙酸 (C₂H₅COOH) 和丁酸 (C₃H₇COOH)。
3. Naming Carboxylic Acids | 羧酸的命名
IUPAC names are based on the parent alkane, with the final ‘e’ replaced by ‘–oic acid’. The carboxyl carbon is always number 1 in the chain. Common names, such as acetic acid for ethanoic acid, are also accepted in some question contexts, but IGCSE CCEA expects systematic names. For example, CH₃CH₂COOH is propanoic acid, not propionic acid.
IUPAC 名称以母体烷烃为基础，将词尾的 ‘e’ 改为 ‘–oic acid’。羧基碳永远是链中的 1 号位。俗名如乙酸 (acetic acid) 在某些题目背景下也可接受，但 CCEA IGCSE 期望使用系统命名。例如 CH₃CH₂COOH 应称作丙酸 (propanoic acid)，而不是 propionic acid。
4. Physical Properties | 物理性质
The first few carboxylic acids are colourless liquids with sharp, sour smells. They have relatively high boiling points compared to similar‑sized alkanes or alcohols due to hydrogen bonding between carboxyl groups, which can form dimers. Solubility in water decreases as the hydrocarbon chain lengthens; methanoic and ethanoic acids are fully miscible, while larger acids become increasingly oily and less soluble.
前几个羧酸是具有刺激性酸味的无色液体。与同样大小的烷烃或醇相比，它们的沸点较高，这是因为羧基之间可形成氢键，甚至形成二聚体。随着碳链增长，在水中的溶解度降低；甲酸和乙酸可与水完全互溶，而更大的羧酸则变得偏油性，溶解度下降。
5. Carboxylic Acids as Weak Acids | 羧酸作为弱酸
Carboxylic acids are weak acids – they partially dissociate in water to give carboxylate ions and H⁺. This is often represented as an equilibrium: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Because the dissociation is incomplete, their pH is higher (around 3–4 for 0.1 mol dm⁻³ ethanoic acid) than that of strong mineral acids. The term ‘weak’ refers to the degree of ionisation, not concentration.
羧酸是弱酸——在水中只能部分电离，生成羧酸根离子和 H⁺。这通常用平衡表示：CH₃COOH ⇌ CH₃COO⁻ + H⁺。由于电离不完全，相同浓度下它们的 pH（如 0.1 mol dm⁻³ 乙酸约 3–4）高于强无机酸。’弱’ 指电离程度，而非浓度。
6. Reaction with Metals | 与金属的反应
Like other acids, carboxylic acids react with reactive metals (above hydrogen in the reactivity series) to produce a salt and hydrogen gas. For example: 2CH₃COOH + Mg → Mg(CH₃COO)₂ + H₂. The fizzing is slower than with strong acids, reflecting their weak acidic nature. The salt formed is a carboxylate (here magnesium ethanoate).
就像其他酸一样，羧酸能与活泼金属（金属活动性顺序中排在氢之前）反应，生成盐和氢气。例如：2CH₃COOH + Mg → Mg(CH₃COO)₂ + H₂。其冒泡速度比强酸慢，反映出其弱酸性。生成的盐是羧酸盐（此处为乙酸镁）。
7. Reaction with Bases and Alkalis | 与碱的反应
Neutralisation with metal oxides or hydroxides yields a carboxylate salt and water. For instance: CH₃COOH + NaOH → CH₃COONa + H₂O. This is a typical acid–base reaction. The resulting solutions contain the carboxylate ion, which is the conjugate base, often used in buffers.
与金属氧化物或氢氧化物发生中和反应，生成羧酸盐和水。例如：CH₃COOH + NaOH → CH₃COONa + H₂O。这是一个典型的酸碱反应。所得溶液含有羧酸根离子，即共轭碱，常用于缓冲溶液。
8. Reaction with Carbonates and Hydrogencarbonates | 与碳酸盐和碳酸氢盐的反应
Carboxylic acids react with carbonates to form a salt, water and carbon dioxide gas. The test for CO₂ (limewater turns milky) confirms the reaction. Example: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O. Hydrogencarbonates follow a similar pattern but with a 1:1 ratio for the acid. This reaction is useful for distinguishing carboxylic acids from other organic compounds.
羧酸与碳酸盐反应生成盐、水和二氧化碳气体。用石灰水变浑浊的检验可确认 CO₂ 的生成。例：2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O。与碳酸氢盐的反应类似，但酸与盐的摩尔比为 1:1。这一反应常用于区分羧酸和其他有机化合物。
9. Esterification (Reaction with Alcohols) | 酯化反应（与醇的反应）
One of the most important reactions at IGCSE is esterification. A carboxylic acid reacts with an alcohol in the presence of a strong acid catalyst (usually concentrated H₂SO₄), under heating, to form an ester and water. The reaction is reversible; the ester has a sweet, fruity smell. General equation: RCOOH + R’OH ⇌ RCOOR’ + H₂O. For example, ethanoic acid + ethanol ⇌ ethyl ethanoate + water.
IGCSE 最重要的反应之一是酯化。在强酸催化剂（通常是浓硫酸）存在下加热，羧酸与醇反应生成酯和水。该反应可逆；酯具有甜美的果香味。通式：RCOOH + R’OH ⇌ RCOOR’ + H₂O。例如，乙酸 + 乙醇 ⇌ 乙酸乙酯 + 水。
10. Esters – Naming and Uses | 酯的命名与用途
The ester name is derived from the alcohol (alkyl part) and the carboxylic acid (with the ending –oate). Ethyl ethanoate comes from ethanol and ethanoic acid. Esters are used as solvents, plasticisers, and in food flavourings and perfumes. Recognising esters and their formation from carboxylic acids is a common CCEA exam question.
酯的名称来自醇（烷基部分）和羧酸（酸的部分改为 –oate 结尾）。乙酸乙酯来自乙醇和乙酸。酯可用作溶剂、增塑剂，以及食品调味剂和香水。识别酯以及从羧酸生成酯是 CCEA 常考题目。
11. Comparing Carboxylic Acids with Mineral Acids | 羧酸与无机酸的比较
Carboxylic acids are weaker electrolytes, have higher pH at the same concentration, and react more slowly with metals and carbonates than strong acids like HCl or H₂SO₄. However, they share typical acid behaviour: turning blue litmus red, neutralising bases, and releasing CO₂ from carbonates. Understanding the concept of ‘weak’ vs ‘strong’ as well as ‘dilute’ vs ‘concentrated’ is vital.
与盐酸、硫酸等强酸相比，羧酸是较弱的电解质，在相同浓度下 pH 更高，与金属和碳酸盐的反应更慢。但它们仍具有酸的典型行为：使蓝色石蕊试纸变红、中和碱、与碳酸盐反应放出 CO₂。理解 ‘强/弱’ 与 ‘浓/稀’ 的区别至关重要。
12. Summary of Key Reactions and Exam Tips | 关键反应总结与应试技巧
Remember the core equations: acid + metal → salt + H₂; acid + base → salt + H₂O; acid + carbonate → salt + CO₂ + H₂O; acid + alcohol ⇌ ester + H₂O. In CCEA exams, be prepared to draw displayed formulae of the first few acids and esters, label the functional group, and explain why carboxylic acids are weak acids. Always write balanced equations and include state symbols if required.
牢记核心方程式：酸 + 金属 → 盐 + H₂；酸 + 碱 → 盐 + H₂O；酸 + 碳酸盐 → 盐 + CO₂ + H₂O；酸 + 醇 ⇌ 酯 + H₂O。在 CCEA 考试中，要准备好画出前几个羧酸和酯的显示式，标出官能团，并解释为什么羧酸是弱酸。始终书写配平的方程式，并根据要求注明状态符号。
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