📚 GCSE Chemistry: Carboxylic Acids – Key Points | GCSE 化学：羧酸 考点精讲
Carboxylic acids are a family of organic compounds containing the functional group –COOH. They appear frequently in GCSE Chemistry exam papers, especially questions on acidity, reactions with metals and carbonates, and esterification. Understanding their structure, naming system, and typical reactions is essential for achieving top marks.
羧酸是一类含有 –COOH 官能团的有机化合物。它们在 GCSE 化学试卷中出现频率很高，尤其常考查酸性、与金属和碳酸盐的反应以及酯化反应。掌握其结构、命名体系和典型反应对于获得高分至关重要。
1. Introduction to Carboxylic Acids | 羧酸简介
Carboxylic acids have the general formula R–COOH, where R represents a hydrogen atom or an alkyl group. The functional group –COOH, called the carboxyl group, consists of a carbonyl (C=O) and a hydroxyl (–OH) bonded to the same carbon atom. This arrangement gives carboxylic acids their characteristic acidic properties.
羧酸的通式为 R–COOH，其中 R 表示氢原子或烷基。官能团 –COOH 称为羧基，由羰基 (C=O) 和羟基 (–OH) 连接在同一个碳原子上构成。这种结构赋予了羧酸特有的酸性。
2. Naming Carboxylic Acids | 羧酸的命名
The first four members of the homologous series are methanoic acid (HCOOH), ethanoic acid (CH₃COOH), propanoic acid (C₂H₅COOH), and butanoic acid (C₃H₇COOH). The ‘-oic acid’ suffix indicates the presence of the –COOH group. In naming, the carbon of the carboxyl group is always counted as carbon number 1.
同系列的前四个成员分别是甲酸 (HCOOH)、乙酸 (CH₃COOH)、丙酸 (C₂H₅COOH) 和丁酸 (C₃H₇COOH)。词尾“-酸”表示 –COOH 基团的存在。命名时，羧基的碳原子始终被编为 1 号碳。
- HCOOH – methanoic acid (甲酸)
- CH₃COOH – ethanoic acid (乙酸)
- C₂H₅COOH – propanoic acid (丙酸)
- C₃H₇COOH – butanoic acid (丁酸)
3. Physical Properties | 物理性质
Carboxylic acids can form hydrogen bonds due to the presence of the –OH group in the carboxyl. As a result, they have higher boiling points than alcohols of similar molecular mass. The first four members are completely miscible with water, but solubility decreases as the hydrocarbon chain lengthens. They often have sharp or pungent smells; for example, ethanoic acid is the main component of vinegar.
由于羧基中含有 –OH 基团，羧酸能够形成氢键。因此，它们的沸点比相对分子质量相近的醇更高。前四种羧酸可与水任意混溶，但随着碳链增长，溶解度降低。它们通常具有强烈或刺鼻的气味；例如，乙酸是食醋的主要成分。
4. Acidity of Carboxylic Acids | 羧酸的酸性
Carboxylic acids are weak acids. In water, they partially ionise to produce hydrogen ions (H⁺) and carboxylate ions. For ethanoic acid, the equilibrium is: CH₃COOH ⇌ CH₃COO⁻ + H⁺. Being weak acids, they have a pH around 3–5 in aqueous solution, and they only partially dissociate, which means the reverse reaction is significant.
羧酸是弱酸。在水中，它们部分电离产生氢离子 (H⁺) 和羧酸根离子。以乙酸为例，平衡方程式为：CH₃COOH ⇌ CH₃COO⁻ + H⁺。由于是弱酸，其水溶液的 pH 值大约在 3–5 之间，且只部分解离，逆反应显著。
CH₃COOH ⇌ CH₃COO⁻ + H⁺
5. Reactions with Metals | 与金属的反应
Carboxylic acids react with reactive metals such as magnesium, zinc and iron, producing a salt and hydrogen gas. For example, ethanoic acid and magnesium give magnesium ethanoate and hydrogen: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. This is a typical acid–metal reaction, and the fizzing confirms the presence of H₂ gas.
羧酸与活泼金属（如镁、锌和铁）反应，生成盐和氢气。例如，乙酸与镁反应生成乙酸镁和氢气：2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂。这是典型的酸与金属的反应，产生的气泡可以证实氢气的存在。
2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂
6. Reactions with Bases and Alkalis | 与碱的反应
Carboxylic acids undergo neutralisation reactions with bases such as sodium hydroxide, forming a salt and water. For instance, ethanoic acid and sodium hydroxide produce sodium ethanoate and water: CH₃COOH + NaOH → CH₃COONa + H₂O. This reaction is identical to other acid–base neutralisations and is evidenced by a rise in pH.
羧酸与氢氧化钠等碱发生中和反应，生成盐和水。例如，乙酸与氢氧化钠反应生成乙酸钠和水：CH₃COOH + NaOH → CH₃COONa + H₂O。该反应与其它酸碱中和反应完全相同，可通过 pH 值的升高来证明。
CH₃COOH + NaOH → CH₃COONa + H₂O
7. Reactions with Carbonates | 与碳酸盐的反应
When carboxylic acids react with carbonates or hydrogencarbonates, they produce a salt, water and carbon dioxide gas. A typical example is ethanoic acid with sodium carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. The effervescence of CO₂ can be tested with limewater, which turns milky. This reaction is often used to distinguish carboxylic acids from alcohols.
羧酸与碳酸盐或碳酸氢盐反应时，会生成盐、水和二氧化碳气体。典型的例子是乙酸与碳酸钠反应：2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂。产生的 CO₂ 气体使澄清石灰水变浑浊，可用于检验。该反应常用来区分羧酸和醇类。
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂
8. Esterification – Making Esters | 酯化反应 – 酯的合成
Carboxylic acids react with alcohols in the presence of a concentrated sulfuric acid catalyst and heat to form esters and water. This reversible reaction is called esterification. For example, ethanoic acid and ethanol produce ethyl ethanoate, a sweet-smelling ester: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. Esters are used in flavourings, perfumes, and as solvents.
羧酸与醇在浓硫酸催化并加热的条件下反应，生成酯和水。这个可逆反应称为酯化反应。例如，乙酸与乙醇反应生成具有果香味的乙酸乙酯：CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O。酯类常用于食用香精、香水以及作为溶剂。
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
In the lab, esters are often prepared by warming a mixture of carboxylic acid and alcohol with a few drops of concentrated H₂SO₄. The ester can be identified by its fruity odour and is separated by distillation.
在实验室中，通常将羧酸和醇的混合物与几滴浓硫酸一起温热来制备酯。生成的酯可根据其水果香味鉴别，并通过蒸馏分离。
9. Comparison of Carboxylic Acids with Alcohols and Alkanes | 羧酸、醇和烷烃的比较
It is important to contrast carboxylic acids with other organic families. The table below summarises key differences in functional groups, physical properties and chemical behaviour.
将羧酸与其它有机家族进行比较十分重要。下表总结了它们在官能团、物理性质和化学行为方面的主要区别。
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