# Carboxylic acids

> Edexcel International GCSE Chemistry · 4CH1 2017
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-carboxylic-acids/

This guide covers all Edexcel IGCSE Chemistry (4CH1) required content for carboxylic acids, including structure, naming, key reactions, and vinegar composition, exclusive to Paper 2C.

**Prerequisites:** [Naming and drawing basic organic compounds](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-introduction-to-organic-chemistry/); [General acid reactions with metals and carbonates](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s2-acids-bases-salts/)

## Learning objectives

- Identify the carboxyl (-COOH) functional group as the characteristic group of carboxylic acids
- Name and draw structural/displayed formulae for unbranched carboxylic acids with up to 4 carbon atoms
- Write balanced equations with state symbols for reactions of carboxylic acids with reactive metals and metal carbonates
- Recall that vinegar is an aqueous solution of ethanoic acid

## 1. Structure and Functional Group of Carboxylic Acids

**Carboxylic acid** — A homologous series of organic compounds defined by the presence of the carboxyl (-COOH) functional group, located at the end of the carbon chain.

*Notation:* General formula for unbranched chains: $C_nH_{2n+1}COOH$ (n ≥ 0)

The carboxyl group counts as one carbon atom when calculating total chain length for naming. Carboxylic acids are weak acids that partially ionise in aqueous solution to release $H^+$ ions, so they exhibit typical acidic properties.

**Worked example:** Identify the functional group and homologous series of the compound with formula HCOOH.

1. Step 1: Locate the terminal group in the formula: -COOH, which is the carboxyl functional group.
2. Step 2: Compounds containing the carboxyl group belong to the carboxylic acid homologous series.

> **tip**
>
> Never forget the double bond between the carbon and one oxygen atom in the carboxyl group when drawing displayed formulae.

*Calculator:* allowed

## 2. Naming and Drawing the First Four Unbranched Carboxylic Acids

Unbranched carboxylic acids are named using a prefix indicating the total number of carbon atoms in the chain (including the carboxyl carbon) followed by the suffix *-oic acid*.

| Total C atoms | Name | Structural formula | Displayed formula shorthand |
| --- | --- | --- | --- |
| 1 | Methanoic acid | HCOOH | $H-C(=O)-OH$ |
| 2 | Ethanoic acid | $CH_3COOH$ | $CH_3-C(=O)-OH$ |
| 3 | Propanoic acid | $CH_3CH_2COOH$ | $CH_3CH_2-C(=O)-OH$ |
| 4 | Butanoic acid | $CH_3CH_2CH_2COOH$ | $CH_3CH_2CH_2-C(=O)-OH$ |

**Worked example:** Give the correct name and draw the displayed formula of the unbranched carboxylic acid with 3 total carbon atoms.

1. Step 1: 3 total carbon atoms uses the prefix *prop-*, paired with the suffix *-oic acid*, so the name is propanoic acid.
2. Step 2: The terminal carboxyl group has a C double-bonded to O, single-bonded to OH, and single-bonded to the second carbon in the 3-carbon chain.
3. Step 3: Complete the structure by adding hydrogen atoms to the remaining carbon atoms to satisfy their 4-bond valency.
4. $$H\thspace{0.5em} H\thspace{0.5em} O\\|\thspace{0.5em} |\thspace{0.5em} ||\H-C-C-C-OH\\|\thspace{0.5em} |\H\thspace{0.5em} H$$

**Check your understanding**

1. What is the correct name of $CH_3CH_2CH_2COOH$?

   - Propanoic acid
   - Butanoic acid
   - Ethanoic acid
   - Methanoic acid

   *Why:* The formula has 4 total carbon atoms, so uses the prefix *but-* and suffix *-oic acid*.

*Calculator:* allowed

## 3. Reactions of Carboxylic Acids

As weak acids, carboxylic acids undergo the same core reaction patterns as inorganic acids with reactive metals and metal carbonates. The salts formed are named with the suffix *-oate* (e.g. ethanoic acid forms ethanoate salts).

- Reaction with reactive metals (e.g. magnesium, sodium): $$\text{carboxylic acid} + \text{metal} \rightarrow \text{carboxylate salt} + \text{hydrogen}$$
- Reaction with metal carbonates (e.g. sodium carbonate, calcium carbonate): $$\text{carboxylic acid} + \text{metal carbonate} \rightarrow \text{carboxylate salt} + \text{water} + \text{carbon dioxide}$$

**Worked example:** Write a balanced symbol equation with state symbols for the reaction between aqueous ethanoic acid ($CH_3COOH(aq)$) and solid sodium carbonate ($Na_2CO_3(s)$).

1. Step 1: Identify products: sodium ethanoate salt, water, and carbon dioxide.
2. Step 2: Write the unbalanced equation: $CH_3COOH(aq) + Na_2CO_3(s) \rightarrow CH_3COONa(aq) + H_2O(l) + CO_2(g)$
3. Step 3: Balance sodium: 2 $CH_3COONa$ on the product side requires 2 $CH_3COOH$ on the reactant side.
4. Step 4: Verify all atoms are balanced: C=4, H=8, O=8, Na=2 on both sides of the equation.
5. $$2CH_3COOH(aq) + Na_2CO_3(s) \rightarrow 2CH_3COONa(aq) + H_2O(l) + CO_2(g)$$

> **Exam tip:** Always include state symbols in balanced reaction equations for these reactions, as marks are explicitly awarded for correct states in Edexcel IGCSE Chemistry exams.

*Calculator:* allowed

## 4. Ethanoic Acid and Vinegar

Ethanoic acid is the most common household carboxylic acid. Vinegar is a dilute aqueous solution of ethanoic acid, typically containing 5-20% ethanoic acid by volume, used for cooking and cleaning.

**Worked example:** A student adds a small piece of magnesium ribbon to a sample of vinegar. Name the gas released and the salt formed in the reaction.

1. Step 1: Vinegar contains ethanoic acid, which reacts with magnesium (a reactive metal) to produce hydrogen gas and a magnesium ethanoate salt.
2. Step 2: Gas = hydrogen; Salt = magnesium ethanoate.

> **note**
>
> The formation of ethanoic acid via oxidation of ethanol is covered in the alcohols sub-topic (S4_T05), so you do not need to recall that reaction for this sub-topic.

*Calculator:* allowed

## Common pitfalls

- **Wrong:** Counting the carboxyl carbon as extra when naming, e.g. naming $CH_3COOH$ as methanoic acid
  - Why it fails: The carboxyl group contains one carbon atom that is part of the main carbon chain length used for naming prefixes
  - Correct: Count all carbon atoms including the one in the -COOH group when choosing the naming prefix, so $CH_3COOH$ (2 C atoms) is ethanoic acid
- **Wrong:** Writing the formula of sodium ethanoate as $CH_3COONa_2$
  - Why it fails: The ethanoate ion has a charge of -1, so only one +1 sodium ion is needed to balance the charge
  - Correct: Balance ionic charges when writing salt formulae: $CH_3COO^- + Na^+ = CH_3COONa$
- **Wrong:** Forgetting $CO_2$ is a product of reactions with carbonates, mixing it up with $H_2$ from metal reactions
  - Why it fails: Confusion between general acid reaction product patterns
  - Correct: Use the mnemonics MASH (Metal + Acid → Salt + Hydrogen) and CASHWAC (Carbonate + Acid → Salt + Water + Carbon dioxide) to remember product sets
- **Wrong:** Drawing the carboxyl group as -C-O-OH instead of -C(=O)-OH in displayed formulae
  - Why it fails: Forgetting the double bond between the carbon and one oxygen atom in the carboxyl group
  - Correct: Always show the C=O double bond when drawing displayed formulae of the carboxyl functional group
- **Wrong:** Stating vinegar is pure ethanoic acid
  - Why it fails: Vinegar is a dilute aqueous solution, not pure acid
  - Correct: Recall that vinegar is an aqueous (water-based) solution containing ethanoic acid as its active acidic component

## Cheatsheet

| Concept | Key Recall Point |
| --- | --- |
| Functional group | -COOH (carboxyl group, counts as 1 C atom) |
| Naming convention | Prefix (total C count) + -oic acid: 1C=methanoic, 2C=ethanoic, 3C=propanoic, 4C=butanoic |
| Salt naming | Metal name + -oate (e.g. sodium ethanoate) |
| Metal reaction | Acid + reactive metal → carboxylate salt + $H_2$ |
| Carbonate reaction | Acid + metal carbonate → carboxylate salt + $H_2O$ + $CO_2$ |
| Vinegar composition | Dilute aqueous solution of ethanoic acid |

## What's next

Now that you have mastered carboxylic acid structure, naming and core reactions, you are ready to progress to related organic chemistry sub-topics in the Edexcel IGCSE Chemistry specification. Esterification, the reaction of carboxylic acids with alcohols to form esters, is the next sub-topic and builds directly on the naming conventions and functional group knowledge you have learned here. You can also revisit the alcohols sub-topic to review how ethanoic acid is formed via oxidation of ethanol, or revise general acid-base reaction rules to reinforce your understanding of the reaction patterns covered in this guide. All content in this sub-topic is exclusive to Paper 2C, so focus your revision on this paper if you are sitting the single Chemistry IGCSE award.

- [Alcohols (S4_T05)](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-alcohols/)

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