Carboxylic acids

Edexcel International GCSE Chemistry· 4.34C–4.37C (Section S4, 4(f))· 15 min read

1. 1. Structure and Functional Group of Carboxylic Acids★★☆☆☆Higher only⏱ 3 min

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📘 Definition

Carboxylic acid

General formula for unbranched chains: $C_{n} H_{2 n + 1} C O O H$ (n ≥ 0)

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

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.

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

2. 2. Naming and Drawing the First Four Unbranched Carboxylic Acids★★★☆☆Higher only⏱ 4 min

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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	$H - C (= O) - O H$
2	Ethanoic acid	$C H_{3} C O O H$	$C H_{3} - C (= O) - O H$
3	Propanoic acid	$C H_{3} C H_{2} C O O H$	$C H_{3} C H_{2} - C (= O) - O H$
4	Butanoic acid	$C H_{3} C H_{2} C H_{2} C O O H$	$C H_{3} C H_{2} C H_{2} - C (= O) - O H$

📐 Worked Example

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

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Step 1: 3 total carbon atoms uses the prefix prop-, paired with the suffix -oic acid, so the name is propanoic acid.
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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.
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Step 3: Complete the structure by adding hydrogen atoms to the remaining carbon atoms to satisfy their 4-bond valency.
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$H\thspace{0.5em} H\thspace{0.5em} O\\|\thspace{0.5em} |\thspace{0.5em} ||\H-C-C-C-OH\\|\thspace{0.5em} |\H\thspace{0.5em} H$

✓ Quick check

What is the correct name of $C H_{3} C H_{2} C H_{2} C O O H$ ?
- Propanoic acid
- Butanoic acid
- Ethanoic acid
- Methanoic acid
Reveal answer
Butanoic acid —
The formula has 4 total carbon atoms, so uses the prefix but- and suffix -oic acid.

3. 3. Reactions of Carboxylic Acids★★★★☆Higher only⏱ 5 min

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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): $carboxylic acid + metal \to carboxylate salt + hydrogen$
Reaction with metal carbonates (e.g. sodium carbonate, calcium carbonate): $carboxylic acid + metal carbonate \to carboxylate salt + water + carbon dioxide$

📐 Worked Example

Write a balanced symbol equation with state symbols for the reaction between aqueous ethanoic acid ( $C H_{3} C O O H (a q)$ ) and solid sodium carbonate ( $N a_{2} C O_{3} (s)$ ).

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

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.

4. 4. Ethanoic Acid and Vinegar★☆☆☆☆Higher only⏱ 2 min

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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.

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

5. Common Pitfalls

Wrong move:

Counting the carboxyl carbon as extra when naming, e.g. naming $C H_{3} C O O H$ as methanoic acid

Why:

The carboxyl group contains one carbon atom that is part of the main carbon chain length used for naming prefixes

Correct move:

Count all carbon atoms including the one in the -COOH group when choosing the naming prefix, so $C H_{3} C O O H$ (2 C atoms) is ethanoic acid

Wrong move:

Writing the formula of sodium ethanoate as $C H_{3} C O O N a_{2}$

Why:

The ethanoate ion has a charge of -1, so only one +1 sodium ion is needed to balance the charge

Correct move:

Balance ionic charges when writing salt formulae: $C H_{3} C O O^{-} + N a^{+} = C H_{3} C O O N a$

Wrong move:

Forgetting $C O_{2}$ is a product of reactions with carbonates, mixing it up with $H_{2}$ from metal reactions

Why:

Confusion between general acid reaction product patterns

Correct move:

Use the mnemonics MASH (Metal + Acid → Salt + Hydrogen) and CASHWAC (Carbonate + Acid → Salt + Water + Carbon dioxide) to remember product sets

Wrong move:

Drawing the carboxyl group as -C-O-OH instead of -C(=O)-OH in displayed formulae

Why:

Forgetting the double bond between the carbon and one oxygen atom in the carboxyl group

Correct move:

Always show the C=O double bond when drawing displayed formulae of the carboxyl functional group

Wrong move:

Stating vinegar is pure ethanoic acid

Why:

Vinegar is a dilute aqueous solution, not pure acid

Correct move:

Recall that vinegar is an aqueous (water-based) solution containing ethanoic acid as its active acidic component

6. Quick Reference 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 + $H_{2} O$ + $C O_{2}$
Vinegar composition	Dilute aqueous solution of ethanoic acid

7. Frequently Asked

Is this topic tested in Edexcel Double Award Science?

No, all carboxylic acid content covered here is exclusive to the single Chemistry IGCSE (4CH1) Paper 2C, and does not appear in the 4SD0 Double Award specification.

Do I need to know esterification reactions for this sub-topic?

No, ester formation from carboxylic acids and alcohols is covered in the next sub-topic (S4_T07: Esters) for this specification. This sub-topic only requires knowledge of reactions with metals and metal carbonates.

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)