Carboxylic Acids | Chemistry

📖 In-Depth Theory

Structure of Carboxylic Acids

CARBOXYLIC ACIDS contain the functional group –COOH (carboxyl group).

First four members:

Methanoic acid: HCOOH

Ethanoic acid: CH₃COOH (the acid in vinegar)

Propanoic acid: C₃H₅COOH

Butanoic acid: C₄H₇COOH

NAMING:

Named from the corresponding alkane — replace '-e' with '-anoic acid'.

meth- → methanoic acid; eth- → ethanoic acid; prop- → propanoic acid.

PROPERTIES:

Acidic — donate H⁺ ions in solution.

Lower members dissolve in water — miscible.

Higher members less soluble.

Ethanoic acid (CH₃COOH) is the main component of vinegar.

Reactions of Carboxylic Acids

1. REACTION WITH CARBONATES:

Carboxylic acid + metal carbonate → salt + water + carbon dioxide

CH₃COOH + Na₂CO₃ → CH₃COONa + H₂O + CO₂

Ethanoic acid + sodium carbonate → sodium ethanoate + water + CO₂

Same pattern as mineral acids reacting with carbonates.

CO₂ gas produced — effervescence (bubbling).

2. REACTION WITH METALS:

Carboxylic acid + metal → salt + hydrogen gas

More dilute/weaker reaction than mineral acids.

3. DISSOLVING IN WATER:

Forms weakly acidic solution — pH 3–5 (not as low as strong acids at the same concentration).

Due to weak dissociation.

4. REACTION WITH ALCOHOLS — ESTERIFICATION:

Carboxylic acid + alcohol → ester + water

CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O

Ethanoic acid + ethanol ⇌ ethyl ethanoate + water

Conditions: acid catalyst (H₂SO₄), warm, reversible reaction.

Esters have characteristic fruity smells — used in flavourings and perfumes.

Uses and Natural Sources

ETHANOIC ACID (CH₃COOH):

Main component of vinegar (typically 4–8% solution).

Preservative — inhibits bacterial growth.

Produced by oxidation of ethanol (e.g. wine left open).

METHANOIC ACID (HCOOH):

Found in ant stings (causes the burning sensation).

Used as a preservative in silage.

ESTERS:

Esters are formed from carboxylic acids + alcohols.

Fruity smells: ethyl ethanoate (nail polish remover), amyl acetate (banana), isoamyl acetate.

Used in flavourings, perfumes, solvents, plasticisers.

FATTY ACIDS:

Long-chain carboxylic acids (e.g. stearic acid, oleic acid).

Components of fats and oils — reacted with glycerol to form triglycerides.

CARBOXYLIC ACIDS AND pH:

Weaker than hydrochloric or sulfuric acid at the same concentration.

pH around 3–5 for typical dilute solutions.

Incompletely dissociate in water — equilibrium mixture of HA ⇌ H⁺ + A⁻.

⚠️ Common Mistake

Carboxylic acids are WEAK acids — they do not fully dissociate in water unlike HCl or H₂SO₄. At the same concentration, carboxylic acid has a HIGHER pH than a strong acid. Esterification is REVERSIBLE — both ester + water and acid + alcohol are present at equilibrium.

📐 Key Equations

Acid + carbonate → salt + water + CO₂

Acid + alcohol → ester + water (acid catalyst)

📌 Key Note

Carboxylic acids: –COOH group. Ethanoic acid = vinegar. Reactions: + carbonate (salt + H₂O + CO₂), + metals (salt + H₂), + alcohol (ester + water, acid catalyst, reversible). Weak acids — partial dissociation. Esters: fruity smells, used in flavourings and perfumes.

🎯 Matching Activity — Carboxylic Acid Reactions

Match each reaction to the products formed. — drag the symbols on the right to match the component names on the left.

Ethanoic acid + sodium carbonate

Drop here

Ethanoic acid + ethanol

Drop here

Ethanoic acid dissolved in water

Drop here

Ethanol oxidised by bacteria

Drop here

Weakly acidic solution — partial dissociation, pH ~3–4

Ethyl ethanoate + water — esterification with acid catalyst

Sodium ethanoate + water + CO₂ — effervescence observed

Ethanoic acid — how wine turns to vinegar

⭐ Higher Tier Only

Explain carboxylic acids as weak acids — partial ionisation: RCOOH ⇌ RCOO⁻ + H⁺. Compare pH of carboxylic acid with equivalent strong acid concentration. Write equations for esterification. Explain the reversibility of esterification and how equilibrium can be shifted.

🔬 Triple Science Only

Carboxylic acids (4.7.2.4) is chemistry-only — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. What type of compound forms when ethanoic acid reacts with ethanol, and what are the conditions?

2. Why does ethanoic acid have a higher pH than hydrochloric acid at the same concentration?

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🧪 Carboxylic Acids

Structure of Carboxylic Acids

Reactions of Carboxylic Acids

Uses and Natural Sources

Mr. Badmus AI