Carboxylic Acids

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CognitoCarboxylic acids and esters

Key facts & methods

Carboxylic acid structure and properties

Functional group: -COOH (carboxyl group).
Methanoic acid HCOOH, Ethanoic acid CH3COOH, Propanoic acid C2H5COOH.
Weak acids — partially ionise in water: CH3COOH ⇌ CH3COO- + H+.
Lower pH than strong acids of same concentration.
Soluble in water. Have distinctive sharp/vinegary smell.
Ethanoic acid is the acid in vinegar.

Reactions of carboxylic acids

With metals: carboxylic acid + metal → metal salt + hydrogen. E.g. 2CH3COOH + Mg → Mg(CH3COO)2 + H2.
With carbonates: carboxylic acid + carbonate → salt + water + CO2. E.g. CH3COOH + Na2CO3 → salt + H2O + CO2.
With alkalis: neutralisation. CH3COOH + NaOH → CH3COONa + H2O.
React like other acids — same reaction types, but weaker (less vigorous).

Esterification

Carboxylic acid + alcohol → ester + water.
E.g. ethanoic acid + ethanol ⇌ ethyl ethanoate + water. Acid catalyst (H2SO4).
Esters have fruity smells — used in flavourings and perfumes.
The reaction is reversible (note ⇌ sign).
Ester name: first part from alcohol (ethyl from ethanol), second from acid (ethanoate from ethanoic acid).

Exam questions — 4 questions · 10 marks · AQA 8462 style

1 markFunctional groupAQA 8462

Which functional group is present in carboxylic acids?

A -OH only

B -COOH (carboxyl group)

C -C=C-

D -CHO

3 marksReaction with carbonateAQA 8462

Describe what you would observe when ethanoic acid is added to sodium carbonate. Write a word equation. (3 marks)

+30 XP

1 markEster formationAQA 8462

Which two functional groups react together to form an ester?

A Two alcohol groups (-OH)

B An alcohol (-OH) and a carboxylic acid (-COOH)

C An alkene and a carboxylic acid

D Two carboxylic acid groups

3 marksWeak acid comparisonAQA 8462 Higher

Ethanoic acid and hydrochloric acid both have concentration 1 mol/dm3. Explain why the ethanoic acid solution has a higher pH than the hydrochloric acid solution. (3 marks)

Hint: Strong acid = fully ionises. Weak acid = partially ionises. Same concentration but different H+ concentration.

+30 XP

Carboxylic acid structure and properties

Reactions of carboxylic acids

Esterification

Module complete! 🎉