9701 · 18.1
Carboxylic acids flashcards
Revision flashcards for Cambridge 9701 Carboxylic acids (syllabus 18.1). Flip, recall, then mark a real past-paper question.
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What is the functional group of a carboxylic acid?
The carboxyl group, $-\text{COOH}$, which consists of a carbonyl group ($C=O$) and a hydroxyl group ($-OH$) attached to the same carbon atom.
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What is the general formula for a straight-chain carboxylic acid?
$C_n H_{2n+1} COOH$ (for $n \ge 0$). More generally, $R-COOH$ where R is a hydrogen atom or an alkyl group.
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Why are carboxylic acids classified as weak acids?
They only partially dissociate (ionise) in aqueous solution, establishing an equilibrium. For example: $CH_3COOH(aq) \rightleftharpoons CH_3COO^-(aq) + H^+(aq)$.
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Why do carboxylic acids have higher boiling points than alkanes of similar relative molecular mass?
They can form strong intermolecular hydrogen bonds. They can also form stable dimers, effectively doubling their molecular size and increasing the energy needed to vaporise them.
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What are the products when a carboxylic acid reacts with a reactive metal like magnesium?
A carboxylate salt and hydrogen gas. Example: $2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2$.
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What are the products when a carboxylic acid reacts with a carbonate, like sodium carbonate?
A carboxylate salt, water, and carbon dioxide gas. This effervescence is a characteristic test for carboxylic acids.
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What are the conditions required for esterification?
A carboxylic acid is heated under reflux with an alcohol in the presence of a strong acid catalyst, typically concentrated sulfuric acid ($H_2SO_4$).
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What is a dimer in the context of carboxylic acids?
A pair of carboxylic acid molecules held together by two hydrogen bonds between their carboxyl groups. This occurs in the gaseous state and in non-polar solvents.
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How does the solubility of carboxylic acids in water change as the carbon chain length increases?
Solubility decreases. Short-chain acids (e.g., methanoic, ethanoic) are fully miscible as they can form hydrogen bonds with water. The long, non-polar hydrocarbon chain in larger acids disrupts this interaction.
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What is the IUPAC name for the salt formed from the reaction of propanoic acid and potassium hydroxide?
Potassium propanoate. The '-oic acid' suffix is replaced with '-oate'.
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What is the observation when aqueous sodium hydrogencarbonate is added to ethanoic acid?
Effervescence (fizzing) is observed as carbon dioxide gas is produced.