CCEA A Level Chemistry
CCEA A Level Chemistry 4.8 Carboxylic Acids Exam Questions
Free CCEA A Level Chemistry revision for 4.8 Carboxylic Acids: what you need to know, common mistakes and exam-style questions with answers. Covers properties, preparation by oxidation and hydrolysis, salt formation, esterification and reactions with PCl5 and LiAlH4.
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About CCEA A Level Chemistry 4.8 Carboxylic Acids
Section 4.8 of Unit A2 1 covers carboxylic acids. You need to name and draw them with up to six carbons, explain their high boiling points and solubility using hydrogen bonding, and prepare them by oxidation of primary alcohols and aldehydes, including practical details such as heating under reflux. CCEA also includes hydrolysis of esters and nitriles with acid or alkali to give acids or their salts. Reactions include salt formation with sodium carbonate, sodium hydroxide and ammonia, esterification with alcohols, conversion to acyl chlorides with phosphorus pentachloride, and reduction with lithal. This links to alcohols (2.6), aldehydes (4.7) and acid derivatives (4.9), and to Ka in 4.5.
What you need to know
- Properties Carboxylic acids form strong hydrogen bonds, including dimers in the pure liquid, so they have high boiling points. Short chain acids are soluble in water. They are weak acids.
- Preparation by oxidation Heat a primary alcohol under reflux with excess acidified potassium dichromate, then distil off the acid. Aldehydes are also oxidised to acids.
- Hydrolysis Esters and nitriles are hydrolysed by heating with dilute acid to give the carboxylic acid, or with alkali to give the carboxylate salt (the nitrile also releases ammonia).
- Salt formation With sodium carbonate: 2CH3COOH + Na2CO3 gives 2CH3COONa + H2O + CO2 (effervescence, a test for COOH). With NaOH: a salt and water. With ammonia: an ammonium salt.
- Other reactions With an alcohol and a concentrated sulfuric acid catalyst, heated under reflux, an ester forms reversibly. With PCl5, an acyl chloride forms with POCl3 and HCl. LiAlH4 reduces the acid to a primary alcohol.
Common mistakes in 4.8 Carboxylic Acids
- Saying carboxylic acids are fully dissociated. They are weak acids, so only partially dissociated.
- Forgetting that esterification is reversible. Use reflux and an acid catalyst, and note the equilibrium.
- Writing the salt formula wrongly, for example CH3COONa2. Ethanoate has a 1- charge, so sodium ethanoate is CH3COONa.
- Saying alkaline hydrolysis of an ester gives the acid. It gives the carboxylate salt; acidify to obtain the acid.
- Forgetting the extra carbon from a nitrile. Hydrolysis of CH3CH2CN gives propanoic acid, CH3CH2COOH.
Try three 4.8 Carboxylic Acids exam-style questions
1. Write an equation for the reaction of propanoic acid with sodium carbonate and give one observation. (2 marks)
Show the mark scheme answer
2CH3CH2COOH + Na2CO3 gives 2CH3CH2COONa + H2O + CO2; effervescence.
2. Write an equation for the reaction of ethanoic acid with PCl5. (1 mark)
Show the mark scheme answer
CH3COOH + PCl5 gives CH3COCl + POCl3 + HCl.
3. Explain why ethanoic acid has a higher boiling point than propan-1-ol, although they have similar Mr. (2 marks)
Show the mark scheme answer
Ethanoic acid forms more extensive hydrogen bonding, including dimers, through both C=O and O-H; more energy needed to overcome these intermolecular forces.
4.8 Carboxylic Acids FAQs
What does CCEA A Level Chemistry 4.8 Carboxylic Acids cover?
It covers properties, preparation by oxidation and hydrolysis, salt formation, esterification and reactions with PCl5 and LiAlH4.
Which unit is 4.8 Carboxylic Acids examined in?
It is examined in Unit A2 1: Further Physical and Organic Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
How can I test for a carboxylic acid?
Add sodium carbonate or sodium hydrogencarbonate solution. Effervescence of a gas that turns limewater milky shows a carboxylic acid.
