Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Carboxylic Acids Exam Questions
Free Edexcel International A Level Chemistry exam questions for Carboxylic Acids, with a mark scheme for every pack. Covers carboxylic acid structure and properties, preparation by oxidation and nitrile hydrolysis, reactions with bases, carbonates, LiAlH4, PCl5 and alcohols.
Carboxylic Acids exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel International A Level Chemistry Carboxylic Acids
Carboxylic acids in Unit 4 of Edexcel International A Level Chemistry focuses on the COOH group, its physical properties and its typical reactions. You learn how acids are prepared by oxidising primary alcohols or aldehydes and by hydrolysing nitriles, and how they react as weak acids with bases and carbonates. They are reduced by LiAlH4, converted into acyl chlorides with PCl5 and form esters with alcohols in the presence of an acid catalyst. The topic connects back to Alcohols and forward to Carboxylic acid derivatives. Examiners test equations, reagents and conditions, explanations of boiling temperature and solubility, and titrations to identify an acid.
What you need to know
- Physical properties Carboxylic acids form hydrogen bonds, so they have relatively high boiling temperatures and can form dimers. Short-chain acids are soluble in water, but solubility decreases as the hydrocarbon chain lengthens.
- Preparation Oxidise a primary alcohol or aldehyde with excess acidified potassium dichromate(VI) under reflux, or hydrolyse a nitrile by heating under reflux with dilute acid.
- Acid reactions Carboxylic acids are weak acids. They react with metals, alkalis and carbonates. Effervescence with sodium carbonate or sodium hydrogencarbonate distinguishes them from alcohols and phenols.
- Reduction LiAlH4 in dry ether reduces carboxylic acids to primary alcohols.
- Reaction with PCl5 PCl5 converts a carboxylic acid into an acyl chloride, giving steamy fumes of HCl and POCl3 as by-products.
- Esterification Heating with an alcohol and a concentrated sulfuric acid catalyst forms an ester and water. The reaction is reversible, so the yield is limited.
Common mistakes in Carboxylic Acids
- Saying carboxylic acids are strong acids. They are weak acids that only partially dissociate in water.
- Using NaBH4 to reduce a carboxylic acid. Edexcel uses LiAlH4 in dry ether for this reduction.
- Forgetting water as a product of esterification, or omitting the acid catalyst in conditions.
- Writing the salt formula incorrectly, for example CH3COONa2. The ethanoate ion is singly charged.
- Giving the wrong observation for the carbonate test. You see effervescence, and the gas turns limewater cloudy.
Try three Carboxylic Acids exam-style questions
1. Give the reagents and conditions to convert propanenitrile into propanoic acid. (2 marks)
Show the mark scheme answer
Dilute hydrochloric (or sulfuric) acid; heat under reflux.
2. 0.150 g of a monoprotic carboxylic acid is titrated with 0.100 mol dm-3 NaOH and requires 20.30 cm3. Calculate the Mr of the acid and identify it. (3 marks)
Show the mark scheme answer
Moles NaOH = 0.100 x 20.30 / 1000 = 2.03 x 10^-3 mol = moles acid; Mr = 0.150 / 2.03 x 10^-3 = 73.9; propanoic acid, CH3CH2COOH (Mr 74.0).
3. Describe a chemical test to distinguish ethanoic acid from ethanol. (2 marks)
Show the mark scheme answer
Add sodium carbonate (or sodium hydrogencarbonate) solution; ethanoic acid gives effervescence (CO2 turns limewater cloudy), ethanol gives no change.
Carboxylic Acids FAQs
What does Edexcel International A Level Chemistry Carboxylic acids cover?
It covers the structure and physical properties of carboxylic acids, their preparation by oxidation and nitrile hydrolysis, and their reactions with bases, carbonates, LiAlH4, PCl5 and alcohols.
Which unit is Carboxylic acids examined in?
Carboxylic acids is part of Unit 4, together with carbonyl compounds and carboxylic acid derivatives.
What reaction conditions should I learn for Carboxylic acids?
Learn reflux with excess acidified dichromate for oxidation, reflux with dilute acid for nitrile hydrolysis, LiAlH4 in dry ether for reduction, PCl5 at room temperature and concentrated sulfuric acid with heat for esterification.
