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Carboxylic acids — KCSE Chemistry

KCSE Chemistry · 59 practice questions · 4 syllabus objectives · 4 revision lessons

18 easy21 medium20 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Name and write structural formulae of carboxylic acids and describe their preparation by oxidation of alcohols

Describe the reactions of carboxylic acids: with metals, carbonates, bases and alcohols (esterification)

Write equations for esterification reactions and describe the uses of common esters

Carboxylic acids

Revision Notes

Concise lesson notes for Carboxylic acids, written to the KCSE Chemistry marking standard. Read the first lesson free below.

Carboxylic Acids: Names and Structures

Carboxylic acids are organic compounds characterized by the presence of a carboxyl group (-COOH). The general formula for carboxylic acids is RCOOH, where R represents an alkyl group. Common examples include:

  • Methanoic acid (Formic acid): HCOOH
  • Ethanoic acid (Acetic acid): CH3COOH
  • Propanoic acid: C2H5COOH

These acids can be prepared through the oxidation of alcohols. For instance:

  • Ethanol (C2H5OH) can be oxidized to form Ethanoic acid (CH3COOH) using an oxidizing agent like potassium dichromate (K2Cr2O7) in acidic conditions.

The oxidation process involves the addition of oxygen or the removal of hydrogen from the alcohol, transforming it into a carboxylic acid.

Key points to remember

  • Carboxylic acids contain a carboxyl group (-COOH).
  • General formula: RCOOH, R is an alkyl group.
  • Common examples: Methanoic acid, Ethanoic acid, Propanoic acid.
  • Prepared by oxidizing alcohols using oxidizing agents.
  • Ethanol oxidizes to Ethanoic acid in acidic conditions.

Worked example

Name the carboxylic acid formed from the oxidation of propanol. Answer: Propanoic acid (C2H5COOH) is formed from the oxidation of propanol (C3H7OH).

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Lesson 2: Reactions of Carboxylic Acids

Objective: Describe the reactions of carboxylic acids: with metals, carbonates, bases and alcohols (esterification)

Carboxylic acids are organic compounds containing the carboxyl group (-COOH). They undergo various reactions, which are important in both laboratory and industrial processes.

  1. Reaction with Metals: Carboxylic acids react with reactive metals (like sodium) to produce hydrogen gas and a salt. For example:

    • Equation: RCOOH + Na → RCOONa + H₂↑

  2. Reaction with Carbonates: When carboxylic acids react with carbonates, they produce carbon dioxide, water, and a salt. For example:

    • Equation: RCOOH + Na₂CO₃ → 2 RCOONa + H₂O + CO₂↑

  3. Reaction with Bases: Carboxylic acids react with bases (like sodium hydroxide) to form a salt and water, a neutralization reaction. For example:

    • Equation: RCOOH + NaOH → RCOONa + H₂O

  4. Esterification (Reaction with Alcohols): Carboxylic acids react with alcohols in the presence of an acid catalyst to form esters and water. For example:

    • Equation: RCOOH + R'OH ⇌ RCOOR' + H₂O

Understanding these reactions helps in grasping the versatility of carboxylic acids in organic chemistry.

  • Carboxylic acids react with metals to release hydrogen gas.
  • They react with carbonates to produce carbon dioxide and salts.
  • Neutralization with bases forms salts and water.
  • Esterification produces esters from carboxylic acids and alcohols.

Describe the reaction of ethanoic acid with sodium carbonate.

  • Ethanoic acid reacts with sodium carbonate to produce sodium ethanoate, water, and carbon dioxide.
  • Equation: CH₃COOH + Na₂CO₃ → 2 CH₃COONa + H₂O + CO₂↑.
Lesson 3: Esterification Reactions and Uses of Esters

Objective: Write equations for esterification reactions and describe the uses of common esters

Esterification is a chemical reaction between a carboxylic acid and an alcohol, resulting in the formation of an ester and water. The general equation for this reaction is:

Carboxylic Acid + Alcohol → Ester + Water
For example, when acetic acid (ethanoic acid) reacts with ethanol, the equation is:

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
This reaction is usually catalyzed by an acid, such as sulfuric acid, to increase the reaction rate.

Common esters have various uses:

  • Fragrance and flavoring: Esters are responsible for the fruity smells and tastes in many foods and perfumes.
  • Solvents: Many esters are used as solvents in paints and coatings due to their ability to dissolve various substances.
  • Plasticizers: Esters are added to plastics to improve flexibility and durability.

Understanding these reactions not only helps in writing equations but also in appreciating the practical applications of esters in everyday life.

  • Esterification involves a carboxylic acid and an alcohol.
  • The reaction produces an ester and water.
  • Esters are used in fragrances, solvents, and as plasticizers.
  • The reaction is often catalyzed by an acid.

Write the equation for the esterification of propanoic acid with methanol and state one use of the resulting ester.

Answer:
C₂H₅COOH + CH₃OH → C₂H₅COOCH₃ + H₂O
Use: The resulting ester, methyl propanoate, is used in flavoring and fragrances.

Lesson 4: Understanding Carboxylic Acids

Objective: Carboxylic acids

Carboxylic acids are organic compounds containing a carboxyl group (-COOH). They are known for their acidic properties and are commonly found in nature. Key characteristics include:

  • Functional Group: The carboxyl group consists of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-OH).
  • Naming: Carboxylic acids are named by replacing the '-e' ending of the corresponding alkane with '-oic acid'. For example, ethane becomes ethanoic acid.
  • Properties: They are polar and can form hydrogen bonds, leading to higher boiling points compared to hydrocarbons of similar size.
  • Reactions: Carboxylic acids can undergo various reactions, including neutralization with bases to form salts and esters.

Understanding the structure and properties of carboxylic acids is crucial for their application in various fields, including pharmaceuticals and food chemistry.

  • Carboxylic acids contain the -COOH functional group.
  • They are named by replacing -e with -oic acid.
  • They exhibit acidic properties and can form salts.
  • They have higher boiling points due to hydrogen bonding.

Question: Write the formula and name for the carboxylic acid derived from butane. Answer: The formula is C4H8O2, and it is named butanoic acid.

Sample Questions

Read 3 questions and answers free. Sign up to access all 59 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER2 marks

Identify two characteristics of carboxylic acids that distinguish them from alcohols. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
Carboxylic acids contain a carboxyl group (-COOH), whereas alcohols contain a hydroxyl group (-OH) (1 mk)
Carboxylic acids are typically more acidic than alcohols due to the presence of the carboxyl group (1 mk)
2
easySHORT ANSWER4 marks

State four uses of alkanoic acids (carboxylic acids) in everyday life or industry.

Answer & marking scheme

Part (a) — 4 marks
Ethanoic acid (vinegar) is used as a food preservative and seasoning/flavouring (1 mk)
In the manufacture of aspirin (acetylsalicylic acid) and other pharmaceuticals (1 mk)
In the manufacture of esters used in perfumes, flavourings, and solvents (1 mk)
In the textile industry for dyeing and printing fabrics / in the manufacture of plastics (e.g., PVA from ethanoic acid) (1 mk)
3
easySHORT ANSWER1 mark

Give the IUPAC name for the carboxylic acid with the structural formula: (CH₃)₂CHCH₂COOH

Answer & marking scheme

Part (a) — 1 mark
IUPAC name: propanoic acid (1 mk)
4

Name the ester formed and state one industrial or domestic use for the ester produced from ethanol (C₂H₅OH) and butanoic acid (C₃H₇COOH).

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Frequently asked questions

What does the KCSE Chemistry topic "Carboxylic acids" cover?

Carboxylic acids covers Name and write structural formulae of carboxylic acids and describe their preparation by oxidation of alcohols; Describe the reactions of carboxylic acids: with metals, carbonates, bases and alcohols (esterification); Write equations for esterification reactions and describe the uses of common esters, and more, all aligned to the official KNEC KCSE Chemistry syllabus.

How many practice questions are available for Carboxylic acids?

HighMarks has 59 Carboxylic acids practice questions for KCSE Chemistry, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Chemistry syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Carboxylic acids for the KCSE exam?

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