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4.6.1Electrochemistry

Carboxylic Acids — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 11 of 20 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5Functional Group and General Formula

Functional group
  • Functional group Carboxylic acids contain the –COOH group, which is responsible for their acidic properties and reactions.
General formula
  • General formula The general formula CₙH₂ₙ₊₁COOH uses n for the carbons in the alkyl group, not counting the carboxyl carbon.
Carbon counting
  • Carbon counting The total number of carbon atoms is n + 1, so for methanoic acid n = 0 and the formula is HCOOH.
Naming
  • Naming Acid names use the alkane stem followed by -oic acid, for example propanoic acid for three carbon atoms.
Vinegar
  • Vinegar Vinegar is an aqueous solution of ethanoic acid containing about 5% of the acid by volume.

Section 2 of 5The First Four Carboxylic Acids

1
Methanoic acid has no alkyl carbons, so n = 0 and it is the simplest carboxylic acid.
2
Ethanoic acid has one alkyl carbon (n = 1) and gives the acid in vinegar.
3
Propanoic acid and butanoic acid follow the same pattern, with one extra CH₂ unit each time.
NameStructural formulaMolecular formula
Methanoic acidHCOOHCH₂O₂
Ethanoic acidCH₃COOHC₂H₄O₂
Propanoic acidCH₃CH₂COOHC₃H₆O₂
Butanoic acidCH₃CH₂CH₂COOHC₄H₈O₂

Section 3 of 5Acidic Behaviour

Acidic Behaviour
Ionisation
  • Ionisation Carboxylic acids partly ionise in water to release H⁺ ions, which is why they are acidic.
Weak acids
  • Weak acids Carboxylic acids are weak acids, so only a small proportion of their molecules ionise at any moment.
pH
  • pH Their solutions have a lower pH than neutral water, but they are less corrosive than strong mineral acids.
Shared reactions
  • Shared reactions Carboxylic acids react with metals, carbonates and bases in the same way as other acids.

Section 4 of 5Worked Example

Question Use the general formula CₙH₂ₙ₊₁COOH to find the molecular formula of pentanoic acid.

  1. Pentanoic acid has 5 carbon atoms in total, so the alkyl group has n = 5 − 1 = 4
  2. Substitute n = 4 to give C₄H₉COOH
  3. Count the atoms5 carbon, 10 hydrogen (9 + 1 from the –COOH group) and 2 oxygen

Answer C₅H₁₀O₂

Section 5 of 5Exam Checklist & Tips

4.6.1 – Key Points for the Exam

  • Carboxyl group The –COOH functional group.
  • Carboxylic acid An organic acid containing the –COOH group.
  • Alkyl group A group of carbon and hydrogen atoms that is attached to the carboxyl group in a carboxylic acid.
  • Weak acid An acid that is only partly ionised in aqueous solution.
  • Ethanoic acid The carboxylic acid CH₃COOH, the main acid in vinegar.

Common Mistakes

  • Using n for the total number of carbons; n counts only the carbons in the alkyl group.
  • Writing the formula of methanoic acid as CH₃COOH; it is HCOOH.
  • Forgetting to subtract the carboxyl carbon when working out n from the name of the acid.
  • Describing carboxylic acids as strong acids.

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