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11.7Organic Chemistry

Carboxylic Acids — Cambridge O Level Chemistry 5070 Revision Notes & Topical Questions

5 min read 13 parts Lesson 7 of 8

📚 Cambridge O Level Chemistry 5070 Hub / Topical Revision
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Lesson summary

Describe the reactions of carboxylic acids, how they are made, and how esters are formed.

Part 1 of 11Reactions of carboxylic acids

  • Behave like acids
    • react with metals, hydroxides and carbonates.
  • Salt names
    • end in -anoate, such as methanoate and ethanoate.
  • Metal
    • acid + metal → salt + hydrogen.
  • Example
    • 2CH3COOH + Mg → (CH3COO)2Mg + H2.
  • Hydroxide
    • acid + hydroxide → salt + water.
  • Example
    • CH3CH2COOH + KOH → CH3CH2COOK + H2O.
  • Carbonate
    • acid + carbonate → salt + water + carbon dioxide.
  • Example
    • 2CH3CH2CH2COOH + K2CO3 → 2CH3CH2CH2COOK + H2O + CO2.
    • Skill
      • name the salts and give their formulae.
  • Ethanoic acid
    • also called acetic acid, giving vinegar that is about 5% ethanoic acid by volume.

Part 2 of 11Making ethanoic acid by fermentation

  • Source
    • bacteria such as acetobacter in air oxidise ethanol in wine using oxygen.
  • Equation
    • C2H5OH + O2 → CH3COOH + H2O.
    • Observation
      • opened wine tastes vinegary because ethanoic acid forms.

Part 3 of 11Making ethanoic acid with an oxidising agent

  1. Heat ethanol with acidified potassium manganate(VII).
  2. Use reflux, with a condenser fitted above the flask.
  3. The condenser stops the volatile alcohol escaping.
  4. Purple changes to colourless as manganate(VII) is reduced.
  • Equation
    • CH3CH2OH + [O] → CH3COOH + H2O.
  • Symbol
    • [O] represents the oxidising agent.

Part 4 of 11Esterification

  • Definition
    • alcohols and carboxylic acids react to form esters.
  • Ester group
    • R–COO–R.
    • Properties
      • sweet-smelling oily liquids.
    • Uses
      • food flavourings and perfumes.
  • Example
    • ethanoic acid with ethanol using concentrated sulfuric acid as catalyst.
  • Equation
    • CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O.
    • Product
      • ethyl ethanoate.
    • Reversible
      • the reaction is an equilibrium.

Part 5 of 11Names and formulae of acids and salts

AcidFormulaSalt with sodium
Methanoic acidHCOOHSodium methanoate, HCOONa
Ethanoic acidCH3COOHSodium ethanoate, CH3COONa
Propanoic acidCH3CH2COOHSodium propanoate, CH3CH2COONa
Butanoic acidCH3CH2CH2COOHSodium butanoate, CH3CH2CH2COONa
  • Rule
    • replace the H of –COOH with the metal and change -oic acid to -oate.

Part 6 of 11Carboxylic acids as weak acids

  • Ionisation
    • they only partly ionise in water, so they are weak acids.
  • pH
    • their solutions have pH values closer to 7 than strong acids.
    • Reaction speed
      • they react more slowly than hydrochloric acid of the same concentration.
    • Observation
      • with a carbonate there is bubbling of carbon dioxide.

Part 7 of 11Tests and observations

  1. Add magnesium ribbon to the acid and see bubbles of hydrogen.
  2. Add sodium carbonate to the acid and see carbon dioxide turn limewater milky.
  3. Add universal indicator and see a pH below 7.
  • Vinegar
    • contains about 5% ethanoic acid by volume.

Part 8 of 11Ester formation in detail

  • Reactants
    • a carboxylic acid and an alcohol.
  • Catalyst
    • a few drops of concentrated sulfuric acid.
    • Conditions
      • warm gently, and the mixture reaches equilibrium.
    • Product 1
      • an ester with a sweet, fruity smell.
    • Product 2
      • water.
  • Naming
    • ethanol with propanoic acid gives ethyl propanoate.

Part 9 of 11Equations to know

ReactionEquation
Ethanoic acid with magnesium2CH3COOH + Mg → (CH3COO)2Mg + H2
Propanoic acid with potassium hydroxideCH3CH2COOH + KOH → CH3CH2COOK + H2O
Butanoic acid with potassium carbonate2CH3CH2CH2COOH + K2CO3 → 2CH3CH2CH2COOK + H2O + CO2
Oxidation of ethanolCH3CH2OH + [O] → CH3COOH + H2O
EsterificationCH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
  • Tip
    • name the salt from the metal and the acid, then write its formula.

Part 10 of 11Uses of carboxylic acids and esters

  • Ethanoic acid
    • vinegar for food, and preserving and flavouring.
  • Industry
    • raw material for making other chemicals.
  • Esters
    • used in perfumes and food flavourings because of their sweet, fruity smells.
  • Natural source
    • many fruit smells are esters.
    • Polymers
      • polyesters such as PET are built from the same ester link.

Part 11 of 11Worked examples

  1. Write the equation for ethanoic acid with magnesium and name the salt. [2]2 marks

  2. Describe how ethanol is oxidised to ethanoic acid in the laboratory. [3]3 marks

  3. Name the ester formed from ethanol and propanoic acid and state the catalyst. [2]2 marks

  4. Explain why wine left open tastes sour. [2]2 marks

Key terms

TermMeaning
Carboxylic acidA compound containing –COOH.
Ethanoic acidThe acid in vinegar, CH3COOH.
EsterificationReaction of an alcohol and a carboxylic acid to make an ester.
RefluxHeating with a condenser so vapours return to the flask.
EsterA compound with the group –COO–, often with a sweet smell.

Avoid these mistakes

  1. Writing the salt of ethanoic acid as ethanate — it is ethanoate.
  2. Forgetting the water produced in neutralisation and ester formation.
  3. Leaving out the condenser in the oxidation.
  4. Thinking esterification goes to completion.

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