CIE IGCSE Chemistry

Revision Notes

14.2.4 Carboxylic Acids

Ethanoic Acid: Basics

Carboxylic acids

  • These are a homologous series of organic compounds that all contain the same functional group: –COOH
  • They are colourless liquids which are weakly acidic and have typical acidic properties
  • They react with alkaline solutions, turn blue litmus red and form salts called ethanoates

 

The first three carboxylic acids

The-first-three-Carboxylic-Acids table, IGCSE & GCSE Chemistry revision notes

Exam Tip

The carbon atom in the -COOH functional group is counted as part of the molecule and not just the functional group. Eg. CH3CH2CH2COOH has 4 carbon atoms so is called butanoic acid, not propanoic acid.

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Ethanoic Acid

  • Ethanoic acid is a typically weak acid and dissociates slightly in water, producing a mildly acidic solution
  • The equilibrium lies far to the left during ionization:
CH3COOH ⇌ H+ + CH3COO-
  • Ethanoic acid reacts with the more reactive metals, hydroxides and carbonates

Reactions of ethanoic acid

  • In the reaction with metals a metal salt and hydrogen gas are produced
  • For example in reaction with magnesium the salt magnesium ethanoate is formed:
2CH3COOH + Mg → (CH3COO)2Mg + H2
  • In the reaction with hydroxides a salt and water are formed in a neutralisation reaction
  • For example in reaction with potassium hydroxide the salt potassium ethanoate is formed:
CH3COOH + KOH → CH3COOK + H2O
  • In the reaction with carbonates a metal salt, water and carbon dioxide gas are produced
  • For example in reaction with potassium carbonate the salt potassium ethanoate is formed:
2CH3COOH + K2CO3 → 2CH3COOK + H2O + CO2
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Ethanoic Acid & Esterification Reactions

Making carboxylic acids

Oxidation by fermentation

  • The microbial oxidation of ethanol will produce a weak solution of vinegar (ethanoic acid).
  • This occurs when a bottle of wine is opened as bacteria in the air (acetobacter) will use atmospheric oxygen from air to oxidise the ethanol in the wine:
C2H5OH + O2 → CH3COOH + H2O
  • The acidic, vinegary taste of wine which has been left open for several days is due to the presence of ethanoic acid

Oxidation with potassium manganate (VII)

  • Alcohols can also be oxidised to carboxylic acids by heating with acidified potassium manganate (VII)
  • The heating is performed under reflux which involves heating the reaction mixture in a vessel with a condenser attached to the top
  • The condenser prevents the volatile alcohol from escaping the reaction vessel as alcohols have low boiling points

 

Reflux-Apparatus, IGCSE & GCSE Chemistry revision notesDiagram showing the experimental setup for the oxidation with K2MnO4 using reflux apparatus

 

Making esters

  • Alcohols and carboxylic acids react to make esters in esterification reactions
  • Esters are compounds with the functional group R-COO-R
  • Esters are sweet-smelling oily liquids used in food flavourings and perfumes
  • Ethanoic acid will react with ethanol in the presence of concentrated sulfuric acid (catalyst) to form ethyl ethanoate:
CH3COOH + C2H5OH → CH3COOC2H5 + H2O

 

Preparing-Ethyl-Ethanoate_, IGCSE & GCSE Chemistry revision notesDiagram showing the formation of ethyl ethanoate

 

Naming esters

  • An ester is made from an alcohol and carboxylic acid
  • The first part of the name indicates the length of the carbon chain in the alcohol, and it ends with the letters ‘- yl’
  • The second part of the name indicates the length of the carbon chain in the carboxylic acid, and it ends with the letters ‘- oate’
  • g. the ester formed from pentanol and butanoic acid is called pentyl butanoate

 

Parts-of-an-Ester, IGCSE & GCSE Chemistry revision notesDiagram showing the origin of each carbon chain in ester

 

Examples of esters

Examples of esters table, IGCSE & GCSE Chemistry revision notes

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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