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Carboxylic AcidsAQA GCSE Chemistry: Revision notes

Section 1

What Is a Carboxylic Acid?

A carboxylic acid is an organic compound containing the functional group –COOH (the carboxyl group). Carboxylic acids form a homologous series — a family of compounds with the same general pattern and similar chemical properties, differing by a CH2 unit each time.

The first four members are:

  • Methanoic acid — HCOOH
  • Ethanoic acid — CH3COOH
  • Propanoic acid — C2H5COOH
  • Butanoic acid — C3H7COOH

As with alkanes and alcohols, the names go methan-, ethan-, propan-, butan- according to the number of carbon atoms.

Key termsfunctional grouphomologous seriescarboxyl group
Exam tip

Spot a carboxylic acid instantly by looking for –COOH at the end of the formula.

Section 2

How Do Carboxylic Acids React?

Carboxylic acids show typical acid behaviour:

  • They react with carbonates to produce a salt, water and carbon dioxide (the same pattern as other acids).
  • They dissolve in water to form acidic solutions.
  • They react with alcohols (in the presence of an acid catalyst) to form an ester plus water — this reaction is called esterification.

For example, ethanoic acid + ethanol → ethyl ethanoate (an ester) + water.

Key termsesteresterification
Example

Methanoic acid + methanol → methyl methanoate + water.

Section 3

Why Are Carboxylic Acids Weak Acids? (Higher Tier)

Carboxylic acids are classed as weak acids because they only partially ionise in aqueous solution — only some of the acid molecules split up into ions, so the solution contains a mixture of molecules and ions.

This contrasts with strong acids (such as hydrochloric, nitric and sulfuric acid), which fully ionise in solution.

Acid typeIonisationExampleTypical pH (same concentration)
Strong acidCompleteHydrochloric acidLower (more acidic)
Weak acidPartialEthanoic acidHigher (less acidic)

Remember: as pH decreases by 1 unit, the hydrogen ion concentration increases by a factor of 10.

Key termsweak acidstrong acidionisation
Common mistake

Weak acid does not mean dilute acid. Weak/strong refers to how much the acid ionises; dilute/concentrated refers to how much water it is mixed with.

Section 4

How Are Alcohols Related to Carboxylic Acids?

Alcohols have the functional group –OH. The first four members are methanol, ethanol, propanol and butanol.

Alcohols:

  • React with sodium, releasing hydrogen gas
  • Burn in air (combustion)
  • Are added to water, in which they dissolve
  • React with an oxidising agent to be oxidised into the corresponding carboxylic acid

Alcohols are widely used as solvents and fuels.

Key termsalcoholoxidising agent
Exam tip

Oxidising an alcohol adds an oxygen atom to its –OH group, converting it into a –COOH group — this is how ethanol becomes ethanoic acid (vinegar going off).

Section 5

How Is Ethanol Made by Fermentation?

Ethanol can be produced by fermentation of sugar using yeast. The yeast's enzymes convert sugar into ethanol and carbon dioxide:

glucose → ethanol + carbon dioxide

Fermentation requires:

  • Anaerobic conditions (absence of oxygen)
  • A warm temperature (around 30–40°C) to keep the enzymes working efficiently without denaturing them

This process is used in brewing and biofuel production.

Key termsfermentationyeastanaerobic

Must Know

  • Carboxylic acids contain the functional group –COOH; the first four are methanoic, ethanoic, propanoic and butanoic acid
  • Carboxylic acids react with carbonates (salt + water + CO2), dissolve in water, and react with alcohols to form esters (esterification)
  • Carboxylic acids are weak acids — they only partially ionise, unlike strong acids like HCl which fully ionise (HT)
  • Alcohols (–OH) react with sodium, burn in air, dissolve in water, and are oxidised to carboxylic acids
  • Ethanol is made by fermentation of sugar with yeast, under anaerobic, warm conditions

That's the notes covered.

Carry on to the next subtopic.