4CH1

Carboxylic Acids

Organic Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable
Rare
Stable5%

Properties and reactions with carbonates and alcohols tested as shorter questions.

  • Carboxylic acids behave like the inorganic acids met in topic 16: they donate a proton (the H of the O–H) into solution, and they react with the same partners — metals, bases and carbonates — to give a salt as one of the products

Naming the salts

  • A salt formed from a carboxylic acid ends in "-anoate" instead of "-anoic acid"
  • The pattern:
    • Methanoic acid → methanoate salts (HCOO−)
    • Ethanoic acid → ethanoate salts (CH3COO−)
    • Propanoic acid → propanoate salts (CH3CH2COO−)
    • Butanoic acid → butanoate salts (CH3CH2CH2COO−)
  • The cation (sodium, potassium, calcium, ammonium, etc.) is named first, exactly as for inorganic salts

Reaction with reactive metals

  • Carboxylic acids react with metals above hydrogen in the reactivity series to give a salt plus hydrogen gas:

carboxylic acid + metal → metal carboxylate + hydrogen

  • Worked example: ethanoic acid with magnesium ribbon

2 CH3COOH(aq) + Mg(s) → (CH3COO)2Mg(aq) + H2(g)

  • The acid loses its acidic hydrogens to form the magnesium ethanoate salt
  • The gas given off can be tested with a lit splint — the characteristic squeaky pop confirms hydrogen
Exam tip

Observations when a carboxylic acid reacts with a metal

What comes up: "give two observations" for the reaction of ethanoic acid with magnesium (or another reactive metal).

Write (two marks): (1) effervescence (or bubbles/fizzing); (2) the metal becomes smaller and disappears (or dissolves).

Watch out: writing only "gas is given off" or "a gas is produced" scores zero for that mark — the mark scheme requires effervescence/fizzing/bubbles, not a vague reference to gas. Both marks are independent, so state both clearly.

Reaction with metal carbonates

  • Carboxylic acids react with carbonates to give a salt plus water plus carbon dioxide gas:

carboxylic acid + metal carbonate → metal carboxylate + water + carbon dioxide

  • Worked example: propanoic acid with sodium carbonate

2 CH3CH2COOH(aq) + Na2CO3(s) → 2 CH3CH2COONa(aq) + H2O(l) + CO2(g)

  • Worked example: methanoic acid with calcium carbonate

2 HCOOH(aq) + CaCO3(s) → (HCOO)2Ca(aq) + H2O(l) + CO2(g)

  • Visible features:
    • as carbon dioxide bubbles off
    • The carbonate dissolves into the acid solution
    • Confirming the gas: bubble it through limewater — the limewater turns milky, confirming CO2
Exam tip

Chemical test to show a solution contains a carboxylic acid

What comes up: "describe a chemical test, other than using an indicator, to show the mixture contains a carboxylic acid" (2 marks).

Write (two marks): (1) add a carbonate (any named carbonate, e.g. sodium carbonate or calcium carbonate); (2) effervescence/fizzing/bubbles are seen. To score both marks for the gas-test route, you can instead state: bubble the gas through limewater — the limewater turns cloudy/milky.

Watch out: the second mark depends on the first — if you skip naming the reagent and go straight to "bubbles form", you score zero. Both marks are linked; name the reagent and give the observation.

Reaction with metal hydroxides (neutralisation)

  • Like any acid, a carboxylic acid is neutralised by a metal hydroxide to give a salt plus water:

carboxylic acid + metal hydroxide → metal carboxylate + water

  • Worked example: ethanoic acid with sodium hydroxide

CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)

  • Sodium ethanoate (the salt) is fully soluble in water; the reaction is the same as for the inorganic acid–alkali reactions in topic 15

Why carboxylic acids are classed as weak acids

  • An aqueous carboxylic acid only partially dissociates — most of the acid molecules sit in solution as the un-dissociated −COOH form, with only a small fraction releasing H+
  • This makes carboxylic acids weak acids — a 1 mol dm−3 ethanoic acid solution has a pH of about 2.4, well above the pH 0–1 of a 1 mol dm−3 strong acid like HCl
  • All the reactions in this section still happen — they are simply slower than the equivalent reactions of HCl or H2SO4 at the same concentration
Exam tip

Ester formation: catalyst and recognising the product

What comes up: two very common 1-mark questions — (a) "name a suitable catalyst for the reaction between a carboxylic acid and an alcohol"; (b) "how would you know an ester has formed?"

Write: (a) concentrated sulfuric acid (concentrated phosphoric acid is also accepted); (b) a distinctive sweet or fruity smell is detected (alternatively, an oily layer forms on the surface of the reaction mixture).

Watch out: writing "dilute sulfuric acid" loses the mark — the mark scheme ignores "dilute". The catalyst must be a named acid; writing only "acid catalyst" is not sufficient.