Carboxylic Acids

• 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

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:
  • Effervescence 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

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