4fCarboxylic acids

Separate Chemistry only.

The functional group

The carboxyl group, −COOH.

It contains two oxygens bonded quite differently: one joined to the carbon by a double bond, and one joined by a single bond and carrying a hydrogen.

That hydrogen is the acidic one. It is what makes the compound an acid.

Naming

The ending is -anoic acid:

The carbon in the −COOH group counts towards the total. Ethanoic acid has two carbons altogether — one in the CH₃ and one in the COOH.

That is why methanoic acid is HCOOH, not CH₃COOH. Its single carbon is already part of the carboxyl group, so a hydrogen sits where a carbon chain would otherwise be.

Why an alcohol is not acidic

Ethanol contains −OH. Ethanoic acid contains −COOH. Both have an oxygen bonded to a hydrogen, yet only one is an acid.

The difference is the C=O next to the O−H. That neighbouring double-bonded oxygen makes the hydrogen able to leave as an H⁺ ion, and releasing H⁺ in solution is what makes a substance an acid.

An −OH group on its own does not do this. The whole carboxyl group is needed.

Vinegar

Vinegar is a dilute aqueous solution of ethanoic acid.

It connects directly to the alcohols topic: the ethanoic acid in vinegar is what ethanol becomes when microorganisms oxidise it in air.

Separate Chemistry only.

Carboxylic acids behave like any other acid. The same three reaction patterns apply, and the only new thing to learn is what the salts are called.

With metals

acid + metal → salt + hydrogen

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

Magnesium forms a 2+ ion, so two ethanoate groups are needed — which fixes the coefficient of 2 on the acid, and also supplies the two hydrogen atoms for H₂.

Test the gas with a lighted splint: a squeaky pop confirms hydrogen.

With metal carbonates

acid + carbonate → salt + water + carbon dioxide

Three products, as always. The fizzing you see is carbon dioxide, and bubbling it through limewater turns it milky.

Naming the salts

The salts of ethanoic acid are ethanoates. Magnesium and ethanoic acid give magnesium ethanoate.

The pattern follows the acid's name: methanoic acid gives methanoates, propanoic acid gives propanoates.

Recognising an acid from its reactions

If a substance fizzes with both a metal and a carbonate, it is behaving as an acid.

But the two gases are different: hydrogen from the metal, carbon dioxide from the carbonate. Treating the fizzing as the same in both cases misses the chemistry the question is testing.

Everyday chemistry

Vinegar removes limescale from a kettle. The limescale is mainly calcium carbonate, and the ethanoic acid reacts with it to form calcium ethanoate, which is soluble and simply rinses away.

The limescale is not dissolved unchanged — it is converted into a different, soluble compound. That distinction is what a question about the chemistry is looking for.