Carboxylic acids — the –COOH series
The functional group is –COOH (carboxyl): a C=O and an –OH on the same carbon — not –OH alone (alcohol) nor –COO– (ester). The first four members share the general formula : methanoic acid ( 46), ethanoic acid (60), propanoic acid (74) and butanoic acid (88). Each step up the series adds CH, so rises by 14. Lower members are liquids; ethanoic acid is the acid in vinegar.
Reactions — metals, bases, carbonates
The acidic proton of –COOH drives three reactions. With a metal (e.g. Mg): salt + hydrogen — effervescence, metal dissolves. With a base (oxide or hydroxide): salt + water only, no gas. With a carbonate (e.g. NaCO): salt + water + carbon dioxide — effervescence. This fizzing with a carbonate distinguishes a carboxylic acid from an alcohol, which does not react. Salt naming: ethanoic acid → ethanoates, methanoic acid → methanoates.
(Extended) Weak acid & esterification
Weak acid: ethanoic acid is only partially ionised (), so at equal concentration to a strong acid it has lower , higher pH and reacts more slowly with Mg. Esterification: carboxylic acid + alcohol react reversibly (concentrated acid catalyst) to give ester + water. Name it alkyl-first then acid-root + -anoate: ethanol + ethanoic acid give ethyl ethanoate.
Drawn from real examiner reports.
–COOH is not the –OH group
Asked for the functional group of a carboxylic acid, candidates write –OH — the hydroxyl group of alcohols. The carboxyl group is –COOH: a C=O (carbonyl) and an –OH on the same carbon, giving two oxygen atoms. It is not the ester linkage –COO– either. The required answer is always –COOH.
W22 Paper 31 Q7(a)(i); W22 Paper 41 Q6(c)
(Extended) Propyl ethanoate vs ethyl propanoate
These two esters are constantly swapped. Split the name: the alkyl part comes from the alcohol, the -anoate part from the acid. Ethyl propanoate = ethanol + propanoic acid; propyl ethanoate = propan-1-ol + ethanoic acid. Identify the acid and alcohol separately, then build the name with the alcohol part first.
W22 Paper 42 Q6(e)(i); S22 Paper 41 Q5(g)(i)
Observations, not product names
"Describe what you observe" wants what can be seen, heard or felt — never a product name. "Magnesium ethanoate and hydrogen are formed" scores zero; the credited observations are effervescence and the metal dissolves. With a carbonate: effervescence (carbon dioxide) and the solid dissolves. Name a product only when the question tells you to.
W22 Paper 31 Q6(a); W22 Paper 32 Q6(a); S23 Paper 31 Q4(d)(ii)
(Extended) Weak acid is not dilute acid
Weak and dilute describe different things. Weak = only partially ionised (few molecules donate H); dilute = low concentration. A 1 mol/dm³ solution of ethanoic acid is fairly concentrated yet still a weak acid; a very dilute HCl is still strong. Weak/strong is about degree of ionisation, never concentration.
Molecular vs structural formula
Asked for the molecular formula, candidates give a structural formula. For ethanoic acid the molecular formula is while the structural formula is . Both describe the same molecule but answer different commands: "molecular formula" wants every atom counted; "structural formula" shows how they are joined.
W22 Paper 42 Q6(c); W22 Paper 41 Q6(a)(ii)
Wrong gas named for the reaction
Do not mix up the gases. A carboxylic acid with a metal gives hydrogen (squeaky-pop test). With a carbonate it gives carbon dioxide (limewater turns milky) — not hydrogen. With a base (metal oxide or hydroxide) it gives no gas at all, only salt and water, so "bubbles" is wrong for that neutralisation.
AMC product scaffold
Predict products with AMC: Acid + Metal → salt + hydrogen; Acid + base (metal oxide/hydroxide) → salt + water; Acid + Carbonate → salt + water + carbon dioxide. Fix the products first, then give observations (bubbles for metal and carbonate; none for a hydroxide).
(Extended) Name esters in two steps
Build an ester name in order: (1) alcohol → alkyl stem (ethanol → ethyl); (2) acid → change -ic acid to -anoate (propanoic → propanoate); (3) combine, alcohol part first: ethyl propanoate.
(Extended) Esterification is reversible
Esterification is reversible: write the equilibrium arrow , not a single arrow, and state a concentrated acid catalyst, not dilute. The two products are the ester and water — never carbon dioxide. Dropping the reversible arrow costs a mark.
(Extended) Weak-acid rate chain
For "why does ethanoic acid react more slowly than HCl at the same concentration", give the chain: weak acid → partially ionised → lower → fewer H collisions with the metal each second → slower rate.
Carboxylic acid — a member of the homologous series characterised by the –COOH (carboxyl) functional group. The carboxyl group contains both a C=O bond (carbonyl) and an –OH on the same carbon atom.
Weak acid (Extended) — an acid that is only partially ionised in aqueous solution. The equilibrium lies to the left: . "Weak" describes the degree of ionisation, NOT the concentration.
Esterification (Extended) — the reversible reaction between a carboxylic acid and an alcohol, using a concentrated acid catalyst (usually concentrated , sulfuric acid), to form an ester and water.
Ester — a compound with the functional group –COO– (ester linkage), formed from a carboxylic acid and an alcohol.
Key formula: where = amount (mol), = mass (g), = relative formula mass.
Define carboxylic acid and state its functional group.
Ethanoic acid has the molecular formula .
(a) Calculate the relative formula mass () of ethanoic acid. (: C = 12, H = 1, O = 16)
(b) A sample of ethanoic acid contains 3.0 g. Calculate the number of moles in this sample.
