Carboxylic acids: the -COOH group
Carboxylic acids contain the carboxyl group, -COOH: a carbon joined to oxygen by a double bond (C=O) plus an -O-H group. Names end in -anoic acid; general formula (or ). The first four are methanoic (), ethanoic (), propanoic and butanoic acid.
React like acids: metals and carbonates
Aqueous carboxylic acids act as typical acids, forming a carboxylate salt (name ends -anoate). With a reactive metal (e.g. magnesium): salt + hydrogen (fizzing; a lit splint gives a squeaky pop): With a carbonate: salt + water + carbon dioxide (limewater turns milky). The ethanoic-acid salt is an ethanoate.
Weak acids; vinegar is ethanoic acid
Carboxylic acids are weak acids — in water only partially ionised, so only a few molecules release at any moment: So at equal concentration ethanoic acid has a higher pH and reacts more slowly than a strong acid like hydrochloric. Vinegar is dilute aqueous ethanoic acid, from ethanol oxidised by air.
Drawn from real examiner reports.
Drawing -COOH without every bond
A displayed formula must show every bond. The two classic omissions in -COOH are (1) leaving out the O-H bond (writing "OH" as a unit instead of -O-H) and (2) drawing a single C-O instead of the C=O double bond. Check: every carbon shows 4 bonds, every oxygen 2, every hydrogen 1.
June 2024 Paper 2C Q5(b)(iii): candidates lost a mark in the displayed formula by not showing the bond between the O and the H; the report also stresses showing the C=O double bond.
"A carboxylic acid", not ethanoic acid
Left open to the air, oxygen oxidises ethanol to ethanoic acid (the souring that makes vinegar). "A carboxylic acid is formed" is too vague to score — that could be any member of the family, so name ethanoic acid (or vinegar). The same applies to salts: name the ethanoate, not "a salt".
June 2023 Paper 2C Q3(c): the mark was withheld because "carboxylic acid is formed" was too vague; the report states it needs to be ethanoic acid or vinegar.
"Weak" does not mean "dilute"
"Weak" and "dilute" are not interchangeable. A weak acid is only partially ionised in water (a property of the acid) — the mark-earning description of a carboxylic acid. Dilute means a small amount of acid per unit volume (how it was mixed). Ethanoic acid is weak even when concentrated; "weak because it is dilute" scores 0.
Forgetting the acid needs a coefficient of 2
When a carboxylic acid reacts with a metal or carbonate carrying two positive charges or two metal atoms, the salt has two carboxylate ions, so two acid molecules are needed, e.g. . Leaving the acid as 1 leaves the equation unbalanced.
(common Edexcel chemistry error)
Mixing up -anoic (acid) and -anoate (salt)
The acid ends in -anoic (ethanoic acid, ); its salt ends in -anoate (sodium ethanoate, ). In a reaction with a metal or carbonate the product salt is a carboxylate — do not call it "ethanoic" or just "a salt".
Giving the wrong gas or gas test
A carboxylic acid with a reactive metal gives hydrogen — a lit splint gives a squeaky pop. With a carbonate it gives carbon dioxide — bubble through limewater, which turns milky/cloudy. Do not swap the tests (limewater for carbon dioxide, lit splint for hydrogen), and give both the method and the result.
(common Edexcel chemistry error)
Explain weak-acid behaviour by a chain
For "why does ethanoic acid react more slowly than hydrochloric acid of the same concentration", chain it: partially ionised → fewer ions → lower concentration → slower rate, higher pH. Name the real substances (ethanoic acid, not "the acid").
On "show by calculation", show every step
"Show that" and "show by calculation" award marks for the working, so set out every step — , moles, the mole ratio, then the answer. Keep full figures and round only at the end; no working, or rounding too early, forfeits marks even if the final number is right.
Give a method and a result for gas tests
To identify a gas from these reactions, give a method and a result, not a conclusion. Hydrogen: lit splint → squeaky pop. Carbon dioxide: bubble through limewater → turns milky. "It is hydrogen" with no test, or a result with no method, loses full marks.
The first four unbranched carboxylic acids:
| Name | Condensed formula | Molecular formula | |
|---|---|---|---|
| Methanoic acid | 46 | ||
| Ethanoic acid | 60 | ||
| Propanoic acid | 74 | ||
| Butanoic acid | 88 |
What functional group do all carboxylic acids contain?
Calculate the relative formula mass () of propanoic acid, . (: C = 12, H = 1, O = 16)
