Carboxylic acids — AQA GCSE Chemistry

Carboxylic acids constitute a homologous series of organic molecules defined by the functional group -COOH, including common compounds such as methanoic acid and ethanoic acid. They exhibit typical acidic properties but are classified as weak acids because they only partially dissociate into hydrogen ions in water.

This topic bridges organic chemistry with fundamental concepts of acid-base equilibria, preparing students to understand condensation polymerisation and ester formation. In industry and daily life, carboxylic acids are essential in food preservation, vinegar production, pharmaceutical synthesis, and solvents.

• →Functional Group: All carboxylic acids contain the carboxyl group (-COOH), written structurally with a C=O double bond and a C-OH single bond.
• →Weak Acid Behaviour: Carboxylic acids partially ionise in aqueous solutions, setting up a reversible reaction that yields a lower H+ concentration and higher pH than strong acids of identical concentration.
• →Reactions with Bases: Carboxylic acids react with carbonates to yield a metal carboxylate salt, water, and carbon dioxide; they react with metals to yield a salt and hydrogen.
• →Esterification: Carboxylic acids react reversibly with alcohols in the presence of an acid catalyst (e.g. concentrated sulfuric acid) to produce sweet-smelling esters and water.

• State that –COOH is the functional group present in every carboxylic acid.
• Identify the group as a carbon atom double-bonded to an oxygen atom and single-bonded to an –OH group.
• Explain that the –COOH carbon also bonds to one carbon of the rest of the molecule, or to hydrogen in methanoic acid.
• Recognise –COOH within a displayed, structural or molecular formula and name the compound as a carboxylic acid.
• Link the –OH part of –COOH to the release of H⁺ ions in water, giving a weakly acidic solution.
• Use the group to predict reactions with carbonates and, at higher tier, with alcohols.
• Name the first four carboxylic acids as methanoic, ethanoic, propanoic and butanoic acid.
• State that they form a homologous series with the same –COOH functional group.
• Explain that successive members differ by a CH₂ unit.
• Use the prefixes meth-, eth-, prop- and but- to show one, two, three and four carbon atoms.
• Count the –COOH carbon as part of the chain when working out the name.
• Write the molecular formulae HCOOH, CH₃COOH, CH₃CH₂COOH and CH₃CH₂CH₂COOH.
• Represent carboxylic acids using displayed, structural or molecular formulae such as CH₃COOH.
• Describe the reaction with a carbonate as producing a salt, water and carbon dioxide, with effervescence.
• Name the salt formed from the acid and the metal in the carbonate, for example sodium ethanoate.
• Describe dissolution in water as forming an acidic solution that ionises only slightly.
• Explain weak acidity in terms of partial ionisation giving few H⁺ ions and a higher pH than a strong acid.
• At higher tier, describe reaction with an alcohol to form an ester, usually with an acid catalyst.
• Recognise the '-anoic acid' ending in a name as showing a carboxylic acid.
• Use the stem of the name to count the carbon atoms in the chain.
• Identify the –COOH unit in displayed, structural or molecular formulae.
• Count the –COOH carbon when working out the name from a formula.
• Distinguish carboxylic acids from alcohols and esters by the presence of the complete –COOH group.
• Link recognition of the group to the reactions and weak acidity expected of the compound.

• 💡When asked to identify a functional group, circle the whole –COOH unit in the formula before naming it.
• 💡Practise converting between displayed, structural and molecular formulae of the first four carboxylic acids so recognition is quick.
• 💡If a question asks why a compound is acidic, refer to the –COOH group releasing H⁺ ions in water rather than saying it 'contains acid'.
• 💡Count every carbon atom, including the one in –COOH, before choosing the stem of the name.
• 💡Learn the four names and formulae as a sequence so you can move up or down the series by adding or removing CH₂.
• 💡When comparing members, separate chemical similarity (same –COOH reactions) from physical trends (boiling point and solubility).
• 💡For carbonate reactions, write the general word equation acid + carbonate → salt + water + carbon dioxide before substituting names.
• 💡When explaining weak acidity, compare the extent of ionisation and the resulting pH with a strong acid of the same concentration.
• 💡At higher tier, link the alcohol and acid names directly to the ester name, remembering the acid provides the '-oate' ending.
• 💡Scan a formula for the –COOH unit first, then count carbons to choose the correct stem.
• 💡Check names for the '-anoic acid' ending before deciding a compound is a carboxylic acid.
• 💡Practise mixed examples of alcohols, carboxylic acids and esters so you can justify your identification by the functional group present.
• 💡Count all carbon atoms when naming carboxylic acids; methanoic has one carbon (HCOOH), ethanoic has two (CH3COOH), and propanoic has three (C2H5COOH).
• 💡When asked to write an esterification equation, clearly identify that water (H2O) is eliminated alongside the ester product.
• 💡Use equilibrium arrows (<=>) when writing the aqueous ionisation of carboxylic acids to clearly signify partial, reversible dissociation.

• Treating –COOH as an alcohol –OH group; correct this by noting that the –OH is attached to a carbon that is also double-bonded to oxygen, which makes the compound acidic rather than an alcohol.
• Writing the group as COOH without showing the double bond to oxygen; correct this by drawing C double-bonded to O and single-bonded to O–H.
• Assuming every molecule containing oxygen is a carboxylic acid; correct this by checking for the complete –COOH unit, since esters and alcohols contain oxygen but not this group.
• Naming CH₃COOH as methanoic acid by counting only the carbon outside –COOH; correct this by counting the –COOH carbon too, giving ethanoic acid.
• Writing formulae that omit the –COOH carbon from the chain length; correct this by checking the total carbon count against the prefix.
• Assuming all members have identical physical properties; correct this by noting that boiling point and solubility change as the chain lengthens, even though the functional group is the same.
• Saying carboxylic acids fully ionise in water; correct this by stating that ionisation is partial, so only a small proportion of molecules release H⁺ ions.
• Predicting hydrogen gas when a carboxylic acid reacts with a carbonate; correct this by naming carbon dioxide, water and a salt as the products.
• Confusing the ester product with the alcohol or acid reactant; correct this by identifying the ester formed from the acid and the alcohol, for example ethyl ethanoate from ethanoic acid and ethanol.
• Mistaking an alcohol for a carboxylic acid because both contain –OH; correct this by checking whether the –OH is part of a –COOH group.
• Ignoring the –COOH carbon when counting chain length; correct this by including it so the name matches the total number of carbon atoms.
• Reading a formula such as CH₃COOH as an ester; correct this by identifying the –COOH unit, which is absent from esters.
• Equating 'weak' with 'dilute': Students often think a weak acid means a solution with lots of water. In reality, 'weak' describes incomplete ionisation, while 'dilute' refers to a low amount of solute per unit volume.
• Misnaming the salts: Students frequently name the salt produced by ethanoic acid as 'ethanoate acid' or 'ethane salt' instead of using the metal name followed by 'ethanoate' (e.g., sodium ethanoate).
• Writing incorrect molecular formulas: Forgetting that the carbon in the -COOH functional group counts toward the total carbon chain, leading to errors like writing propanoic acid as C3H7COOH instead of C2H5COOH.

1. 1Day 1-2: Memorise the names, molecular formulas, and displayed structures of the first four carboxylic acids (methanoic, ethanoic, propanoic, butanoic).
2. 2Day 3-4: Practise writing balanced symbol equations for carboxylic acid reactions with carbonates, oxides, and hydroxides.
3. 3Day 5: Master the difference between strong vs weak acids and concentrated vs dilute solutions using particle diagrams and pH values.
4. 4Day 6-7: Complete exam-style questions on ester formation and revise mark schemes for commonly missed terms.

• 📋Displayed formula drawing: Questions asking you to draw the displayed structure of an named carboxylic acid showing all bonds.
• 📋Comparison extended response: Comparing the reactions and pH of a strong acid (like HCl) and a weak acid (like CH3COOH) at equal concentrations.
• 📋Equation writing: Balanced symbol equations involving the reaction of carboxylic acids with carbonates or alcohols.