Carboxylic acids

The alcohols lesson ended with a sour note: oxidise ethanol — microbially in open air or sharply in the lab with dichromate(VI) — and the product is ethanoic acid. That product is not a one-off. It is the representative of a whole family of organic acids, the carboxylic-acids, and this lesson gives that family the same treatment the alcohols received: structure first, then names, then reactions (u4:c26).

Every member of the family carries the same functional-group: the carboxyl group, –COOH. Read the arrangement carefully, because it is two groups welded onto one carbon — a carbonyl (C=O) and a hydroxyl (O–H) sharing the same end carbon atom:

      O
      ‖
—C—O—H

That shared carbon is what makes the chemistry of this family different from both the alcohols (O–H alone) and the aldehydes (C=O alone). It is also why the group is always found at the end of a chain: the bonding arrangement needs a carbon with a spare bond for the rest of the molecule.

The most familiar member is ethanoic-acid, which carried the old name acetic acid — and the everyday face of the family is vinegar: an aqueous solution containing ethanoic acid (4.37C). The sour taste of vinegar and the sharpness of the family are the same chemistry at different dilutions.

The family is built by oxidation. A carboxylic acid forms when its corresponding alcohol is oxidised — leave ethanol exposed to air and its resident microorganisms, or oxidise it deliberately with potassium dichromate(VI) in dilute sulfuric acid, and the –OH-bearing carbon climbs one rung up the oxidation ladder to –COOH (Sections 4–5 of the alcohols lesson). The rung above it on that ladder does not exist at IGCSE: the carboxylic acids are where the oxidation story of the alcohols stops.

Statement 4.35C narrows the task to the four simplest members of the homologous-series — methanoic-acid, ethanoic, propanoic and butanoic acid — and asks for the three levels of formula plus the name of each. The naming engine is the same one the alkanes, alkenes and alcohols used: meth- means one carbon, eth- two, prop- three, but- four, and the acid ending -anoic acid replaces the alcohol's -anol.

The molecular-formula counts each element's atoms; the structural-formula shows the order of the atoms unambiguously — every member ends in COOH with the hydrogen-bearing chain before it. The molecular formulae share the pattern CnH2nO2: each member is its corresponding alcohol with one extra oxygen atom — the oxidation that builds the carboxyl group. Methanol CH₄O oxidised gives methanoic acid CH₂O₂; ethanol C₂H₆O gives ethanoic acid C₂H₄O₂. The carbon count never changes on the way up.

The displayed-formula level is where the discipline lives. Every bond must be drawn — the chain as C–C and C–H lines, and the carboxyl end with the C=O double bond and the O–H single bond shown explicitly. Methanoic acid's displayed formula is H–C(=O)–O–H with both bonds on the same carbon visible. The condensed COOH is a convenience for equations; the drawn-out arrangement is what displayed means.

Ethanoic acid and the other carboxylic acids are weak-acids — a label this section uses and Section 5 finally explains. In practical terms: their solutions have a pH of about 3–5 (the exact value depends on the concentration), they turn blue litmus paper red, and they react with everything you expect an acid to react with. Weak does not mean inactive — the family passes every classical test of an acid; it just runs the reactions more gently than hydrochloric or sulfuric acid do.

Everything that follows takes ethanoic-acid as the worked example, because it is the member you are most likely to meet in a laboratory or a kitchen. The other members behave the same way: if a question switches the acid, all you change is the formula — CH₃COOH becomes CH₃CH₂COOH, and the reaction writing itself is unchanged. That is the practical payoff of a homologous series: learn one member's reactions, own the family's.

Two reaction families deliver Statement 4.36C, and each gets its own section: the reaction with metals (next section, salts plus hydrogen) and the reaction with metal carbonates (the section after, salts plus carbon dioxide plus water). Both are re-runs of acid chemistry met in Unit 2 — what is new here is only the acid, and the ethanoate salts it leaves behind.

The Unit 2 rule still stands: a metal above hydrogen in the reactivity-series reacts with a dilute acid to give a salt and hydrogen (u2:c14, u2:c16). Dilute ethanoic acid obeys it exactly as hydrochloric acid does — only more slowly, which is the weak-acid behaviour of the last section showing through. The new thing is not the reaction type; it is the salt, because every carboxylic acid has its own family of salts built on the ethanoate ion.

Drop magnesium ribbon into dilute ethanoic acid and it fizzes vigorously: hydrogen gas streams off and the solid disappears into a colourless solution of magnesium ethanoate:

Mg(s) + 2CH₃COOH(aq) → (CH₃COO)₂Mg(aq) + H₂(g)

That formula deserves a slow read, because the salt formulae are where students reliably stumble. Magnesium ethanoate is an ionic-compound: it contains magnesium ions, Mg²⁺, and ethanoate ions, CH₃COO⁻. Each ethanoate-ion carries a single negative charge, so a doubly-charged magnesium ion needs two of them to balance the charges — hence the subscript 2 and hence the 2 in front of the acid in the equation. The metal is written after the ethanoate groups because that order reflects the structure: the positive magnesium ion is attracted to the negative charge sitting on the oxygen atoms of the ethanoate ions.

The ethanoate ion itself is worth a picture in words: a CH₃–C(=O) unit with the spare oxygen carrying the negative charge — written CH₃COO⁻. It is the carboxylic-acid molecule with its acidic hydrogen removed; the O–H proton is what the metal takes away, and the electron it leaves behind becomes the ion's charge. Draw it as the carboxyl group minus one H, with a minus sign on the single-bonded oxygen.

Other metals behave exactly as the reactivity-series predicts. Sodium is dangerously reactive with the acid — do not try it outside a demonstration. Zinc produces bubbles of hydrogen noticeably more slowly than it does with hydrochloric acid, a direct visual of the weak-acid speed difference. Metals below hydrogen in the series — copper, silver — do not react at all. Wherever a reaction does take place, a metal ethanoate forms alongside the hydrogen: zinc ethanoate is (CH₃COO)₂Zn, built by the same two-ethanoates-per-ion arithmetic.

This section is the why behind the weak-acid label — it runs one step beyond the statement list, which is exactly why the family's acid behaviour has been described so far only in outcomes. The distinction is about dissociation: how completely an acid splits into ions when it dissolves in water.

Ethanoic acid is a weak acid. The acids met earlier in the course — hydrochloric acid, sulfuric acid — are strong-acids. The word measures ionisation, and nothing else. A strong acid dissociates completely in water: a solution of hydrochloric acid contains mostly H⁺ and Cl⁻ ions and hardly any intact HCl molecules at all. A weak acid dissociates only partially: for ethanoic acid, only about 1 molecule in every 75 splits into H⁺ and CH₃COO⁻ — the rest stay whole, sitting in the solution dissolved but not ionised.

That single sentence explains every observation in this chapter. Same concentration of ethanoic acid and hydrochloric acid: the ethanoic solution holds far fewer H⁺ ions per litre, so it reacts with metals or metal carbonates much more slowly (fewer protons available to collide per second), and its pH sits higher. The acid still reacts — the reactions are just protracted, because each dissociated H⁺ that reacts nudges the equilibrium of the remaining molecules to dissociate a little more. Weak acids do the whole of an acid's job; they do it at a lower H⁺ concentration.

The second reaction family of Statement 4.36C is the carbonates: a metal-carbonate reacts with an acid to give a salt, carbon dioxide and water — the Unit 2 rule again (u2:c16), and dilute ethanoic acid obeys it like any other acid. What changes is only the salt, and the same ethanoate arithmetic as the metals section runs underneath it.

With sodium carbonate the reaction fizzes vigorously and leaves a colourless solution of sodium ethanoate:

Na₂CO₃(s) + 2CH₃COOH(aq) → 2CH₃COONa(aq) + CO₂(g) + H₂O(l)

With calcium carbonate the story repeats: lots of fizzing and a colourless solution — this time containing calcium ethanoate:

2CH₃COOH(aq) + CaCO₃(s) → (CH₃COO)₂Ca(aq) + CO₂(g) + H₂O(l)

Notice that both equations need two acid molecules per carbonate unit. The arithmetic is the metals section's again, viewed from the other side: the carbonate ion is doubly charged (CO₃²⁻), so it needs either two singly-charged metal ions or one doubly-charged one — and it needs two protons to become water and carbon dioxide. Each proton leaves behind one ethanoate-ion in solution, so two protons means two CH₃COOH molecules on the left. The salt side then just counts charges: two Na⁺ (each with one ethanoate) or one Ca²⁺ (with two ethanoates).

This is also the reaction that makes the family's fizz a diagnostic: any solution that fizzes on a carbonate and turns limewater milky with the gas produced is behaving as an acid — and for the carboxylic acids, gently. Ethanoic acid plus calcium carbonate is, in fact, a household-scale example of acid-on- limescale chemistry, run at vinegar strength rather than laboratory strength.

You now hold the complete carboxylic-acid story. The family is defined by the carboxyl group –COOH (4.34C); its four named members up to four carbons come with the three-level formula discipline and the count-them-all naming rule (4.35C); as weak-acids they react with metals and metal carbonates — slower than the strong acids, leaving ethanoate-ion salts and running the same charge arithmetic every time (4.36C); and vinegar, the everyday solution of ethanoic-acid, is the family's flagship (4.37C). Three equations anchor the chapter: the magnesium reaction, the sodium carbonate reaction and the calcium carbonate reaction.

Every specification statement of this chapter — 4.34C to 4.37C — has been taught, and each now joins the platform question bank through the links below, with practice items keyed to exactly those statements.

Before opening the bank, audit yourself against the chapter's core moves. Can you draw the carboxyl group with both bonds explicit? Can you produce molecular, structural and displayed levels for the four named acids? Can you write any metal ethanoate or hydrogen-carbonate product from ion charges alone? Can you explain the weak-acid rate difference through dissociation? Any wobble is a section to revisit, not a reason to start over.

The question bank for this chapter trains a predictable set of shapes, so rehearse the moves rather than the answers. Drawing and naming items open the set: structural formulae for named acids, displayed-formula drawings with the COOH bonding explicit, and naming from condensed structures. Identification items follow: a drawn structure to be tested against a tick-table of the alkene, alcohol and carboxylic-acid groups — read the drawing bond by bond, because a molecule can legitimately tick two boxes at once. A long oxidation item then reaches back to the alcohols lesson: propan-1-ol heated under reflux with potassium dichromate(VI) in dilute sulfuric acid, asking for the product's full displayed formula (the oxidation lands on this chapter's family), the orange-to-green colour change, a hazard critique of an unsafe apparatus choice, and then the product tested in two test-tubes — one with magnesium ribbon, one with sodium carbonate solution — so both of this chapter's reaction families run inside a single item. A finishing item balances the vinegar-plus-sodium-hydrogencarbonate frame (a salt plus carbon dioxide plus water) and asks for the reaction type and the observation. The shapes are the chapter; the acid names are just costumes.