Carboxylic Acids

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.34 — Carboxylic acids contain the –COOH functional group (bold C statement — Paper 2 only)
• 4.35 — Structural and displayed formulae for unbranched carboxylic acids up to four carbons (bold C statement — Paper 2 only)
• 4.36 — Reactions of aqueous carboxylic acids with metals and metal carbonates (bold C statement — Paper 2 only)
• 4.37 — Vinegar as an aqueous solution containing ethanoic acid (bold C statement — Paper 2 only)

Specification section 4(e) Alcohols and (f) carboxylic acids — also taught in Alkenes, Alkene Reactions And The Bromine Test, Alcohols, Making Ethanol.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A CARBOXYLIC ACID contains the group –COOH. That is a carbon atom carrying a double-bonded oxygen and an –OH, all on the end of a chain, and it is written as one unit because it behaves as one. These are the compounds that ethanol becomes when it is oxidised, so this family is the destination of two of the three routes on the alcohols page.

The first four carboxylic acids, and the counting trap

1. METHANOIC ACID: one carbon in total. HCOOH — the –COOH group with nothing but a hydrogen attached.
2. ETHANOIC ACID: two carbons in total. CH₃COOH — a methyl group attached to –COOH.
3. PROPANOIC ACID: three carbons in total. CH₃CH₂COOH, also written C₂H₅COOH.
4. BUTANOIC ACID: four carbons in total. CH₃CH₂CH₂COOH, also written C₃H₇COOH.
5. THE TRAP: the carbon inside the –COOH group counts. Ethanoic acid has ONE carbon outside the group and TWO altogether — which is why it is eth- and not meth-.
6. The general formula is CₙH₂ₙ₊₁COOH, where n counts the carbons OUTSIDE the group. For ethanoic acid n = 1.

Think of it like counting the driver as part of the crew

Ask how many people are on a bus and the answer includes the driver, even though the driver is doing a different job from everyone else. Ask how many passengers and it does not. The carbon in –COOH is the driver: it is doing something different from the carbons in the chain, so it feels as though it should be counted separately, and it is exactly this feeling that produces the wrong answer. When you name a carboxylic acid, count everybody on the bus. Ethanoic acid has two carbons, and one of them is the driver.

Dissolved in water, these compounds behave as acids, and the reactions the specification asks for are the same two acid reactions met earlier in the course. What is worth noticing is that they behave exactly as any other acid does — the chemistry of an acid is not changed by its being an organic one.

Reaction one — with a metal

1. Add a reactive metal such as magnesium to an aqueous carboxylic acid.
2. The metal fizzes and gradually disappears as it dissolves.
3. A gas is given off, which burns with a squeaky pop when a lit splint is held to it — it is hydrogen.
4. The general pattern is: acid + metal → salt + hydrogen.
5. For ethanoic acid and magnesium: 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂.
6. The salt formed from ethanoic acid is called an ETHANOATE — magnesium ethanoate in this case.

Reaction two — with a metal carbonate

1. Add a metal carbonate such as sodium carbonate to an aqueous carboxylic acid.
2. The mixture effervesces vigorously and the solid dissolves.
3. The gas given off turns limewater milky, which identifies it as carbon dioxide.
4. The general pattern is: acid + metal carbonate → salt + water + carbon dioxide.
5. For ethanoic acid and sodium carbonate: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂.
6. Three products here rather than two — forgetting the water is the commonest error on this equation.

Statement 4.37 says something very simple and worth taking literally: VINEGAR is an aqueous solution containing ethanoic acid. It is not ethanoic acid; it is a dilute solution of it in water, typically around five per cent. This is why vinegar can be poured on food and drunk in a salad dressing while pure ethanoic acid is corrosive and would burn the mouth. Concentration is the whole difference.

It also explains the everyday observations that this page sits behind. Vinegar fizzes when it is poured on baking powder, because sodium hydrogencarbonate reacts with the ethanoic acid to give carbon dioxide. Vinegar cleans limescale off a kettle, because limescale is calcium carbonate and it reacts away. And vinegar is what a bottle of wine becomes if you leave it open, which is the microbial oxidation from the alcohols page arriving at its destination.

One qualification, so that the picture is honest. Carboxylic acids are WEAK acids: in solution only a small proportion of their molecules release a hydrogen ion at any moment, unlike hydrochloric acid where essentially all of them do. This is why vinegar reacts more gently than a laboratory acid of the same concentration, and why it is safe on food. The reactions are the same; the vigour is not.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Carboxylic acid

A member of the homologous series whose functional group is –COOH, with the general formula CₙH₂ₙ₊₁COOH.

The first four carboxylic acids

2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂

Conditions: Aqueous ethanoic acid with magnesium. Effervescence, the metal dissolves, and the gas gives a squeaky pop with a lit splint.

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Conditions: Aqueous ethanoic acid with sodium carbonate. Effervescence, and the gas turns limewater milky. THREE products — do not lose the water.

The words this page is marked on

–COOH

The carboxylic acid functional group: a carbon with a double-bonded oxygen and an –OH.

Ethanoate

The salt formed from ethanoic acid, for example sodium ethanoate, CH₃COONa.

Vinegar

An aqueous solution containing ethanoic acid, typically around five per cent.

Weak acid

One in which only a small proportion of the molecules release a hydrogen ion in solution.

The two reactions and their observations

• ACID + METAL → salt + hydrogen. Fizzing, the metal dissolves, and a lit splint gives a squeaky pop
• ACID + METAL CARBONATE → salt + water + carbon dioxide. Effervescence, and the gas turns limewater milky
• The salts of ethanoic acid are called ethanoates
• Both reactions are the ordinary reactions of any acid — being organic changes nothing about them

Model answer [4 marks]

Describe what is seen when magnesium ribbon is added to aqueous ethanoic acid, and name the products. [4]

The magnesium fizzes and gradually dissolves as it reacts. A colourless gas is given off which burns with a squeaky pop when a lit splint is held to it, identifying it as hydrogen. The products are magnesium ethanoate, which stays in solution, and hydrogen gas, and the equation is 2CH₃COOH + Mg → (CH₃COO)₂Mg + H₂. The reaction follows the general pattern for any acid with a reactive metal, giving a salt and hydrogen, although it is less vigorous than with a strong acid of the same concentration.

Model answer [3 marks]

Explain why vinegar can be used on food while pure ethanoic acid cannot. [3]

Vinegar is not ethanoic acid itself but an aqueous solution containing it, typically at around five per cent, so the ethanoic acid is present in a very dilute form. Pure ethanoic acid is corrosive and would burn the mouth and skin. Because the reactions of an acid become less vigorous the more dilute the solution is, the same substance that is dangerous when concentrated is harmless when diluted this far — the difference is entirely one of concentration, not of chemistry.

Not this: Ethanoic acid has three carbon atoms, since ethane has two and the –COOH adds another.

This: The carbon inside the –COOH group is COUNTED in the name. Ethanoic acid, CH₃COOH, has two carbons in total: one in the methyl group and one in the –COOH. That is exactly why it is called eth- and not prop-.

Mark-losing trap. Count the carbon in –COOH. Ethanoic acid has TWO carbons altogether, not three.

Mark-losing trap. The carbonate reaction gives THREE products — salt, water AND carbon dioxide. The water is the one that gets forgotten.

Mark-losing trap. Vinegar is a dilute aqueous SOLUTION containing ethanoic acid, not pure ethanoic acid.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is the main acid present in vinegar?
2. Grade 7 · Deduce [2 marks] — How many carbon atoms are there in one molecule of propanoic acid, CH₃CH₂COOH?
3. Grade 8 · Predict [3 marks] — Predict the products when aqueous ethanoic acid is added to calcium carbonate, and state the observation.
4. Grade 9 · Describe [4 marks] — Select every statement that belongs in a full answer describing how you would show that an unlabelled colourless liquid is aqueous ethanoic acid rather than water.
5. 9+ · Explain [6 marks] — A student finds that vinegar removes limescale from a kettle more slowly than dilute hydrochloric acid of the same concentration. Explain this observation and identify the gas produced.

The people behind this science

Two ways into the same idea — the one who worked out which element actually makes something an acid, and the one who decided a substance should be identified by the reactions it gives. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Humphry Davy — the one who worked out which element actually makes something an acid

This page assumes something that took a long argument to settle: that what makes –COOH an acid group is the hydrogen it can release. Lavoisier had named oxygen from the Greek for acid-former, believing every acid must contain it, and Davy demolished that by showing that muriatic acid — hydrochloric acid — contains no oxygen whatever. Acidity, he argued, belongs to hydrogen. That conclusion is why a group written –COOH is recognised as acidic on sight, and he is the right person to ask how a definition of a whole class of substances gets overturned.

• “How did you show that an acid need contain no oxygen?”
• “What convinced you hydrogen was the element that mattered?”
• “How do you argue against a name that already assumes the answer?”
• “Does every substance containing hydrogen count as an acid?”
• “What did Lavoisier's supporters say when you were proved right?”

Robert Boyle — the one who decided a substance should be identified by the reactions it gives

The two reactions on this page are diagnostic ones: effervescence with a carbonate and a gas that turns limewater milky, or a metal dissolving with a gas that pops. Boyle is where that way of working comes from. In the 1660s he set out the first systematic tests for acids and alkalis, using plant dyes that changed colour, and argued that a substance should be identified by what it DOES with known reagents rather than by its origin or appearance. He is the right figure to ask what makes a set of observations amount to an identification.

• “What tests did you use to recognise an acid?”
• “Why identify a substance by its reactions rather than its look?”
• “How many tests does it take before you are certain?”
• “Could two different substances pass the same test?”
• “What did you make of acids that act far more gently than others?”

Carry on through the course

• Previous lesson: Making Ethanol
• Next lesson: Esters And How To Make One
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification