The shape of the lesson
By the end of the lesson, learners can
- Three ways to make one: Rotate the carboxyl group in the simulator and switch between the acid and its anion.
- Weak, but still an acid: Compare the acids in the simulator, then take one to the bench and add a metal, an alkali and a carbonate.
- Tests and reduction: Two more ideas finish the topic: telling an acid from an alcohol, and turning the acid back into an alcohol.
- Make: primary alcohol or aldehyde + K2Cr2O7/H+, reflux; nitrile or ester + dilute acid, reflux.
- Metal: salt + H2; alkali: salt + H2O.
- Carbonate: salt + CO2 + H2O, the test against alcohols.
- LiAlH4: reduces RCOOH to RCH2OH.
One picture every idea returns to
A carboxylic acid contains the carboxyl group, –COOH. It is made by oxidising a primary alcohol or an aldehyde with acidified K2Cr2O7, heating under reflux; by hydrolysing a nitrile with dilute acid under reflux (the C of the CN group becomes the COOH carbon); or by hydrolysing an ester. Carboxylic acids are weak acids: they only partly dissociate, RCOOH ⇌ RCOO− + H+. They still show all the typical acid reactions: reactive metals give a salt and hydrogen, alkalis give a salt and water, and carbonates give a salt, water and carbon dioxide. That fizzing with sodium carbonate is the test that separates them from alcohols. LiAlH4 in dry ether reduces them to primary alcohols.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, The –COOH group, a weak acid.. Oxidise a primary alcohol under reflux, or hydrolyse a nitrile or an ester, and you get a carboxylic acid. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Three ways to make one | Rotate the carboxyl group in the simulator and switch between the acid and its anion. Project tab 1 of the lab (Carboxylic acids lab, the carboxyl group) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Weak, but still an acid | Compare the acids in the simulator, then take one to the bench and add a metal, an alkali and a carbonate. Project tab 2 of the lab (Carboxylic acids lab, how acidic and why) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Tests and reduction | Two more ideas finish the topic: telling an acid from an alcohol, and turning the acid back into an alcohol. | Jigsaw |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Tests and reduction, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Only carbonates and hydrogencarbonates separate acids from alcohols: sodium and PCl 5 react with any –OH group. | Carboxylic acids are weak (partly dissociated), not dilute; a weak acid still reacts completely with excess metal, alkali or carbonate. Keep the 2: 1 acid to Mg and acid to Na 2 CO 3 ratios. Reduction needs LiAlH 4, not NaBH 4, and gives a primary alcohol. A nitrile keeps its CN carbon, so count carbons carefully. |
Carboxylic acids: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Give the reagents and conditions for making ethanoic acid from (a) ethanol, (b) ethanenitrile, CH3CN, (c) ethyl ethanoate.
Write balanced equations for the reactions of propanoic acid with (a) magnesium, (b) sodium hydroxide, (c) sodium carbonate.
Two colourless liquids are ethanol and ethanoic acid. Describe a chemical test to tell them apart, giving the result for each.
25.0 cm3 of a solution of ethanoic acid is neutralised by 20.0 cm3 of 0.125 mol dm−3 NaOH. Calculate the concentration of the ethanoic acid in mol dm−3.
4.40 g of butanoic acid, CH3CH2CH2COOH (Mr 88.0), is reduced completely with LiAlH4. Name the product and calculate its maximum mass (Mr 74.0).
Explain what is meant by saying that ethanoic acid is a weak acid, and write the equation for the equilibrium involved.
Answer key and marking notes
- (a) Excess acidified potassium dichromate(VI), heat under reflux.
- (b) Dilute hydrochloric acid, heat under reflux (CH3CN + 2H2O + HCl → CH3COOH + NH4Cl).
- (c) Dilute acid, heat under reflux (or NaOH(aq) under reflux, then acidify the sodium ethanoate).
- (a) 2CH3CH2COOH + Mg → (CH3CH2COO)2Mg + H2.
- (b) CH3CH2COOH + NaOH → CH3CH2COONa + H2O.
- (c) 2CH3CH2COOH + Na2CO3 → 2CH3CH2COONa + CO2 + H2O.
- Add sodium carbonate (or sodium hydrogencarbonate) solution.
- Ethanoic acid fizzes and the gas turns limewater milky (CO2); ethanol shows no change.
- n(NaOH) = 0.125 × 20.0 ÷ 1000 = 0.00250 mol.
- Ratio 1 : 1, so n(CH3COOH) = 0.00250 mol.
- Concentration = 0.00250 ÷ 0.0250 = 0.100 mol dm−3.
- The product is butan-1-ol, CH3CH2CH2CH2OH (a primary alcohol).
- n = 4.40 ÷ 88.0 = 0.0500 mol; mass = 0.0500 × 74.0 = 3.70 g.
- A weak acid only partially dissociates into ions in water; most molecules stay un-ionised.
- CH3COOH ⇌ CH3COO− + H+, with the position of equilibrium well to the left.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which reaction produces propanoic acid, CH3CH2COOH?
- Aheating propanenitrile, CH3CH2CN, under reflux with dilute hydrochloric acid
- Bheating ethanenitrile, CH3CN, under reflux with dilute hydrochloric acid
- Cwarming propan-1-ol with a limited amount of acidified K2Cr2O7 and distilling the product off as it forms
- Dheating propan-2-ol under reflux with excess acidified K2Cr2O7
Which reagent could be used to distinguish ethanoic acid from ethanol?
- Asodium carbonate solution: only ethanoic acid fizzes, giving CO2
- Bsodium metal: only ethanoic acid gives hydrogen
- Cphosphorus(V) chloride: only ethanoic acid gives steamy fumes
- Dbromine water: only ethanoic acid decolourises it
Which equation correctly shows the reaction of ethanoic acid with magnesium?
- A2CH3COOH + Mg → (CH3COO)2Mg + H2
- BCH3COOH + Mg → CH3COOMg + H2
- C2CH3COOH + Mg → (CH3COO)2Mg + H2O
- D2CH3COOH + Mg → (CH3COO)2Mg + CO2 + H2
Butanoic acid, CH3CH2CH2COOH, is reduced with LiAlH4 in dry ether. What is the organic product?
- Abutan-1-ol, CH3CH2CH2CH2OH
- Bbutan-2-ol, CH3CH2CH(OH)CH3
- Cbutane, CH3CH2CH2CH3
- Dno reaction, because carboxylic acids cannot be reduced
0.100 mol dm−3 hydrochloric acid has pH 1.0, but 0.100 mol dm−3 ethanoic acid has pH 2.9. Which statement explains this?
- Aethanoic acid only partially dissociates, CH3COOH ⇌ CH3COO− + H+, so [H+] is much lower
- Bethanoic acid is more dilute than the hydrochloric acid
- Ceach ethanoic acid molecule releases four H+ ions, one from each H atom, but more slowly
- Dethanoic acid reacts with water to form hydroxide ions, which raise the pH
Use the reaction scheme. Compound A fizzes with sodium carbonate solution. What is compound B?
- Apropan-1-ol, CH3CH2CH2OH
- Bethanol, CH3CH2OH
- Cpropylamine, CH3CH2CH2NH2
- Dpropanoic acid, CH3CH2COOH
0.600 g of ethanoic acid (Mr 60.0) reacts with excess sodium carbonate. What volume of carbon dioxide, measured at room conditions, is produced? (molar gas volume 24.0 dm3)
- A0.120 dm3
- B0.240 dm3
- C0.0600 dm3
- D14.4 dm3
25.0 cm3 of a solution of ethanoic acid is exactly neutralised by 20.0 cm3 of 0.125 mol dm−3 sodium hydroxide. What is the concentration of the ethanoic acid?
- A0.100 mol dm−3
- B0.156 mol dm−3
- C0.200 mol dm−3
- D0.00250 mol dm−3
Quiz key
Acid hydrolysis turns the CN group into COOH and keeps every carbon: CH3CH2CN + 2H2O + HCl → CH3CH2COOH + NH4Cl. Ethanenitrile has only two carbons, so it gives ethanoic acid; thinking the CN carbon is lost is the classic error. Distilling the product off as it forms stops the oxidation of propan-1-ol at propanal; reflux with excess oxidant is needed for the acid. Propan-2-ol is a secondary alcohol, so it is oxidised only to the ketone propanone.
Carboxylic acids are acidic enough to react with carbonates: 2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O; the gas turns limewater milky. Ethanol is far too weak an acid to do this. Sodium reacts with both, giving hydrogen in each case, so it cannot distinguish them. PCl5 gives steamy HCl fumes with any O-H group, in alcohols and acids alike. Neither compound has a C=C bond, so neither decolourises bromine water.
A reactive metal and an acid give a salt and hydrogen. Mg2+ needs two CH3COO− ions, so two acid molecules react and one H2 forms. CH3COOMg ignores the 2+ charge of magnesium and leaves hydrogen unbalanced. Water forms when an acid reacts with an alkali or a base, not a metal, and that equation is unbalanced. CO2 forms only with a carbonate; there is no carbonate here.
LiAlH4 reduces the COOH group to CH2OH: RCOOH + 4[H] → RCH2OH + H2O, so a primary alcohol forms with the OH on the end carbon. Butan-2-ol would need the oxygen on carbon 2, but the functional group is on carbon 1. Reduction stops at the alcohol; the C-O bond is not removed, so no alkane forms. Carboxylic acids can be reduced, but only by the powerful LiAlH4; the milder NaBH4 has no effect on them, which is where this misconception comes from.
A weak acid is only partly ionised in water; the equilibrium lies well to the left, so [H+] is only about 1.3 × 10−3 mol dm−3, giving pH 2.9. HCl is a strong acid and is fully ionised, so [H+] = 0.100 mol dm−3 and pH = 1.0. Both acids have the same concentration, so 'more dilute' confuses weak with dilute; this is the classic error. Only the H on the O-H is acidic; the three C-H hydrogens are not released. Ethanoic acid does not produce hydroxide ions in water; it produces H+ ions.
Hydrolysis of propanenitrile with dilute acid under reflux gives propanoic acid, A, which is why A fizzes with carbonate. LiAlH4 then reduces the COOH group to CH2OH, so B is the primary alcohol propan-1-ol. The carbon count stays at three throughout: losing the CN carbon to give ethanol is the classic error. Propylamine would form if LiAlH4 reduced the nitrile itself, but the nitrile has already been hydrolysed. Propanoic acid is A, not B; it is the reactant in the second step.
2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O. n(acid) = 0.600 / 60.0 = 0.0100 mol, so n(CO2) = 0.00500 mol and V = 0.00500 × 24.0 = 0.120 dm3. 0.240 dm3 assumes a 1 : 1 ratio, which would be correct for NaHCO3 but not for Na2CO3. 0.0600 dm3 divides by 2 twice. 14.4 dm3 multiplies the mass, not the moles, by 24.0.
CH3COOH + NaOH → CH3COONa + H2O (1 : 1). n(NaOH) = 0.125 × 20.0 / 1000 = 0.00250 mol = n(acid). Concentration = 0.00250 / 0.0250 = 0.100 mol dm−3. 0.156 mol dm−3 swaps the two volumes. 0.200 mol dm−3 assumes ethanoic acid releases two H+ ions; only the O-H hydrogen is acidic, so it is monoprotic. 0.00250 is the number of moles, not the concentration.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.