The shape of the lesson
By the end of the lesson, learners can
- Substituents change acid strength: Change the substituents in the simulator and watch the pKa and the charge on the carboxylate ion.
- Acids that can be oxidised: Run Tollens' and Fehling's tests in the simulator.
- Order: RCOOH > phenol > water > alcohol.
- More Cl, closer Cl: lower pKa, stronger acid.
- Alkyl groups: electron-donating, weaker acid.
- Oxidised: HCOOH (Tollens', Fehling's, KMnO4) and H2C2O4 (warm KMnO4), both to CO2.
One picture every idea returns to
Carboxylic acids are the strongest of the O–H acids: carboxylic acid > phenol > water > alcohol, because the charge of the carboxylate ion is shared equally over two O atoms. Anything that stabilises RCOO− further makes the acid stronger. Electron-withdrawing Cl atoms pull electron density away from the COO− group and spread its charge: the more Cl atoms, and the closer they are to the COOH group, the stronger the acid (pKa ethanoic 4.76, chloroethanoic 2.86, dichloroethanoic 1.29, trichloroethanoic 0.65). Electron-donating alkyl groups destabilise the anion, so ethanoic acid is weaker than methanoic acid. Methanoic acid, HCOOH, still has an H on its C=O carbon, so it is oxidised to CO2 by Tollens', Fehling's and acidified KMnO4; ethanedioic acid decolourises warm acidified KMnO4, giving CO2.
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, Pull electrons away, make a stronger acid.. All carboxylic acids are weak, but some are far weaker than others. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Substituents change acid strength | Change the substituents in the simulator and watch the pKa and the charge on the carboxylate ion. Project tab 1 of the lab (Carboxylic acids and derivatives (A Level) lab, substituents and acid strength) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Acids that can be oxidised | Run Tollens' and Fehling's tests in the simulator. Project tab 2 of the lab (Carboxylic acids and derivatives (A Level) lab, Tollens' and Fehling's bench) and let a student drive it while the class predicts. | Connect-Extend-Challenge |
| 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 Acids that can be oxidised, 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.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
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 |
|---|---|
| Electron- withdrawing groups make an acid stronger; electron-donating groups make it weaker. | The explanation must be about the stability of the carboxylate anion, not about the O–H bond alone. More Cl is stronger, and Cl closer to the COOH group is stronger. A smaller pKa means a stronger acid. Of the simple carboxylic acids, only methanoic gives positive Tollens' and Fehling's tests; ethanedioic acid needs warm acidified KMnO 4. |
Relative acidity of carboxylic acids: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Explain why trichloroethanoic acid is a much stronger acid than ethanoic acid.
Using the pKa values 4.76 (ethanoic acid) and 0.65 (trichloroethanoic acid), calculate Ka for each and how many times stronger trichloroethanoic acid is.
Put these in order of increasing acid strength and explain the position of the first one: propanoic acid, 2-chloropropanoic acid, 3-chloropropanoic acid.
Describe what is seen when methanoic acid is warmed with (a) Tollens' reagent, (b) Fehling's solution, (c) acidified KMnO4, and write an equation using [O].
25.0 cm3 of ethanedioic acid solution needs 20.0 cm3 of 0.0200 mol dm−3 KMnO4 in warm acid. Calculate the concentration of the ethanedioic acid.
Describe a test to distinguish methanoic acid from ethanoic acid, and explain why only one reacts.
Answer key and marking notes
- Cl is more electronegative than H, so each Cl withdraws electron density along the σ bonds.
- This spreads the negative charge of Cl3CCOO− and stabilises it.
- Three Cl atoms have a large combined effect, so the acid ionises far more (pKa 0.65 against 4.76).
- Ka = 10−4.76 = 1.74 × 10−5 and 10−0.65 = 0.224 mol dm−3.
- Ratio = 0.224 ÷ (1.74 × 10−5) = 1.29 × 104.
- Propanoic acid < 3-chloropropanoic acid < 2-chloropropanoic acid.
- Propanoic acid has no electron-withdrawing group, only an electron-donating ethyl group, so its anion is least stable.
- (a) a silver mirror; (b) a brick-red precipitate of Cu2O from a blue solution; (c) purple to colourless.
- HCOOH + [O] → CO2 + H2O.
- n(MnO4−) = 0.0200 × 0.0200 = 4.00 × 10−4 mol.
- n(H2C2O4) = 4.00 × 10−4 × 5/2 = 1.00 × 10−3 mol.
- Concentration = 1.00 × 10−3 ÷ 0.0250 = 0.0400 mol dm−3.
- Warm each with Tollens' reagent: methanoic acid gives a silver mirror, ethanoic acid gives no change.
- Methanoic acid, HCOOH, has an H attached to the C=O carbon, which can be oxidised (to CO2); in ethanoic acid that carbon is bonded to CH3, OH and =O, with no H.
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 acid has the lowest pKa?
- Atrichloroethanoic acid, Cl3CCOOH
- Bdichloroethanoic acid, Cl2CHCOOH
- Cchloroethanoic acid, ClCH2COOH
- Dethanoic acid, CH3COOH
Why is chloroethanoic acid a stronger acid than ethanoic acid?
- Athe electronegative Cl withdraws electron density, spreading the charge on the carboxylate ion and stabilising it
- Bthe Cl atom donates electrons to the COO− group
- Cthe Cl atom ionises to release H+
- Dchloroethanoic acid forms more hydrogen bonds with water
The chart shows the pKa values of butanoic acid and its 2-, 3- and 4-chloro isomers, labelled A to D. Which is 2-chlorobutanoic acid, CH3CH2CHClCOOH?
- AB
- BA
- CC
- DD
Which compound gives a silver mirror when warmed with Tollens' reagent?
- Amethanoic acid, HCOOH
- Bethanoic acid, CH3COOH
- Cethanedioic acid, HOOCCOOH
- Dpropanone, CH3COCH3
Which list shows the compounds in order of DECREASING acid strength?
- Aethanoic acid, phenol, water, ethanol
- Bphenol, ethanoic acid, water, ethanol
- Cethanoic acid, water, phenol, ethanol
- Dethanol, water, phenol, ethanoic acid
The pKa of ethanoic acid is 4.76 and that of dichloroethanoic acid is 1.29. How many times larger is Ka for dichloroethanoic acid?
- A2.95 × 103
- B3.47
- C3.69
- D3.39 × 10−4
25.0 cm3 of 0.0400 mol dm−3 ethanedioic acid is titrated with 0.0200 mol dm−3 KMnO4 in warm acid: 5H2C2O4 + 2MnO4− + 6H+ → 2Mn2+ + 10CO2 + 8H2O. What volume of KMnO4 is needed?
- A20.0 cm3
- B50.0 cm3
- C125 cm3
- D8.00 cm3
Methanoic acid (pKa 3.75) is a stronger acid than ethanoic acid (pKa 4.76). Which explanation is correct?
- Athe methyl group in ethanoate is electron-donating, increasing the charge density on COO− and destabilising it
- Bmethanoic acid has two O–H bonds
- Cthe methyl group is electron-withdrawing, stabilising ethanoate
- Dmethanoic acid can be oxidised, so it loses H+ more easily
Quiz key
Three electron-withdrawing Cl atoms stabilise Cl3CCOO− most, giving pKa 0.65. Dichloroethanoic acid (1.29) and chloroethanoic acid (2.86) have fewer Cl atoms, so their anions are less stabilised. Ethanoic acid (4.76) has no Cl and an electron-donating methyl group, so it is the weakest and has the highest pKa.
Cl pulls electron density along the σ bonds away from COO−, reducing its charge density, so ClCH2COO− is more stable and the acid ionises more (pKa 2.86 against 4.76). Cl is electron-withdrawing, not donating; a donating group would weaken the acid. The covalent C–Cl bond does not ionise, and Cl gives no H+. Hydrogen bonding to water affects solubility, not the position of the ionisation equilibrium in this way.
In 2-chlorobutanoic acid the Cl is on the carbon next to COOH, so its electron-withdrawing effect is largest and the pKa is lowest (2.86): B. D (4.05) is the 3-chloro isomer and A (4.52) is the 4-chloro isomer; the inductive effect fades with distance. C (4.82) is butanoic acid itself, with no Cl.
Methanoic acid has an H–C=O group, like an aldehyde, so it is oxidised to CO2 and reduces Ag+ to silver. Ethanoic acid has no H on the carboxyl carbon and is not oxidised. Ethanedioic acid is oxidised by warm acidified MnO4−, but not by the weaker oxidant Tollens' reagent. Propanone is a ketone and gives no silver mirror.
Ethanoic acid (4.76) shares its anion charge over two O atoms; phenol (10.0) spreads it into the ring; water (15.7) keeps it on one O; ethanol (about 16) has an alkyl group that destabilises the anion. Phenol is not stronger than a carboxylic acid: it cannot release CO2 from carbonates. Water is weaker than phenol, not stronger. The last list is the order of INCREASING strength.
Ratio = 10(4.76 − 1.29) = 103.47 = 2.95 × 103. 3.47 is the pKa difference itself, not the ratio. 3.69 divides the pKa values (4.76 ÷ 1.29), which has no meaning on a log scale. 3.39 × 10−4 is the inverted ratio, which would make dichloroethanoic acid the weaker acid.
n(H2C2O4) = 0.0250 × 0.0400 = 1.00 × 10−3 mol; n(MnO4−) = 4.00 × 10−4 mol; volume = 4.00 × 10−4 ÷ 0.0200 = 0.0200 dm3 = 20.0 cm3. 50.0 cm3 assumes a 1 : 1 ratio. 125 cm3 uses 5/2 instead of 2/5. 8.00 cm3 applies the 2/5 factor twice.
Alkyl groups push electron density towards the carboxylate group, so CH3COO− is less stable than HCOO−, which has only H attached; ethanoic acid therefore ionises less. Methanoic acid, HCOOH, has only one O–H bond. A methyl group donates electrons; if it withdrew them, ethanoic acid would be the stronger. Ease of oxidation (the H–C=O group) is a separate property and does not decide acid strength.
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.