The shape of the lesson
By the end of the lesson, learners can
- Proton donors and acceptors: Extended content.
- Strong and weak: Compare hydrochloric acid and ethanoic acid of the same concentration in the simulator.
- Acid: proton donor. Base: proton acceptor.
- Strong: completely dissociated (→).
- Weak: partially dissociated (⇌).
- Strength ≠ concentration.
One picture every idea returns to
Extended. An acid is a proton donor and a base is a proton acceptor, because an H+ ion is a proton. A strong acid is completely dissociated in water: HCl(aq) → H+(aq) + Cl−(aq). A weak acid is only partially dissociated: CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq). At the same concentration a weak acid has fewer H+ ions, a higher pH and reacts more slowly.
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, Giving away a proton.. A hydrogen ion is just a proton. 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: Proton donors and acceptors | Extended content. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Strong and weak | Compare hydrochloric acid and ethanoic acid of the same concentration in the simulator. Project tab 5 of the lab (Equilibria lab, acids and bases) 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, 14 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 Strong and weak, 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 |
|---|---|
| Strong and concentrated are different ideas. | Strength is about how completely the acid dissociates; concentration is about how much acid is dissolved per dm 3. A dilute solution of a strong acid and a concentrated solution of a weak acid are both possible. |
Acids as proton donors, strong and weak acids: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
(Extended) Define an acid and a base in terms of proton transfer.
(Extended) Write equations to show the dissociation of (a) hydrochloric acid and (b) ethanoic acid in water.
(Extended) Explain why 0.1 mol/dm3 ethanoic acid has a higher pH than 0.1 mol/dm3 hydrochloric acid.
(Extended) Describe two experiments, other than measuring pH, that would show hydrochloric acid is stronger than ethanoic acid of the same concentration. Give the expected results.
(Extended) Ammonia reacts with hydrogen chloride: NH3 + HCl → NH4Cl. Explain, in terms of protons, which substance is the acid and which is the base.
(Extended) Explain the difference between a dilute acid and a weak acid.
Answer key and marking notes
- An acid is a proton (H+) donor.
- A base is a proton (H+) acceptor.
- (a) HCl(aq) → H+(aq) + Cl−(aq)
- (b) CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq)
- Ethanoic acid is a weak acid; it is only partially dissociated.
- Hydrochloric acid is a strong acid and is completely dissociated.
- So ethanoic acid has a lower concentration of H+ ions, giving a higher pH.
- Add equal pieces of magnesium: hydrochloric acid fizzes faster.
- Measure electrical conductivity: hydrochloric acid conducts better (more ions).
- (Adding a carbonate is also acceptable: faster fizzing with hydrochloric acid.)
- HCl donates a proton, so it is the acid.
- NH3 accepts the proton to form NH4+, so it is the base.
- Dilute means a low concentration: little acid dissolved per dm3 of solution.
- Weak means the acid is only partially dissociated into ions, whatever its concentration.
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.
Ethanoic acid reacts with hydroxide ions: CH3COOH + OH− → CH3COO− + H2O. Which species acts as the proton donor?
- ACH3COOH
- BOH−
- CCH3COO−
- DH2O
When an acid reacts with a carbonate, the ionic equation is CO32− + 2H+ → CO2 + H2O. How should the carbonate ion be described?
- Aas a base, because it accepts protons
- Bas an acid, because it accepts protons
- Cas a base, because it donates protons
- Das an acid, because it donates hydroxide ions
Which solution contains the highest concentration of hydrogen ions?
- A1.0 mol/dm3 hydrochloric acid
- B1.0 mol/dm3 ethanoic acid
- C0.1 mol/dm3 hydrochloric acid
- D0.1 mol/dm3 ethanoic acid
Apart from water molecules, which particles are present in the largest numbers in a solution of ethanoic acid?
- ACH3COOH molecules
- BH+ ions
- CCH3COO− ions
- DOH− ions
Solution P is 2.0 mol/dm3 ethanoic acid. Solution Q is 0.01 mol/dm3 hydrochloric acid. Which description is correct?
- AP is concentrated and weak; Q is dilute and strong.
- BP is concentrated and strong; Q is dilute and weak.
- CP is dilute and weak; Q is concentrated and strong.
- DP and Q are both strong, because both are acids.
Sulfuric acid is a strong acid. Which equation shows how it behaves in water?
- AH2SO4(aq) → 2H+(aq) + SO42−(aq)
- BH2SO4(aq) ⇌ 2H+(aq) + SO42−(aq)
- CH2SO4(aq) → H+(aq) + SO42−(aq)
- DH2SO4(aq) → H2(g) + SO4(aq)
A 0.1 mol/dm3 solution of methanoic acid, HCOOH, has a pH of about 2.4. A 0.1 mol/dm3 solution of hydrochloric acid has a pH of 1. What is the best explanation for the difference?
- AMethanoic acid is only partially dissociated, so it produces fewer H+ ions.
- BThe methanoic acid solution is more dilute than the hydrochloric acid.
- CMethanoic acid molecules contain fewer hydrogen atoms than hydrogen chloride molecules.
- DMethanoic acid releases hydroxide ions as well as hydrogen ions.
Carbonated water contains carbonic acid. It is compared with dilute nitric acid of the same concentration using a simple conductivity circuit. Which result and reason are correct?
- ACarbonated water conducts less well, because carbonic acid is only partially dissociated.
- BDilute nitric acid conducts less well, because nitric acid is a weak acid.
- CBoth conduct equally well, because they have the same concentration.
- DCarbonated water does not conduct at all, because it contains no ions.
Quiz key
Ethanoic acid loses an H+ ion (a proton) and becomes the ethanoate ion, CH3COO−, so CH3COOH is the proton donor, which is the acid. The hydroxide ion gains that proton to become water, so OH− is the proton acceptor, the base. The ethanoate ion and water are products, not the species that donate the proton in this reaction.
The carbonate ion takes two H+ ions (protons) from the acid, and a proton acceptor is a base. So the carbonate ion acts as a base, even though it contains no hydroxide ions. An acid is a proton donor, not an acceptor, so the options calling it an acid have the definition reversed. It does not donate protons: it has no hydrogen to give. Hydroxide ions are not involved in this equation.
Hydrochloric acid is strong, so it is completely dissociated, and 1.0 mol/dm3 is the higher concentration, so 1.0 mol/dm3 hydrochloric acid gives the most H+ ions (about 1.0 mol/dm3). Ethanoic acid at the same concentration is weak and only partially dissociated, so it releases far fewer H+ ions. The 0.1 mol/dm3 hydrochloric acid is fully dissociated but has only one tenth as much acid. The 0.1 mol/dm3 ethanoic acid is both dilute and weak, so it has the fewest.
Ethanoic acid is only partially dissociated: CH3COOH ⇌ H+ + CH3COO−. Only a small fraction of the molecules form ions at any moment, so undissociated CH3COOH molecules are by far the most common solute particles. H+ and CH3COO− ions are present, but in much smaller numbers. The solution is acidic, so OH− ions are very scarce.
Concentration is the amount of acid per dm3: 2.0 mol/dm3 is concentrated and 0.01 mol/dm3 is dilute. Strength depends on how completely the acid dissociates: ethanoic acid is always weak and hydrochloric acid is always strong, whatever their concentrations. Calling P strong confuses a high concentration with strength. Swapping the concentrations ignores the numbers. Not all acids are strong.
A strong acid is completely dissociated into ions, shown with a single one-way arrow; one H2SO4 gives two H+ ions and one SO42− ion, so atoms and charges balance. The reversible sign would mean only partial dissociation, which describes a weak acid. The equation with only one H+ is unbalanced in both hydrogen and charge. An acid releases hydrogen ions in water, not hydrogen gas.
Both solutions have the same concentration, so the higher pH of methanoic acid must mean a lower H+ concentration: it is a weak acid, only partially dissociated. It is not more dilute, since both are 0.1 mol/dm3. The number of hydrogen atoms in the molecule does not decide strength; HCOOH actually has two hydrogen atoms but only one can be released, and only a small fraction of molecules release it. Acids do not release hydroxide ions.
Carbonic acid is a weak acid: only a small fraction of it dissociates, so its solution contains few ions and conducts poorly. Nitric acid is a strong acid, completely dissociated, so it has many more ions and conducts well. Nitric acid is strong, not weak. Equal concentrations do not mean equal numbers of ions when one acid is weak. Carbonated water does conduct slightly, because a few ions are present.
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.