The shape of the lesson
By the end of the lesson, learners can
- Donors and acceptors: In the simulator, drop acids and bases into water and follow the protons.
- Strong and weak: Compare 0.1 mol dm −3 hydrochloric acid with 0.1 mol dm −3 ethanoic acid.
- Conjugate pairs differ by one H+.
- Water can act as acid or base.
- Strong = fully dissociated; weak = partially.
- Strong ≠ concentrated.
One picture every idea returns to
A Brønsted–Lowry acid is a proton (H+) donor; a base is a proton acceptor. When an acid donates a proton it becomes its conjugate base; when a base accepts one it becomes its conjugate acid. Each pair differs by one H+. A strong acid dissociates fully in water (HCl, HNO3, H2SO4); a weak acid dissociates only partially, setting up an equilibrium (CH3COOH). Strength is about the extent of dissociation, not concentration.
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, Pass the proton.. An acid is not defined by tasting sour or turning litmus red. 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: Donors and acceptors | In the simulator, drop acids and bases into water and follow the protons. Project tab 5 of the lab (Equilibria lab, acids and bases) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Strong and weak | Compare 0.1 mol dm −3 hydrochloric acid with 0.1 mol dm −3 ethanoic acid. | Anticipation / Reaction Guide |
| 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, 13 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.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
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 mean different things. | Strong means fully dissociated; concentrated means a lot of acid per dm 3. A dilute solution of HCl is still a strong acid. Also, in a conjugate pair the formulae differ by exactly one H +: the conjugate base of H 2 SO 4 is HSO 4 −, not SO 4 2−. |
Brønsted–Lowry acids and bases: worksheet
6 questions, 13 marks. Written in the style of the papers; no past paper question is reproduced.
Define a Brønsted–Lowry acid and a Brønsted–Lowry base.
Identify the two conjugate acid-base pairs in CH3COOH + H2O ⇌ CH3COO− + H3O+.
Use equations to show that water can act as both an acid and a base.
Explain the difference between a strong acid and a weak acid, using HCl and CH3COOH as examples with equations.
Describe two experiments that would distinguish 0.10 mol dm−3 hydrochloric acid from 0.10 mol dm−3 ethanoic acid, giving the result for each acid.
Give the conjugate base of (a) HSO4− and (b) NH4+, and the conjugate acid of (c) CO32−.
Answer key and marking notes
- Acid: a proton (H+) donor.
- Base: a proton (H+) acceptor.
- CH3COOH (acid) and CH3COO− (conjugate base).
- H2O (base) and H3O+ (conjugate acid).
- As a base: HCl + H2O → H3O+ + Cl− (water accepts H+).
- As an acid: NH3 + H2O ⇌ NH4+ + OH− (water donates H+).
- HCl is fully dissociated: HCl → H+ + Cl−.
- CH3COOH is only partially dissociated: CH3COOH ⇌ CH3COO− + H+, with the equilibrium well to the left.
- Measure pH: HCl about 1, ethanoic acid about 3 (higher [H+] in the strong acid).
- Add magnesium ribbon: faster effervescence with HCl.
- Or measure conductivity: higher for HCl because it has more ions.
- (a) SO42−; (b) NH3.
- (c) HCO3−.
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.
According to the Brønsted-Lowry theory, what is an acid?
- Aa proton (H+) donor
- Ba proton (H+) acceptor
- Ca substance that produces OH− ions in water
- Dan electron pair acceptor
In the equilibrium NH3 + H2O ⇌ NH4+ + OH−, which pair of species is a conjugate acid-base pair?
- ANH4+ and NH3
- BNH3 and H2O
- CNH4+ and OH−
- DH2O and NH4+
Consider the reactions HCl + H2O → H3O+ + Cl− and NH3 + H2O ⇌ NH4+ + OH−. What is the role of water in each reaction?
- Aa base with HCl and an acid with NH3, so water is amphoteric
- Ban acid with HCl and a base with NH3
- Can acid in both reactions
- Da base in both reactions
What is meant by a strong acid?
- Aan acid that dissociates completely into ions in aqueous solution
- Ban acid with a high concentration
- Can acid that has a low pH
- Dan acid that can donate more than one proton per molecule
Solutions of hydrochloric acid and ethanoic acid are compared, both at 0.100 mol dm−3 and 25.0 cm3. Which property is the same for both?
- Athe volume of 0.100 mol dm−3 NaOH needed to neutralise them
- Bthe pH
- Cthe electrical conductivity
- Dthe initial rate of reaction with magnesium ribbon
In a mixture of concentrated acids, the reaction HNO3 + H2SO4 ⇌ H2NO3+ + HSO4− occurs. What is the role of HNO3?
- Ait acts as a base, because it accepts a proton from H2SO4
- Bit acts as an acid, because it is an acid in water
- Cit acts as an acid, because it donates a proton to H2SO4
- Dit acts as neither, because two acids cannot react together
What is the conjugate base of the hydrogencarbonate ion, HCO3−?
- ACO32−
- BH2CO3
- COH−
- DCO2
Which statement about the solutions below is correct? Solution X: 0.00100 mol dm−3 HCl. Solution Y: 2.00 mol dm−3 ethanoic acid.
- AX is a dilute solution of a strong acid and Y is a concentrated solution of a weak acid
- BX is a weak acid because it is dilute, and Y is a strong acid because it is concentrated
- Cboth are strong acids, because both have a pH below 7
- DX is a concentrated strong acid and Y is a dilute weak acid
Quiz key
A Brønsted-Lowry acid donates a proton (H+) and a base accepts one, so every acid-base reaction is a proton transfer. A proton acceptor is the definition of a base, not an acid. Producing OH− ions is the older Arrhenius description of an alkali. An electron pair acceptor is the Lewis definition of an acid, which is a different, wider theory.
NH3 accepts a proton to become NH4+, so NH4+ (acid) and NH3 (base) are a conjugate pair; the other pair is H2O (acid) and OH− (base). NH3 and H2O are the two reactants, a base and an acid, but they do not differ by one proton. NH4+ and OH− are the two products. H2O and NH4+ are an acid and an acid from different pairs.
With HCl, water gains a proton to form H3O+, so it is a base. With NH3, water loses a proton to form OH−, so it is an acid. A substance that can act as both an acid and a base is amphoteric. The second option has the roles reversed: water cannot donate a proton to HCl, a much stronger acid. Water does not act the same way in both reactions; in one it is the proton acceptor and in the other the donor.
Strength describes the extent of dissociation: a strong acid such as HCl is fully dissociated in water, while a weak acid such as ethanoic acid is only partly dissociated and exists mainly as molecules. Concentration is how much acid is dissolved per dm3; a strong acid can be dilute and a weak acid concentrated. A low pH depends on concentration too, so a concentrated weak acid can have a lower pH than a very dilute strong acid. Donating more than one proton makes an acid dibasic (for example H2SO4), which is not the same as strong.
Both solutions contain 2.50 × 10−3 mol of a monobasic acid. As NaOH removes H+, the ethanoic acid equilibrium shifts to release more, so both need the same 25.0 cm3 of NaOH. The pH is lower for HCl because it is fully dissociated, giving a higher [H+]. The conductivity is higher for HCl, which has many more ions in solution. The initial rate with magnesium is faster for HCl, again because of its higher [H+].
HNO3 gains a proton to become H2NO3+, so here it is a Brønsted-Lowry base, and H2SO4, the stronger acid, is the proton donor. The role of a species depends on the reaction it is in, not on how it behaves in water. HNO3 does not donate a proton here; H2SO4 loses one to become HSO4−. Two acids can react when one is a much stronger proton donor than the other.
Removing one H+ from HCO3− gives CO32−, its conjugate base. H2CO3 has one proton more, so it is the conjugate acid of HCO3− (which is itself amphoteric). OH− is the conjugate base of water, not of hydrogencarbonate. CO2 is formed when carbonic acid decomposes; it does not differ from HCO3− by a single proton.
HCl is always fully dissociated, so X is a strong acid, but at 0.00100 mol dm−3 it is dilute (pH 3.00). Ethanoic acid is only partly dissociated, so Y is a weak acid, but at 2.00 mol dm−3 it is concentrated (its pH is about 2.2, lower than X). Calling X weak because it is dilute, and Y strong because it is concentrated, is the classic strong = concentrated error. A pH below 7 shows a solution is acidic, not that the acid is strong. X has the lower concentration, so it cannot be the concentrated one.
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.