Which species donates the proton?
You will be able to: Identify Brønsted–Lowry roles and conjugate acid–base pairs.
Which species donates the proton?
When ammonia reacts with water, a hydrogen nucleus moves from one species to another. Following that one H⁺ is more useful than guessing that every substance containing hydrogen must act as an acid.
A useful starting point: How do oxidation numbers reveal electron transfer? →
Words and symbols before equations
- Brønsted–Lowry acid
- A proton donor in the specified reaction direction.
- Brønsted–Lowry base
- A proton acceptor.
- Conjugate base
- Species left after an acid donates H⁺.
- Conjugate acid
- Species formed after a base accepts H⁺.
What this picture assumes
Equations identify proton donors/acceptors and conjugate pairs. They do not depict equilibrium proportions. H⁺ is transferred; no electron or nucleus is created.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. H₂O donates H⁺ → OH⁻: acid / conjugate base. NH₃ accepts H⁺ → NH₄⁺: base / conjugate acid. Pairs: H₂O/OH⁻ and NH₃/NH₄⁺
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, ammonia accepts H⁺, so it is the base in the forward direction. Water donates H⁺, so it is the acid.
NH₃/NH₄⁺ and H₂O/OH⁻ are conjugate pairs. Each pair differs by exactly one proton and one unit of charge; NH₃ and OH⁻ are not a conjugate pair.
In the reverse direction, NH₄⁺ donates and OH⁻ accepts a proton. The reversible arrow does not mean the two sides are present in equal amounts.
A stronger acid has a weaker conjugate base, and vice versa, when comparing pairs in the same solvent. The qualitative relationship does not mean every conjugate base is strong; quantitative equilibrium treatment comes later.
A worked example, step by step
For HF(aq) + H₂O(l) ⇌ H₃O⁺(aq) + F⁻(aq), label acid, base and conjugate pairs in the forward direction.
- HF loses H⁺ and becomes F⁻, so HF is the acid.
- H₂O gains H⁺ and becomes H₃O⁺, so water is the base.
- Pair HF with F⁻ and H₂O with H₃O⁺.
- Check each pair differs by one H⁺; total charge is zero before and after. This identification does not assume complete ionization.
Pair an acid with its own deprotonated form, not simply with the base beside it in the equation.
Are NH₄⁺ and NH₃ a conjugate pair?
Compare with an explanation
Yes. NH₄⁺ differs from NH₃ by one H⁺ and one positive charge.
Predict. Change one thing. Explain.
Switch between ammonia accepting a proton and water accepting a proton. Identify donor, acceptor and both conjugate pairs; do not infer equilibrium amounts from the schematic.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. H₂O donates H⁺ → OH⁻: acid / conjugate base. NH₃ accepts H⁺ → NH₄⁺: base / conjugate acid. Pairs: H₂O/OH⁻ and NH₃/NH₄⁺
Equations identify proton donors/acceptors and conjugate pairs. They do not depict equilibrium proportions. H⁺ is transferred; no electron or nucleus is created.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using conserved atoms/charge, reaction ratios, particle identity or electron/proton transfer. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionFor HCN + H₂O ⇌ H₃O⁺ + CN⁻, label the forward acid/base and both conjugate pairs. Explain why the arrow alone does not establish complete ionization.
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Compare with the answer and four-point rubric
- 1 point: HCN donates H⁺, so it is the acid.
- 1 point: H₂O accepts H⁺, so it is the base.
- 1 point: Pairs are HCN/CN⁻ and H₂O/H₃O⁺.
- 1 point: The reversible equation identifies possible changes; it does not specify equal amounts or complete reaction.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1How do you identify the acid?
Track which species donates H⁺ in the stated direction.
RECALL 2How much differs within a conjugate pair?
Exactly one proton and one unit of charge.
RECALL 3How does stronger acid relate to its conjugate base?
It has a weaker conjugate base in the same solvent.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
Which species donates the proton?
- Acid → conjugate base + H⁺; base + H⁺ → conjugate acid.
- Each conjugate pair differs by one H⁺.
Remember: Pair an acid with its own deprotonated form, not simply with the base beside it in the equation.
Conditions: Equations identify proton donors/acceptors and conjugate pairs. They do not depict equilibrium proportions. H⁺ is transferred; no electron or nucleus is created.
Refresh Kid · AP Chemistry Unit 4 · Objectives 4.8.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 4.8, objective 4.8.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 4: Chemical Reactions, Topics 4.1–4.9. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Solubility facts for sodium, potassium, ammonium and nitrate salts are included; other precipitation cases give the needed information. Lewis acid-base theory and the labels oxidizing/reducing agent are not treated as required exam content. Quantitative pH, equilibrium and electrochemical potentials are developed in later units. Stoichiometric models state complete-reaction assumptions; they are not mechanisms or equilibrium simulations.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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