Proton transfer and strong versus weak acids
Separate dissociation from concentration and use the different HCl and ethanoic-acid equations.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Read reversible equations and aqueous hydrogen-ion notation.
- Distinguish solution concentration from a species' chemical behaviour.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In NH₃+H⁺→NH₄⁺, what does NH₃ do?
What you will learn
- Define acids as proton donors and bases as proton acceptors.
- Explain complete versus partial dissociation using HCl and CH₃COOH without confusing strength with concentration.
Follow the proton
In the proton-transfer definition, an acid donates a proton, H⁺, and a base accepts a proton. In NH₃+H⁺→NH₄⁺, ammonia accepts a proton and acts as a base even though NH₃ is not a metal hydroxide. In H⁺+OH⁻→H₂O, hydroxide accepts the proton. Identify the direction of proton transfer rather than looking only for OH in a formula.
A proton is a hydrogen nucleus in this acid–base model, not an electron. Acid–base proton transfer is therefore different from the electron-transfer criterion for redox. The syllabus writes H⁺(aq) for aqueous acid accounting; this notation does not mean isolated, unsolvated protons float in the water without interacting with it.
Strength asks how far dissociation occurs
A strong acid is completely dissociated in aqueous solution in the syllabus model. Hydrochloric acid is represented by HCl(aq)→H⁺(aq)+Cl⁻(aq). A weak acid is partially dissociated: ethanoic acid is represented by CH₃COOH(aq)⇌H⁺(aq)+CH₃COO⁻(aq). Undissociated ethanoic-acid molecules remain alongside the ions, and the reversible arrow communicates that balance.
Concentration describes the total acid amount per solution volume; strength describes its dissociation. A strong acid can be dilute and a weak acid concentrated. At equal stated concentrations under comparable conditions, the strong monoprotic acid produces more hydrogen ions than the partially dissociated weak one. Without controlling concentration, acid-strength labels alone do not order two samples' pH. This lesson does not require calculating equilibrium constants or presenting one fixed dissociation fraction for every weak-acid concentration.
Pause and explain
Which statement distinguishes strength from concentration?
Worked example
Compare equal-concentration aqueous HCl and ethanoic acid in the syllabus model, explaining the arrows and hydrogen-ion concentrations.
Show the worked solution
- HCl is strong and uses → for complete dissociation.
- CH₃COOH is weak and uses ⇌ for partial dissociation.
- At the same total monoprotic-acid concentration, HCl has the greater H⁺ concentration; do not confuse this controlled comparison with samples at unspecified concentrations.
Answer HCl completely dissociates; ethanoic acid partially dissociates. Equal total concentration gives more H⁺ from HCl.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the aqueous dissociation equations for HCl and ethanoic acid with their appropriate arrows.
Give me a hint
One acid completely dissociates in this model, while the other partially dissociates.
Compare my reasoning
- HCl(aq)→H⁺(aq)+Cl⁻(aq) represents the strong acid.
- CH₃COOH(aq)⇌H⁺(aq)+CH₃COO⁻(aq) represents the weak acid.
- The different arrows distinguish dissociation behaviour, not solution concentration.
HCl→H⁺+Cl⁻; CH₃COOH⇌H⁺+CH₃COO⁻, all aqueous.
Look for these in your work
- Formulas and charges are conserved.
- The arrows match strong and weak behaviour.
In NH₃+H⁺→NH₄⁺ and OH⁻+H⁺→H₂O, identify both bases and explain the shared definition.
Give me a hint
Each base accepts the same kind of transferred particle.
Compare my reasoning
- NH₃ accepts H⁺ to form NH₄⁺.
- OH⁻ accepts H⁺ to form H₂O.
- Both are proton acceptors, although only one formula contains the hydroxide group.
NH₃ and OH⁻ are both bases because each accepts a proton.
Look for these in your work
- The transferred particle is a proton.
- The definition covers both examples.
One bottle is labelled dilute HCl and another concentrated ethanoic acid. A student says the second must be the stronger acid and the first must have the higher pH. Assess both claims without invented concentrations.
Give me a hint
Strength follows dissociation; pH also depends on concentration.
Compare my reasoning
- HCl is the strong acid in the syllabus model even when dilute.
- Ethanoic acid is weak even when concentrated.
- The pH ordering cannot be fixed from these qualitative labels alone; concentration and the weak acid's dissociation matter.
HCl remains strong and ethanoic acid weak; the supplied labels do not determine their pH order.
Look for these in your work
- Strength and concentration are separated.
- No unsupported pH ordering is invented.
Common mistakes
- Defining a strong acid as necessarily concentrated.
- Giving ethanoic acid a complete-dissociation arrow.
- Calling proton transfer electron transfer.
- Ranking unknown pH values from strength alone without concentrations.
What can you explain now?
Why does CH₃COOH(aq)⇌H⁺(aq)+CH₃COO⁻(aq) use ⇌?
Compare with the explanation
Ethanoic acid is only partially dissociated in aqueous solution.
Undissociated molecules and ions coexist in a reversible balance; it is a weak acid rather than necessarily a dilute one.
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Make it stick
Write both dissociation equations and explain each arrow. Give a proton-acceptor example without OH and a statement that separates strength from concentration.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.