Proton transfer and strong versus weak acids
Separate dissociation from concentration and use the different HCl and ethanoic-acid equations.
Exam scope and assessment notes
Extended only (Supplement) · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
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.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
After trying it yourself, choose your next review. This is your self-assessment.
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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.