Higher Tier: carboxylic acids are only partially ionised
| English | Français |
|---|---|
| acid strength | acid strength |
| partial ionisation | partial ionisation |
What would explain this observation?
- Higher Tier: An ethanoic acid solution can be acidic while most acid particles remain un-ionised. Weak describes the extent of ionisation, not a guarantee of a dilute or harmless solution.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Carboxylic acids are weak acids: in aqueous solution they are only partially ionised. Some acid molecules form hydrogen ions and the corresponding negative carboxylate ions, while others remain un-ionised. At the same stated concentration, a weak acid generally has fewer hydrogen ions and a higher pH than a strong acid in this GCSE comparison. The reversible model reflects a balance between ionisation and recombination.
- acid strength 酸强度: The extent of ionisation of an acid in aqueous solution; partial ionisation 部分电离: Ionisation of only some acid particles in the stated aqueous equilibrium.
Why can a carboxylic acid be acidic yet weak?
Concentration describes the amount of acid in a volume; strength describes its extent of ionisation. These are independent labels. Comparing a dilute strong acid with a concentrated weak acid does not establish the simple same-concentration pH ordering. For whole-number pH differences, a decrease of one pH unit means ten times the hydrogen-ion concentration. No equilibrium constant, pKa calculation or advanced weak-acid approximation is required here.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Concentration describes the amount of acid in a volume; strength describes its extent of ionisation. These are independent labels. Comparing a dilute strong acid with a concentrated weak acid does not establish the simple same-concentration pH ordering. For whole-number pH differences, a decrease of one pH unit means ten times the hydrogen-ion concentration. No equilibrium constant, pKa calculation or advanced weak-acid approximation is required here.
- Use teacher-provided concentrations and pH data or an approved supervised pH measurement. Keep temperature and measurement calibration comparable. State what is measured and what explanation depends on equal concentration. A particle model should conserve total acid-derived units and electrical charge: each ionised unit supplies one positive H⁺ and one negative carboxylate in the illustrated monoprotic example. Particle counts are a model, not a measured degree of ionisation for all samples.
Which two habits make the investigation or model in this case more defensible?
Use teacher-provided concentrations and pH data or an approved supervised pH measurement. Keep temperature and measurement calibration comparable. State what is measured and what explanation depends on equal concentration. A particle model should conserve total acid-derived units and electrical charge: each ionised unit supplies one positive H⁺ and one negative carboxylate in the illustrated monoprotic example. Particle counts are a model, not a measured degree of ionisation for all samples.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: solution A has pH 2 and B pH 4. The difference is 2, so hydrogen-ion concentration ratio A:B=10²=100:1. If a fictional closed model contains 100 acid-derived units with 5 ionised, it has 95 un-ionised acid molecules, 5 carboxylate ions and 5 H⁺ ions. That count illustrates partial ionisation without asserting an actual ethanoic acid percentage.
Compare pH 3 and pH 5. How many times greater is H⁺ concentration at pH 3? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Compare pH 3 and pH 5. How many times greater is H⁺ concentration at pH 3?
The result is 100 times. Known: solution A has pH 2 and B pH 4. The difference is 2, so hydrogen-ion concentration ratio A:B=10²=100:1. If a fictional closed model contains 100 acid-derived units with 5 ionised, it has 95 un-ionised acid molecules, 5 carboxylate ions and 5 H⁺ ions. That count illustrates partial ionisation without asserting an actual ethanoic acid percentage.
Check the conclusion and its limits
- Do not say weak acids cannot react with carbonates or have no hydrogen ions. A concentrated weak acid may still have substantial hazards. The pH ratio concerns free H⁺, not automatically the ratio of total acid concentrations. This whole explanatory clause is Higher-only; recognising acids and describing their water/carbonate/alcohol reactions remain both-tier.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A weak acid must have zero hydrogen-ion concentration. This claim is false: Do not say weak acids cannot react with carbonates or have no hydrogen ions. A concentrated weak acid may still have substantial hazards. The pH ratio concerns free H⁺, not automatically the ratio of total acid concentrations. This whole explanatory clause is Higher-only; recognising acids and describing their water/carbonate/alcohol reactions remain both-tier.
Higher Tier: carboxylic acids are only partially ionised: Concentration describes the amount of acid in a volume; strength describes its extent of ionisation. These are independent labels. Comparing a dilute strong acid with a concentrated weak acid does not establish the simple same-concentration pH ordering. For whole-number pH differences, a decrease of one pH unit means ten times the hydrogen-ion concentration. No equilibrium constant, pKa calculation or advanced weak-acid approximation is required here.
A weak acid must have zero hydrogen-ion concentration.
Do not say weak acids cannot react with carbonates or have no hydrogen ions. A concentrated weak acid may still have substantial hazards. The pH ratio concerns free H⁺, not automatically the ratio of total acid concentrations. This whole explanatory clause is Higher-only; recognising acids and describing their water/carbonate/alcohol reactions remain both-tier.
The extent of ionisation of an acid in aqueous solution: write the technical term.
acid strength means The extent of ionisation of an acid in aqueous solution.