Higher Tier: carboxylic acids are only partially ionised
Diperkenalkan| English | Makna |
|---|---|
| acid strength/ˈæsɪd streŋθ/ | kekuatan asam |
| partial ionisation/ˈpɑːʃl ˌaɪənaɪˈzeɪʃn/ | ionisasi parsial |
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?
Difficulty 1/5 · 100 coins + a small bonus on first completion today
A weak acid is only partially ionised: a fraction of its molecules release H+ while most stay intact. It is still acidic, and "weak" says nothing about how concentrated the solution is.
Match each technical term to its precise meaning.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
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?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
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?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
The result is 100 times. Known: pH 3 versus pH 5. Each pH unit is a factor of 10 in H+ concentration. Difference = 2 units, so the ratio = 10^2 = 100 times greater at pH 3.
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: Weak means FEW particles ionise, not none: ethanoic acid releases enough H+ to fizz with carbonates and lower pH well below 7. Zero H+ concentration would mean no acidity at all.
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Weak means FEW particles ionise, not none: ethanoic acid releases enough H+ to fizz with carbonates and lower pH well below 7. Zero H+ concentration would mean no acidity at all.
The extent of ionisation of an acid in aqueous solution: write the technical term.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
acid strength means The extent of ionisation of an acid in aqueous solution.