Zadanie 1f, Paper 1B, maj 2025, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2025, TZ1, pytanie 1(f). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
A student studied how the concentration of sodium thiosulfate affects the rate of its reaction with dilute sulfuric acid.
Na2S2O3(aq) + H2SO4(aq) → Na2SO4(aq) + S(s) + SO2(g) + H2O(l)
The sulfur is produced as a fine suspension, so the mixture turns cloudy as the reaction proceeds. The student stood the beaker on a card with a black cross printed on it and recorded the time from mixing until the cross, viewed from above through the solution, could no longer be seen.
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A second student repeated the experiment with the same five solutions and the same apparatus and obtained the following times.
| Concentration of Na2S2O3 / mol dm−3 | Reaction time / s (± 0.1 s) |
|---|---|
| 0.2000 ± 0.08 % | 20.1 |
| 0.1000 ± 0.2 % | 37.5 |
| 0.0500 ± 0.3 % | 71.2 |
| 0.0250 ± 0.6 % | 137.9 |
| 0.0125 ± 1.3 % | 270.4 |
Suggest two reasons why these times differ significantly from the average times obtained by the first student.
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Odpowiedź z klucza
Any two of:
stopwatch started/stopped at a different moment «relative to mixing» ✔
different judgement of the moment at which the cross is no longer visible ✔
observed from a different angle/distance «parallax» ✔
different/brighter lighting during the second set of measurements ✔
solutions at a different/lower temperature during the second set of measurements ✔
Schemat punktowania
Any two distinct reasons, [1] each. Apply the list principle. Do not accept human error alone, only one trial, or differences in the solutions or the equipment, which the stem states were the same.
Komentarz. All five times are longer, which points to a systematic difference in judgement or conditions rather than random scatter.