Zadanie 2d, Paper 1B, maj 2026, TZ1, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2026, TZ1, pytanie 2(d). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
In a student experiment, the apparatus shown was used to prepare propanoic acid, CH3CH2COOH, from propan-1-ol, CH3CH2CH2OH.
1.80 g of propan-1-ol, excess oxidizing agent (acidified K2Cr2O7(aq)) and a few anti-bumping granules were placed in the round-bottom flask.
PowiększPolecenie
(d) The student used distillation to obtain pure propanoic acid from the reaction mixture.
Powiększ(i) State two impurities, apart from unreacted propan-1-ol, that were present in the propanoic acid before distillation.
(ii) The approximate experimental yield of propanoic acid was calculated from the mass of propanoic acid collected in the distillation. The student's hypothesis was that impurities collected together with the propanoic acid during the distillation lowered this calculated yield.
Evaluate this statement.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Any two of:
propanal/CH3CH2CHO
water/H2O
potassium dichromate/dichromate(VI) ions/Cr2O72−/potassium ions
sulfuric acid/H2SO4/hydrogen ions/sulfate ions
chromium(III) ions/Cr3+
propyl propanoate/CH3CH2COOCH2CH2CH3
«one mark for two correct impurities» ✔
(ii) incorrect «the impurities would increase the calculated yield, not lower it» AND the impurities that distilled over with the propanoic acid increased the mass «of liquid» collected ✔
Schemat punktowania
(i) [1] for any two of the listed impurities; apply the list principle. Unreacted propan-1-ol is excluded by the question and must not be credited. (ii) [1] for judging the statement incorrect AND giving the reason that the impurities add to the mass collected, so the calculated yield is higher; 'incorrect' alone earns nothing.
Komentarz. Sulfuric acid and the chromium salts are non-volatile and mostly stay behind in the flask, but a yield calculated from mass counts anything that does distil over (for example water or propanal) as product.