Zadanie 2c, Paper 2, listopad 2025, TZ1, Chemia IB SL
Wzorowane na: Chemistry SL Paper 2, November 2025, TZ1, pytanie 2(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Many esters are still known by trivial names. Propyl ethanoate, CH3COOCH2CH2CH3, which has the odour of pears, is manufactured from propan-1-ol and ethanoic acid in the presence of an acid catalyst.
CH3CH2CH2OH(l) + CH3COOH(l) ⇌ CH3COOCH2CH2CH3(l) + H2O(l)
Both reactants can be synthesized from alkanes: propan-1-ol from propane via 1-bromopropane, and ethanoic acid from ethane via bromoethane and ethanol. The overall equation for the synthesis of the ester from the two alkanes is
Polecenie
(i) Calculate the enthalpy change, ΔH, for the overall synthesis of propyl ethanoate from propane and ethane shown above. Use section 12 of the data booklet.
(ii) Two students each calculated the enthalpy change for the reaction between propan-1-ol and ethanoic acid by a different route, and both reached a correct result: one used bond enthalpy data and found 0 kJ mol−1, while the other used enthalpy of formation data and found −3 kJ mol−1.
C3H8O(l) + C2H4O2(l) ⇌ C5H10O2(l) + H2O(l)
Explain how both calculations can be correct even though the values differ.
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Odpowiedź z klucza
(i) «bonds broken» 1½ O=O + 4 C–H + Br–Br / 1.5 × 498 + 4 × 414 + 193 / 2596 «kJ mol−1» ✔
«bonds formed» C=O + 2 C–O + 2 O–H + 2 H–Br / 804 + 2 × 358 + 2 × 463 + 2 × 366 / 3178 «kJ mol−1» ✔
ΔH = «2596 − 3178 =» −582 «kJ mol−1» ✔
OR
«all bonds broken» 3 C–C + 14 C–H + Br–Br + 1½ O=O / 3 × 346 + 14 × 414 + 193 + 1.5 × 498 / 7774 «kJ mol−1» ✔
«all bonds formed» 3 C–C + 10 C–H + C=O + 2 C–O + 2 H–Br + 2 O–H / 3 × 346 + 10 × 414 + 804 + 2 × 358 + 2 × 366 + 2 × 463 / 8356 «kJ mol−1» ✔
ΔH = «7774 − 8356 =» −582 «kJ mol−1» ✔
(ii) the bond enthalpies used are typical values averaged over many compounds, not measurements taken for the actual bonds in propan-1-ol, ethanoic acid or the ester
OR
bond enthalpy data refers to substances in the gas phase, while this reaction happens between liquids
OR
enthalpies of formation are measured separately for each specific substance taking part in the reaction ✔
Schemat punktowania
(i) M1: bonds broken 2596 kJ; M2: bonds formed 3178 kJ; M3: −582 kJ mol⁻¹ (reactants minus products). Award [3] for the correct final answer, [2 max] for +582. Accept breaking and re-forming all bonds (7774 − 8356). (ii) [1] for any one valid reason.