Zadanie 4b, Paper 2, listopad 2025, TZ3, Chemia IB HL
Wzorowane na: Chemistry HL Paper 2, November 2025, TZ3, pytanie 4(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Reacting ethene, C2H4, with hydrogen iodide, HI, produces iodoethane, C2H5I.
C2H4(g) + HI(g) → C2H5I(l)
Polecenie
(i) Using section 12 of the data booklet, determine the standard enthalpy change of the reaction, in kJ mol−1.
(ii) A value obtained from bond enthalpies is less accurate than one obtained from enthalpies of formation. State two reasons why.
(iii) Outline why this reaction has a negative entropy change.
(iv) Using section 13 of the data booklet, your answer to part (b)(i) and the standard entropy of iodoethane(l), S⊖ = +212 J K−1 mol−1, predict, showing your working, whether the reaction is spontaneous at 298 K. If you did not obtain an answer to part (b)(i), use −70 kJ mol−1.
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Odpowiedź z klucza
(i) ALTERNATIVE 1:
«bonds broken» C=C + H–I / 614 + 298 / 912 «kJ mol⁻¹» AND «bonds formed» C–C + C–H + C–I / 346 + 414 + 228 / 988 «kJ mol⁻¹» ✔
«ΔH = 912 − 988 =» −76 «kJ mol⁻¹» ✔
ALTERNATIVE 2:
«bonds broken» 1 C=C + 4 C–H + 1 H–I / 614 + 4(414) + 298 / 2568 «kJ mol⁻¹» AND «bonds formed» 1 C–C + 5 C–H + 1 C–I / 346 + 5(414) + 228 / 2644 «kJ mol⁻¹» ✔
«ΔH = 2568 − 2644 =» −76 «kJ mol⁻¹» ✔
(ii) Any two of:
bond enthalpies are average values «taken from many different compounds» ✔
enthalpies of formation are specific to the compounds involved ✔
bond enthalpies apply to gaseous species AND iodoethane is a liquid ✔
bond enthalpies do not account for the change of state / the enthalpy of vaporization ✔
(iii) two moles / molecules of «gaseous» reactants form one mole / molecule of product
OR
gaseous reactants form a liquid product «so the number of moles of gas decreases» ✔
(iv) «ΔS = 212 − 220 − 207 =» −215 «J K⁻¹ mol⁻¹» ✔
«ΔG⊖ = ΔH⊖ − TΔS⊖ = −76 000 − 298 × (−215) = −76 000 + 64 070» «ΔG⊖ =» −12 000 «J mol⁻¹» / −12 «kJ mol⁻¹» AND spontaneous ✔
Schemat punktowania
(i) M1: bonds broken and bonds formed with their values; M2: −76 kJ mol-1. Award [2] for the correct final answer; award [1 max] for +76 kJ mol-1. (ii) [2 max] from the four points. (iii) [1]. (iv) M1: ΔS = −215 J K-1 mol-1; M2: ΔG negative with the conclusion that the reaction is spontaneous. Award [2] for a correct ΔG with the conclusion; if −70 kJ mol-1 is used, ΔG = −5.9 kJ mol-1 and the conclusion is the same. ECF from an incorrect (b)(i).
Komentarz. The bond-enthalpy value refers to gaseous iodoethane; the true value is more negative because condensing the product releases about 30 kJ mol-1 more, which is exactly the limitation asked about in (ii).