Zadanie 2c
Wzorowane na: Chemistry SL Paper 2, November 2023, TZ1, pytanie 2(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 2a-2f.
Hydrogen iodide forms from its elements in the gas phase according to the equilibrium
H2(g) + I2(g) ⇌ 2 HI(g)
Polecenie
Approximate enthalpy changes of reactions can be found from bond enthalpies.
(i) Determine the enthalpy change, ΔH⊖, of this reaction, in kJ mol−1, using section 12 of the data booklet.
(ii) Each bond enthalpy is uncertain by 0.1 %. Determine the percentage uncertainty that this produces in the calculated enthalpy change of the reaction.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «bond breaking» H–H + I–I / 436 + 151 / 587 «kJ» ✔
«bond forming» 2(H–I) / 2 × 298 / 596 «kJ» ✔
«ΔH⊖ = 587 − 596» = −9 «kJ mol−1» ✔
(ii) sum of absolute uncertainties «= 0.436 + 0.151 + 2(0.298)» = 1.183 «kJ mol−1» ✔
percentage uncertainty «» = 13 «%» ✔
Schemat punktowania
(i) M1: bonds broken; M2: bonds formed; M3: enthalpy change with the correct sign. Award [3] for a correct final answer. (ii) M1: sum of the absolute uncertainties, with the H–I term counted twice; M2: percentage uncertainty relative to the enthalpy change. Award [2] for a correct final answer; accept 13.1 %. ECF from (i).
Komentarz
The percentage uncertainty is large because the enthalpy change is the small difference of two large numbers.