Zadanie 2c
Wzorowane na: Chemistry SL Paper 2, November 2023, TZ2, 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.
Methanol, a possible fuel and chemical feedstock, can be produced by the hydrogenation of carbon dioxide according to the equilibrium
CO2(g) + 3 H2(g) ⇌ CH3OH(g) + H2O(g)
Polecenie
Bond enthalpies give an estimate of the enthalpy change of a reaction. Assume that all species are gases.
(i) Use bond enthalpies from section 12 of the data booklet to determine the standard enthalpy change, ΔH⊖, of the forward reaction.
(ii) Each bond enthalpy is known to within ±0.1 %. Determine the percentage uncertainty in the value of ΔH⊖ that you calculated in (i).
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «bond breaking» 2 C=O + 3 H−H «= 2(804) + 3(436)» = 2916 «kJ» ✔
«bond forming» 3 C−H + C−O + 3 O−H «= 3(414) + 358 + 3(463)» = 2989 «kJ» ✔
«ΔH⊖ = 2916 − 2989 =» −73 «kJ mol−1» ✔
(ii) ALTERNATIVE 1
sum of absolute uncertainties «= 0.001 × (2916 + 2989) =» 5.905 «kJ mol−1» ✔
percentage uncertainty «» 8 «%» ✔
ALTERNATIVE 2
sum of absolute uncertainties, each type of bond counted once «= 0.804 + 0.436 + 0.414 + 0.358 + 0.463 =» 2.475 «kJ mol−1» ✔
percentage uncertainty «» 3 «%» ✔
Schemat punktowania
(i) [3]: M1 bonds broken, M2 bonds formed, M3 the enthalpy change with its sign. Award [3] for a correct final answer. (ii) [2]: award [2] for a correct final answer from either alternative. The percentage is taken relative to the size of ΔH⊖ (73), ignoring its sign. ECF from (i).
Komentarz
The second alternative counts each type of bond once, as the IB markscheme for the original question also accepted; the first alternative is the one that follows the propagation of uncertainties.