Zadanie 2d
Wzorowane na: Chemistry SL Paper 2, May 2022, TZ1, pytanie 2(d). 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-2e.
Methanol, CH3OH, is an important industrial chemical used to make plastics, solvents and fuels. It is also the starting material for methylamine, CH3NH2, a gas used in the manufacture of pesticides and medicines.
Polecenie
Whether a higher temperature favours the formation of methanol depends on the enthalpy change of the reaction
CO(g) + 2 H2(g) → CH3OH(g)
(i) Determine the enthalpy change, ΔH, for the production of methanol, in kJ, using bond enthalpies. Use section 12 of the data booklet.
(ii) The same reaction can be carried out in two steps with the following enthalpy changes:
CO(g) + H2(g) → CH2O(g) ΔH1 = +1.9 kJ
CH2O(g) + H2(g) → CH3OH(g) ΔH2 = −92.4 kJ
Calculate the enthalpy change for the production of methanol from carbon monoxide and hydrogen using these data.
(iii) Suggest why the values obtained in (i) and (ii) differ.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) bonds broken: C≡O + 2(H−H) / «1 mol ×» 1077 «kJ mol−1» + 2«mol» × 436 «kJ mol−1» / 1077 «kJ» + 872 «kJ» / 1949 «kJ» ✔
bonds formed: 3(C−H) + C−O + O−H / 3«mol» × 414 «kJ mol−1» + 358 «kJ mol−1» + 463 «kJ mol−1» / 1242 «kJ» + 358 «kJ» + 463 «kJ» / 2063 «kJ» ✔
ΔH «= 1949 kJ − 2063 kJ =» −114 «kJ» ✔
(ii) ΔH «= ΔH1 + ΔH2 = +1.9 kJ + (−92.4 kJ)» = −90.5 «kJ» ✔
(iii) bond enthalpies are average values «taken over many different compounds», so they do not fit these particular molecules exactly ✔
Schemat punktowania
(i) M1: bonds broken; M2: bonds formed; M3: ΔH. Award [2 max] for +114 kJ. (ii) [1]; accept −90 kJ. (iii) [1]; accept “the data in (ii) are for the specific molecules, those in (i) are averages”.