Zadanie 4b
Wzorowane na: Chemistry SL Paper 2, May 2026, TZ3, pytanie 4(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 4a-4d.
Organic compounds are made from many different starting materials and by a variety of reactions, and some of them are used as fuels.
Polecenie
Propane undergoes complete combustion.
C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
(i) Calculate the enthalpy change of reaction, ΔH, in kJ mol−1, for the combustion of propane. Use section 12 of the data booklet.
(ii) State the oxidation state of carbon-2 in propane and of carbon in carbon dioxide.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) «bonds broken: 2 C–C + 8 C–H + 5 O=O»
2 × 346 + 8 × 414 + 5 × 498 = 6494 «kJ mol−1» ✔
«bonds formed: 6 C=O + 8 O–H»
6 × 804 + 8 × 463 = 8528 «kJ mol−1» ✔
«ΔH = Σ bonds broken −Σ bonds formed = 6494 − 8528 =» −2034 kJ mol−1 ✔
(ii) propane (C-2): −2 ✔
carbon dioxide: +4 ✔
Schemat punktowania
(i) Award [3] for the correct final answer. M1: bonds broken, with the correct number of each bond (2 C–C, 8 C–H, 5 O=O); M2: bonds formed, with the correct number of each bond (6 C=O, 8 O–H); M3: the difference, broken minus formed, with the sign. Award [2 max] for a final answer of +2034 kJ mol-1 (sign reversed). (ii) [1] each; do not accept the sign written after the number (2-, 4+); accept IV for +4.
Komentarz
The commonest error is to reverse the subtraction: for bond enthalpies the energy of the bonds broken minus that of the bonds formed gives the enthalpy change, and an exothermic reaction must come out negative.