Zadanie 3e
Wzorowane na: Chemistry HL Paper 2, May 2024, TZ1, pytanie 3(e). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 3a-3h.
Carbon dioxide can be used to convert methane into synthesis gas, a mixture of carbon monoxide and hydrogen. The process is called dry reforming:
CH4(g) + CO2(g) ⇌ 2 CO(g) + 2 H2(g) ΔH = +246 kJ mol−1
Polecenie
The reaction takes place on the surface of a nickel catalyst. The following mechanism is proposed, in which Ni-C represents carbon atoms bonded to the nickel surface:
CH4 + Ni → Ni−C + 2 H2 (slow)
Ni−C + CO2 → Ni−CO + CO (fast)
Ni−CO → Ni + CO (fast)
(i) Deduce the role played by nickel, with a reason drawn from the mechanism.
(ii) Deduce the rate equation that corresponds to this mechanism.
(iii) Suggest why obtaining this rate equation from experiments would not prove that the mechanism is correct.
(iv) Sketch a graph of the rate of the reaction against the concentration of carbon dioxide, at constant temperature and constant concentration of methane.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) catalyst AND it is regenerated «used in the first step, re-formed in the last» ✔
(ii) rate = k[CH4] ✔
(iii) several different mechanisms can lead to the same rate equation
OR
experiments can rule a mechanism out but never confirm it
OR
the rate equation is silent about every step after the slow one ✔
(iv) a horizontal straight line at a non-zero rate, parallel to the concentration axis «zero order in CO2» ✔
Schemat punktowania
[1] for each part. (i) Accept "Ni is a product" in the sense that it is regenerated. (iii) Accept any valid argument. (iv) The line must not start at the origin and must not slope.