Zadanie 1c
Wzorowane na: Chemistry SL Paper 2, May 2022, TZ1, pytanie 1(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 1a-1g.
Calcium turnings burn in air to form mainly calcium oxide, CaO. A small amount of calcium nitride, Ca3N2, is also formed according to the equation:
3 Ca(s) + N2(g) → Ca3N2(s)
Polecenie
A student heated 3.492 g of calcium turnings in a crucible with a lid and obtained 4.490 g of solid product. If the product is assumed to be calcium oxide only, the percentage yield is 92 %.
(i) Evaluate whether the formation of calcium nitride, rather than the loss of product, could explain this yield.
(ii) Suggest one further reason for this yield, apart from loss of product from the crucible and reaction with nitrogen.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) yes AND «each Ca combines with 2/3 of an N atom, so» the mass gained per mole of Ca is 14.01 × 2/3 = 9.34 g, which is less than the 16.00 g gained when oxygen reacts ✔
OR
0.08713 mol of Ca would give 4.306 g of Ca3N2 but 4.886 g of CaO, so the observed 4.490 g lies in between ✔
(ii) incomplete reaction ✔
OR
the turnings were already partly oxidized / contained an unreactive impurity ✔
Schemat punktowania
(i) [1] for yes with a supporting comparison of masses. Accept yes AND the molar mass of nitrogen is less than that of oxygen. (ii) [1]; accept the crucible weighed before it had cooled fully, or the formation of non-stoichiometric compounds. Blaming the student or the equipment (careless handling, a faulty balance) instead of the chemistry earns nothing.