Zadanie 1d
Wzorowane na: Chemistry HL Paper 2, May 2022, TZ1, pytanie 1(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 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
The presence of calcium nitride in the product can be demonstrated by adding water. The nitride is hydrolysed to form calcium hydroxide and ammonia.
(i) Calculate coefficients that balance the equation for the reaction:
__ Ca3N2(s) + __ H2O(l) → __ Ca(OH)2(s) + __ NH3(aq)
(ii) Ammonia gas is dissolved in water that already contains a few drops of indicator. Identify an indicator that would show a colour change. Use section 18 of the data booklet.
(iii) Determine the oxidation state of nitrogen in Ca3N2 and of oxygen in Ca(OH)2.
(iv) Deduce, with reasons, whether the hydrolysis of calcium nitride is acid-base, redox, both or neither.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «1» Ca3N2(s) + 6 H2O(l) → 3 Ca(OH)2(s) + 2 NH3(aq) ✔
(ii) phenolphthalein ✔
OR
phenol red ✔
(iii) Ca3N2: N is −3 AND Ca(OH)2: O is −2 ✔
(iv) Acid-base: yes AND N3− accepts H+ «from water» / water donates H+ «to form OH−» ✔
Redox: no AND no oxidation states change «Ca +2, N −3, H +1 and O −2 on both sides» ✔
Schemat punktowania
(i) [1]. (ii) [1]; also accept bromothymol blue. Do not accept methyl red or methyl orange, whose colour does not change between pH 7 and the pH of ammonia solution. (iii) [1] for both values; a value written without its sign (3) or as 3− is not accepted. (iv) M1: acid-base with a reason; M2: not redox with a reason. If both verdicts are right but no reasons are given, award [1] in total. Accept “not redox as no electrons are gained or lost”.