Zadanie 1a, Paper 1B, maj 2025, TZ3, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, May 2025, TZ3, pytanie 1(a). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Nitrogen dioxide, NO2, is a toxic and corrosive gas with a brown colour. Small quantities can be prepared in a school laboratory either by heating the nitrate of a group 2 metal, such as magnesium nitrate, or by reacting silver with concentrated nitric acid, HNO3. The gas prepared in this way was collected and sealed in a glass vessel, in which the following equilibrium is established:
2 NO2(g) ⇌ N2O4(g) ΔH⊖ = −57.2 kJ mol−1
Polecenie
(i) Suggest, giving a reason for each, two different safety precautions that should be taken when carrying out either of these preparations.
(ii) Deduce the coefficients needed to balance the following equation for the reaction of silver with concentrated nitric acid.
Ag(s) + HNO3(aq) → AgNO3(aq) + NO2(g) + H2O(l)
(iii) Calculate the mass, in g, of nitric acid, HNO3, consumed when 0.0150 mol of NO2 is produced. Use section 7 of the data booklet.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Any two of:
safety goggles/eye protection AND the acid is corrosive / the gas can damage the eyes ✔
fume cupboard/fume hood AND NO2 is toxic / must not be inhaled ✔
tongs/heat-resistant gloves AND to prevent burns from the hot apparatus or from the acid ✔
(ii) Ag(s) + 2 HNO3(aq) → AgNO3(aq) + NO2(g) + H2O(l) ✔
(iii) «from (ii), n(HNO3) = 2 × n(NO2) = 0.0300 mol; M(HNO3) = 1.01 + 14.01 + 3(16.00) = 63.02 g mol−1, so m = 0.0300 × 63.02 =» 1.89 «g» ✔
Schemat punktowania
(i) [2 max]: one mark for each precaution paired with a matching reason; award [1 max] if two precautions or two reasons are given without pairing. General laboratory rules (lab coat, tied-back hair, 'a well-ventilated room') and 'gas mask' do not score. (ii) [1] for the coefficients 1, 2, 1, 1, 1 or any correct multiple. (iii) [1] for 1.89 g; ECF for a mass consistent with the student's own equation in (ii) (0.945 g from a 1 : 1 ratio scores only with ECF).
Komentarz. The mole ratio of metal to NO2 must be read from the balanced equation in (ii); here it is 1 : 1, so no halving is needed.