Zadanie 1c, Paper 1B, maj 2025, TZ3, Chemia IB SL
Wzorowane na: Chemistry SL Paper 1B, May 2025, TZ3, pytanie 1(c). 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
A sample of 0.0120 mol of NO2 was placed in a sealed vessel of volume 1.00 dm3, which was kept at a constant temperature of 35 °C.
(i) Suggest a simple way of keeping the vessel at a constant temperature in a school laboratory.
(ii) The concentration of NO2 was followed with a colorimeter. One student placed the vessel in the apparatus and took the absorbance reading straight away. Suggest why this reading may not be a reliable measure of the equilibrium concentration.
(iii) Suggest how the students could avoid the problem identified in (ii).
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) thermostatically controlled water bath / water bath with a thermometer and heater / oven or incubator set to 35 °C ✔
(ii) the system has not yet reached equilibrium «so the absorbance is still changing» ✔
OR
the gas has not yet reached 35 °C ✔
(iii) take readings at intervals and use the value obtained once the absorbance stops changing / is constant ✔
Schemat punktowania
(i) [1] for a named piece of apparatus together with a means of temperature control; 'a temperature-controlled room' does not score. (ii) [1] for equilibrium not yet being established, or the absorbance/[NO2] still changing, or the sample not yet at 35 °C; an answer implying that the reaction 'has not finished' or 'not fully occurred', as if it went to completion, does not score. (iii) [1] for waiting or repeating readings until the absorbance is constant; 'wait longer' without reference to a constant reading does not score.
Komentarz. At equilibrium the macroscopic properties, including the absorbance, are constant; a constant reading is the observable sign that the system is ready to be measured.