Zadanie 1e, Paper 1B, maj 2025, TZ3, Chemia IB SL
Wzorowane na: Chemistry SL Paper 1B, May 2025, TZ3, pytanie 1(e). 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
The amount of NO2 prepared was checked by titration. The gas was first absorbed in water:
2 NO2(g) + H2O(l) → HNO3(aq) + HNO2(aq)
The resulting solution was made up to 200.0 cm3 in a volumetric flask, and 20.0 cm3 portions were titrated with a 0.0400 mol dm−3 standard solution of sodium hydroxide, NaOH.
(i) State the most accurate piece of equipment for:
transferring a 20.0 cm3 portion of the solution for titration;
adding the NaOH solution during the titration.
(ii) Draw a table suitable for recording the raw results of this titration. Put the quantities to be recorded, with their units and any other information a reader needs, in the header row(s) or column(s), and leave the data cells empty.
(iii) The burette used to deliver the sodium hydroxide solution had an uncertainty of ± 0.05 cm3. The mean titre was 29.95 cm3. Determine the percentage uncertainty in the titre.
(iv) The equilibrium experiment of 1d was repeated at three further temperatures. The values of K obtained are given below. Calculate the temperatures in kelvin, K, and complete the table.
| T / °C | T / K | K |
|---|---|---|
| 10.0 | value from 1d | |
| 30.0 | 4.63 | |
| 60.0 | 5.99 × 10−1 | |
| 90.0 | 1.09 × 10−1 |
(v) Deduce whether the values of K in (iv) are consistent with the enthalpy change given for the forward reaction.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Transferring 20.0 cm3 of the solution: «volumetric/graduated» pipette/pipet ✔
AND
Adding the NaOH solution: burette/buret ✔
(ii) table with a row (or column) for each of at least three trials ✔
headings for the initial and final burette readings AND for the titre/volume of NaOH added, each with the unit cm3 ✔
For example:
| Trial | Initial burette reading / cm3 (± 0.05) | Final burette reading / cm3 (± 0.05) | Volume of NaOH added / cm3 (± 0.10) |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 |
(iii) «the titre is the difference of two burette readings, so its uncertainty is 2 × 0.05 = 0.10 cm3; =» 0.33 «%» ✔
(iv) 283.2, 303.2, 333.2, 363.2 ✔
(v) yes AND K decreases as the temperature rises, as expected for an exothermic forward reaction ✔
Schemat punktowania
(i) [1] Both required: a «volumetric/graduated» pipette for the fixed 20.0 cm3 portion, and a burette for the variable NaOH volume; either one alone does not score. (ii) M1: space for at least three trials (a rough titration may be included). M2: separate columns (or rows) for the initial and final burette readings and for the titre, with cm3 units; a single 'volume of NaOH' heading without the two readings does not earn M2. Any orientation and extra headings are acceptable; uncertainties are not required. (iii) [1] for 0.33 % (accept 0.3 %); 0.17 %, from using ± 0.05 cm3 once, does not score. (iv) [1] for all four values: 283.2, 303.2, 333.2, 363.2 (accept 283, 303, 333, 363, or values obtained by adding 273.15). (v) [1] for 'yes' supported by the observation that K falls as T rises, which is what an exothermic forward reaction predicts; 'yes' alone does not score.
Komentarz. A titre is the difference between two burette readings, so its absolute uncertainty is twice that of a single reading.