Zadanie 1c, Paper 1B, specimen 2025, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, specimen 2025, 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
An outdoor public swimming pool (a lido) stays open throughout the year. The water is not heated, and in winter, when the pool is used by cold-water swimmers, its temperature falls to about 5 °C. The water is disinfected with chlorine. When chlorine dissolves in water it reacts to form hypochlorous acid, HOCl, which is the species that actually kills microorganisms.
Cl2(aq) + H2O(l) ⇌ HOCl(aq) + H+(aq) + Cl−(aq)
Polecenie
A student writing a report on chlorine as a disinfectant collects, from several websites and reference books, values for how much chlorine dissolves in pure water at various temperatures.
| Source | Temperature / °C | Solubility of chlorine |
|---|---|---|
| A | 5 | 1.18 g per 100 cm3 of water |
| B | 15 | 260 cm3 of gas per 100 cm3 of water |
| C | 20 | 6.9 g per dm3 of water |
| D | 25 | 620 mg per 100 cm3 of water |
| E | 35 | 152 cm3 of gas per 100 cm3 of water |
| F | 35 | 0.46 g per 100 g of water |
(i) Identify one difficulty in comparing the solubility values from these sources.
(ii) The student converts each solubility into mol dm−3.
| Source | Temperature / °C | Solubility of chlorine | Solubility of chlorine / mol dm−3 |
|---|---|---|---|
| A | 5 | 1.18 g per 100 cm3 of water | 0.166 |
| B | 15 | 260 cm3 of gas per 100 cm3 of water | 0.11 |
| C | 20 | 6.9 g per dm3 of water | 0.097 |
| D | 25 | 620 mg per 100 cm3 of water | 0.087 |
| E | 35 | 152 cm3 of gas per 100 cm3 of water | |
| F | 35 | 0.46 g per 100 g of water | 0.065 |
Calculate the value that is missing for source E, taking the density of chlorine gas at 35 °C to be 2.80 g dm−3.
(iii) Suggest an explanation for the trend in solubility with temperature shown by the data.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) Any one of:
different units / solubility expressed in different ways «mass of gas in some sources, volume of gas in others; per volume of water in some, per mass of water in another» ✔
OR
the pressure of the chlorine gas is not stated ✔
OR
different precisions / numbers of significant figures ✔
OR
uncertainties of the values are not given ✔
OR
the density of the gas «needed for sources B and E» is not given ✔
(ii) «mass of chlorine =» 0.152 «dm3» × 2.80 «g dm−3» = 0.426 «g» ✔
«solubility =» 0.426 «g» / 70.90 «g mol−1» / 0.100 «dm3» = 0.0600 «mol dm−3» ✔
OR
« =» 6.02 × 10−3 «mol per 100 cm3» ✔
0.0602 «mol dm−3» ✔
(iii) «solubility decreases as the temperature rises, so» dissolving chlorine in water is exothermic «and the equilibrium shifts towards undissolved gas at higher temperature» ✔
OR
«at higher temperature» the molecules have more kinetic energy and can break free of the attractions holding chlorine molecules among water molecules «so more gas escapes from the solution» ✔
OR
the negative entropy change of dissolving a gas becomes more significant «−TΔS more positive» at higher temperature ✔
Schemat punktowania
(i) [1 max] for any one valid difficulty. (ii) M1: mass of chlorine from the volume of gas and its density (0.426 g); M2: division by M(Cl2) = 70.90 g mol−1 and by 0.100 dm3 to give 0.0600 mol dm−3. Award [2] for a correct final answer with working; accept 0.0594-0.0602 from PV = nRT (0.0594 with P = 100 kPa, 0.0602 with P = 101.3 kPa). ECF from M1. (iii) [1] for any explanation consistent with a decrease in solubility as temperature increases.
Komentarz. Two common slips: dividing by 100 instead of 0.100 (giving 6.00 × 10−4) and using 35.45 g mol−1 instead of 70.90 g mol−1 for Cl2. The value for source F assumes that 100 g of water occupies 100 cm3.