Wzorowane na: Chemistry SL Paper 1B, specimen 2025, pytanie 1(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Informacja do zadań 1a-1g.
Hot tubs and spa pools are often sanitized with bromine compounds rather than with chlorine. The active sanitizer is hypobromous acid, HOBr, which forms when bromine reacts with water.
A student compiles data on the solubility of bromine in water at different temperatures from several websites and textbooks.
| Source | Temperature / °C | Solubility of bromine |
|---|---|---|
| A | 5 | 3.90 g per 100 of water |
| B | 15 | 1.15 of liquid bromine per 100 of water |
| C | 25 | 34.0 g per of water |
| D | 30 | 3.35 g per 100 g of water |
| E | 40 | 1.02 of liquid bromine per 100 of water |
| F | 40 | 3.30 g per 100 of water |
(i) Identify one problem that arises when the values from these sources are compared with one another.
(ii) Each solubility is then expressed in .
| Source | Temperature / °C | Solubility of bromine | Solubility of bromine / |
|---|---|---|---|
| A | 5 | 3.90 g per 100 of water | 0.244 |
| B |
Calculate the missing value for source E and so complete the table. Take the density of liquid bromine to be 3.10 .
(iii) Suggest an explanation for the trend in the solubility of bromine as the temperature rises.
(i) Any one of:
different units / different ways of expressing solubility ✔
OR
some sources give the amount of water by mass, others by volume ✔
OR
different precisions / numbers of significant figures ✔
OR
uncertainties «of the values» not given ✔
OR
«for sources B and E» the density of bromine is needed but not stated ✔
(ii) «mass of bromine =» 1.02 «» × 3.10 «» = 3.16 «g» ✔
«solubility =» 3.16 «g» / 159.80 «» / 0.100 «» = 0.198 «
(iii) «solubility decreases as temperature increases, so» dissolving «bromine in water» is exothermic «and the equilibrium shifts towards undissolved bromine at higher temperature» ✔
OR
«at higher temperature» the kinetic/thermal energy of the molecules overcomes the intermolecular forces between bromine and water «so bromine escapes from the solution» ✔
OR
the negative entropy change of dissolving becomes more significant at higher temperature ✔
(i) [1 max] for any one valid problem. (ii) M1: mass of bromine from volume and density; M2: conversion to mol dm−3 using M(Br2) = 159.80 g mol−1 and 0.100 dm3 of water; award [2] for a correct final answer (0.198, accept 0.20). ECF from M1. (iii) [1] for a valid explanation consistent with a decrease in solubility with temperature.
In (ii) a common slip is to divide by 100 instead of 0.100, giving 0.00198 mol dm−3; another is to use M = 79.90 g mol−1 for Br2.
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| 15 |
| 1.15 per 100 of water |
| 0.22 |
| C | 25 | 34.0 g per of water | 0.213 |
| D | 30 | 3.35 g per 100 g of water | 0.210 |
| E | 40 | 1.02 per 100 of water |
| F | 40 | 3.30 g per 100 of water | 0.207 |