Zadanie 1f, Paper 1B, specimen 2025, Chemia IB HL
Wzorowane na: Chemistry HL Paper 1B, specimen 2025, pytanie 1(f). 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
Bathers release nitrogen-containing compounds, including ammonia, into the pool water. Ammonia reacts with hypochlorous acid to form chloramines.
(i) Deduce an equation for the reaction of ammonia with hypochlorous acid that forms trichloramine, NCl3(aq).
(ii) The graph shows how the composition of the chloramine mixture, expressed as molar percentages, depends on the pH of the water at 5 °C. Trichloramine irritates the eyes and airways of swimmers and causes the characteristic "chlorine" smell of poorly managed pools.
PowiększState two conditions that would minimize the formation of trichloramine in the pool water.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) NH3(aq) + 3 HOCl(aq) → NCl3(aq) + 3 H2O(l) ✔
(ii) pH above «about» 5.5 / high pH «where NH2Cl is the main chloramine» ✔
low concentration of chlorine/HOCl «relative to ammonia»
OR
low concentration of ammonia «swimmers shower before entering the pool» ✔
Schemat punktowania
(i) [1] for a correctly balanced equation; state symbols not required. (ii) [1] for a high pH (any stated minimum from pH 5 upwards, or 'high pH'); [1] for keeping the concentration of HOCl/chlorine or of ammonia low. Vague answers such as 'fewer swimmers' do not score unless they are linked to a concentration or to the pH.
Komentarz. A frequent error in (i) is to write HCl as a product instead of water; hypochlorous acid transfers Cl to nitrogen and the OH ends up in water.