Zadanie 2b
Wzorowane na: Chemistry SL Paper 2, May 2026, TZ2, pytanie 2(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 2a-2c.
Iodine and selenium are two non-metals that occur in trace amounts in seawater.
Polecenie
An ‘iodine clock’ experiment can be based on two reactions that occur at the same time.
S2O82−(aq) + 2 I−(aq) → 2 SO42−(aq) + I2(aq) slow
2 S2O32−(aq) + I2(aq) → S4O62−(aq) + 2 I−(aq) fast
The iodine formed in the slow reaction is removed at once by thiosulfate ions. When all the S2O32− has reacted, iodine accumulates and forms a dark blue complex with starch indicator.
(i) Deduce the oxidation state of sulfur in the peroxodisulfate ion, S2O82−.
(ii) Write a half-equation for the reduction of S2O82− to SO42−.
(iii) Suggest one change to the conditions that would increase the time taken for the dark blue colour to appear.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) +6 ✔ «six oxygen atoms at −2 and the two oxygen atoms of the peroxide link at −1: 2x + 6(−2) + 2(−1) = −2»
(ii) S2O82− + 2 e− → 2 SO42− ✔
(iii) Any one of:
decrease the temperature ✔
decrease the concentration of S2O82− / I− ✔
increase the concentration / amount of S2O32− ✔
Schemat punktowania
(i) [1] +6; do not accept +7 or 6+. (ii) [1] electrons on the left; state symbols ignored. (iii) [1] any one change from the key; do not accept adding a catalyst.