Zadanie 4b, Paper 2, specimen 2025, Chemia IB HL
Wzorowane na: Chemistry HL Paper 2, specimen 2025, pytanie 4(b). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
Wspólne informacje do wiązki zadań
Hydrobromic acid, HBr(aq), is a strong acid used in the manufacture of inorganic bromides and as a reagent in organic synthesis.
Polecenie
Chloride ions can act as ligands towards transition metal ions. Three compounds of chromium that differ in the number of chloride ions bonded directly to the chromium ion have the formulas and colours shown.
| Compound | Colour |
|---|---|
| [Cr(H2O)6]Cl3 | violet |
| [Cr(H2O)5Cl]Cl2 | blue-green |
| [Cr(H2O)4Cl2]Cl | green |
(i) Deduce the charge of the complex ion in [Cr(H2O)4Cl2]Cl and the oxidation state of chromium in it.
(ii) Describe how the three chloride ions in [Cr(H2O)5Cl]Cl2 are bonded.
(iii) Explain the origin of the colours of these complex ions.
(iv) Using the colour wheel (section 15) and the electromagnetic spectrum (section 5) of the data booklet, identify the complex ion in which the electron transition that produces the colour has the largest energy, and explain your choice.
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Odpowiedź z klucza
(i) charge of complex ion: 1+ ✔
oxidation state: +3 ✔
(ii) one chloride ion is a ligand, bonded to the chromium ion by a coordination (covalent) bond «donating a lone pair» ✔
two chloride ions are bonded ionically «to the complex ion, as counter-ions outside the coordination sphere» ✔
(iii) the ligands split the d orbitals of the chromium ion into two sets of different energy ✔
an electron absorbs a photon of visible light whose energy matches the gap and is promoted to the upper set ✔
the light that is not absorbed is transmitted, so the colour seen is the complement of the colour absorbed ✔
(iv) [Cr(H2O)6]3+ / the violet complex ✔
absorbs yellow light «575–585 nm», which has the shortest wavelength / highest frequency «so the largest energy» of the three ✔
Schemat punktowania
(i) [2]: one mark each. (ii) [2]: M1 one chloride as a ligand joined by a coordination bond; M2 two chlorides bonded ionically. (iii) [3]: splitting of the d orbitals by ligands; absorption of light on promotion between the split levels; observed colour complementary to the colour absorbed. (iv) [2]: M1 the hexaaqua (violet) complex; M2 explanation through the complementary colour yellow, shortest wavelength or highest frequency.
Komentarz. Violet is observed because yellow (575–585 nm) is absorbed; the green compounds absorb red light, which lies at the long-wavelength, low-energy end of the visible spectrum.