Zadanie 6c
Wzorowane na: Chemistry HL Paper 2, May 2024, TZ1, pytanie 6(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Informacja do zadań 6a-6c.
Chromium is a first-row transition element that forms several coloured complex ions.
Polecenie
The complex ion [Cr(H2O)6]3+ is violet and absorbs light most strongly at about 580 nm. The complex ion [Cr(NH3)6]3+ is orange and absorbs light most strongly at about 465 nm.
(i) Explain why the two complex ions have different colours. Use section 15 of the data booklet.
(ii) In [Cr(NH3)6]3+, is the chromium(III) ion a Lewis acid or a Lewis base? Identify it and give a reason.
(iii) Describe whether water acts as a Brønsted-Lowry acid or a Brønsted-Lowry base in its reaction with ammonia. Include an equation in your answer.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) the colours arise from electrons being promoted between the split d orbitals «by absorbing visible light»; NH3 ligands cause a greater splitting of the d orbitals / a larger energy gap, so light of a shorter wavelength / higher energy is absorbed ✔
the colour seen is the complement of the colour absorbed: yellow absorbed gives violet, blue absorbed gives orange «colour wheel» ✔
(ii) Lewis acid AND accepts electron pairs from the ligands «forming coordination bonds» ✔
(iii) acid AND donates H+ / a proton «to ammonia» ✔
NH3(g) + H2O(l) ⇌ NH4+(aq) + OH−(aq) ✔
Schemat punktowania
(i) M1: larger d-orbital splitting or higher energy absorbed with NH3; M2: observed colour is complementary to the colour absorbed, supported by the colour wheel. (ii) [1]. (iii) M1: acid with the idea of donating a proton; M2: correct equation; accept either type of arrow. Do not award M2 for an unbalanced equation.