Zadanie 1c, Paper 2, maj 2025, TZ2, Chemia IB HL
Wzorowane na: Chemistry HL Paper 2, May 2025, TZ2, pytanie 1(c). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
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Zadanie
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Hydrogen cyanide, HCN, is an extremely poisonous compound that is nevertheless manufactured on a large scale, because it is a starting material for polymers such as nylon and poly(methyl methacrylate).
Polecenie
The cyanide ion acts as a ligand in many complex ions, for example hexacyanoferrate(III), [Fe(CN)6]3−.
(i) State the type of bond, as precisely as possible, that joins each cyanide ion to the iron ion.
(ii) Deduce the oxidation state of iron in [Fe(CN)6]3−.
(iii) Explain the origin of the colour of transition element complexes such as [Fe(CN)6]3−.
(iv) A 0.500 mol dm−3 solution of the complex ion contains free Fe3+ ions at a concentration of 4.00 × 10−8 mol dm−3. Determine the equilibrium constant, K, for the formation of the complex ion from Fe3+(aq) and CN−(aq).
(v) The standard electrode potential of the half-cell [Fe(CN)6]3−(aq) + e− ⇌ [Fe(CN)6]4−(aq) is +0.36 V. Using this value and section 19 of the data booklet, outline, with a reason, whether [Fe(CN)6]4− is a stronger or a weaker reducing agent than Fe2+(aq).
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) coordination «covalent» bond ✔
(ii) +3 ✔
(iii) Any three of:
iron(III) has a partially filled d-sublevel ✔
the ligands split the d-orbitals into two sets of different energy ✔
an electron is promoted from the lower set to the higher set by absorbing light ✔
the energy difference corresponds to a frequency in the visible region «and the light transmitted is the complementary colour of the light absorbed» ✔
(iv) ✔
[CN−] = 6 × 4.00 × 10−8 = 2.40 × 10−7 «mol dm−3» ✔
✔
(v) E⊖ for the complex couple, +0.36 V, is less positive than +0.77 V for Fe3+/Fe2+ ✔
the less positive the E⊖, the more readily the reduced form gives up electrons, so [Fe(CN)6]4− is the stronger reducing agent ✔
Schemat punktowania
(i) [1] coordination or dative covalent. (ii) [1] +3 or III; do not accept 3+ or 3. (iii) [3 max] one mark per marking point; the statement that the observed colour is complementary does not by itself earn the last mark. (iv) [3] M1 equilibrium expression; M2 [CN-] from the 1:6 stoichiometry; M3 value of K, accept 6.5 to 6.6 × 10^46; award [3] for a correct final answer. (v) [2] M1 comparison of the two E° values; M2 conclusion 'stronger reducing agent' with the reason.
Komentarz. In (iv) the cyanide concentration is six times the iron(III) concentration and is then raised to the sixth power; using 4.00 × 10^-8 for [CN-] gives about 3 × 10^51.