Zadanie 2b, Paper 2, maj 2025, TZ1, Chemia IB SL
Wzorowane na: Chemistry SL Paper 2, May 2025, TZ1, pytanie 2(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
Iron(III) ions form coloured complex ions in aqueous solution.
Polecenie
When potassium thiocyanate solution is added to a pale yellow solution containing iron(III) ions, the following equilibrium is established.
[Fe(H2O)6]3+(aq) + SCN−(aq) ⇌ [Fe(SCN)(H2O)5]2+(aq) + H2O(l)
[Fe(H2O)6]3+(aq) is pale yellow and [Fe(SCN)(H2O)5]2+(aq) is deep red.
(i) State, in terms of equilibrium concentrations, the expression for K, the equilibrium constant of this equilibrium.
(ii) Predict how the value of K and the position of equilibrium are affected when a few crystals of solid potassium thiocyanate, KSCN(s), are dissolved in the mixture at constant temperature.
(iii) When a deep red equilibrium mixture is warmed in a water bath, its colour fades to orange. Deduce, with a reason, whether [Fe(SCN)(H2O)5]2+(aq) forms in an exothermic or in an endothermic process.
(iv) Sketch an energy profile for the forward reaction of this equilibrium. Label the reactants and the products, and mark the activation energy, Ea, and the enthalpy change, ΔH, of the forward reaction.
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Pokaż rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) «K =» ✔
(ii) no change in K AND equilibrium shifts to the right / more [Fe(SCN)(H2O)5]2+ is formed / the solution becomes a deeper red ✔
(iii) exothermic AND warming shifts the equilibrium to the left / favours the reverse «endothermic» reaction ✔
(iv) reactants drawn higher in energy than products «consistent with an exothermic forward reaction» ✔
Ea marked from the reactants level up to the peak ✔
ΔH marked from the reactants level down to the products level ✔
Schemat punktowania
(i) [1] for the complex ion in the numerator and the reactants in the denominator; water is the solvent and is omitted. Square brackets or an equivalent notation for the complex ions are accepted. (ii) [1] for both the unchanged K (K depends only on temperature) and the direction of the shift. (iii) [1] for exothermic together with a reason based on the shift to the left on heating; «exothermic» alone scores zero. (iv) [1] per feature: relative levels of reactants and products; Ea; ΔH. Accept double-headed arrows for Ea and ΔH; the profile must agree with the exothermic answer given in (iii).