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Zadanie 1f

10 pktotwarteśrednie (szac.)Paper 2, listopad 2022

Wzorowane na: Chemistry HL Paper 2, November 2022, pytanie 1(f). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.

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Informacja do zadań 1a-1f.

Ammonium chloride, NH4Cl, is a white crystalline solid used as a nitrogen fertilizer for rice crops and as an electrolyte in dry cells.

Polecenie

Solid ammonium chloride decomposes when heated to form two gases, ammonia and hydrogen chloride.

(i) Write the chemical equation for this decomposition.

(ii) Calculate the volume of ammonia produced at STP when an 8.00 g sample of ammonium chloride decomposes. Use sections 2 and 7 of the data booklet.

(iii) Calculate the standard enthalpy change, ΔH⊖, of the reaction. Use section 13 of the data booklet.

ΔHf⊖ NH4Cl(s) = −314 kJ mol−1

ΔHf⊖ NH3(g) = −46 kJ mol−1

(iv) Predict, with a reason, the sign of the standard entropy change, ΔS⊖, for the decomposition. Deduce, with a reason, whether the decomposition is spontaneous at all temperatures.

(v) Ammonia and hydrogen chloride recombine on cooling to form solid ammonium chloride. Deduce the Lewis (electron dot) structure of the ammonium ion, including its formal charge, and state its shape.

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Odpowiedź z klucza

(i) NH4Cl(s) → NH3(g) + HCl(g) ✔

(ii) «n(NHX4Cl)=8.00 g53.50 g mol−1n(\ce{NH4Cl}) = \dfrac{8.00\ \mathrm{g}}{53.50\ \mathrm{g\,mol^{-1}}} =» 0.1495 «mol» «1 : 1 mole ratio» ✔

«V = 0.1495 mol × 22.7 dm3 mol−1 =» 3.39 «dm3» ✔

(iii) ΔHf⊖ HCl(g) = −92 kJ mol−1 «from section 13» ✔

«ΔH⊖ = (−46) + (−92) − (−314) =» +176 «kJ» ✔

(iv) positive AND one mole of solid forms two moles of gas «so the disorder increases» ✔

no AND ΔH⊖ is positive AND ΔS⊖ is positive, so ΔG⊖ = ΔH⊖ − TΔS⊖ is negative only at high temperature ✔

(v) four N–H bonds around the nitrogen atom «one of them a coordination bond» AND no lone pair on nitrogen «Lewis structure inside brackets» ✔

+ / 1+ / +1 formal charge on the nitrogen atom «charge shown on the ion» ✔

Shape: tetrahedral ✔

Schemat punktowania

(i) 1 mark; accept the equilibrium arrow. (ii) Award [2] for the correct final answer (accept 3.39–3.41 dm3); accept calculations based on PV = nRT. (iii) Award [2] for the correct final answer; M1 is the value for HCl(g) taken from the data booklet. (iv) M1: positive with the reason in terms of gas formed from a solid; M2: not spontaneous at all temperatures, with both signs of ΔH⊖ and ΔS⊖ used. (v) M1: Lewis structure; M2: formal charge on nitrogen; M3: shape. Allow ECF for the shape only if it fits the Lewis structure drawn.

Umiejętności w zadaniu