Zadanie 4d
Wzorowane na: Chemistry HL Paper 2, November 2024, pytanie 4(d). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
Sulfuryl chloride, SO2Cl2, is made industrially from sulfur dioxide and chlorine in the presence of an activated carbon catalyst.
SO2(g) + Cl2(g) ⇌ SO2Cl2(g) ΔH⊖ < 0
(i) Sketch a potential energy profile for the forward reaction, showing the activation energy, Ea, and labelling both axes.
(ii) Explain, in terms of collisions, why raising the temperature increases the rate of this reaction.
(iii) Explain how a catalyst speeds up a reaction.
(iv) Identify the oxidation states of sulfur in SO2 and in SO2Cl2.
(v) State the equilibrium constant expression, Kc, for the reaction.
(vi) Use the absolute entropy values in the table to calculate the entropy change, ΔS⊖, for the reaction as written, in J K−1 mol−1.
| Substance | S⊖ / J K-1 mol-1 |
|---|---|
| SO2(g) | 248.2 |
| Cl2(g) | 223.1 |
| SO2Cl2(g) | 311.9 |
(vii) Outline why the sign of the answer to (vi) could be predicted from the equation.
(viii) Calculate the Gibbs energy change, ΔG⊖, in kJ mol−1, for the reaction at 400 K. Use section 1 of the data booklet and ΔH⊖ = −67.2 kJ mol−1. If you could not answer (vi), use −150 J K−1 mol−1 instead; this is not the correct value.
(ix) Calculate the equilibrium constant for the reaction at 400 K. Use sections 1 and 2 of the data booklet. If you could not answer (viii), use −5.00 kJ mol−1 instead; this is not the correct value.
(x) A flask at 400 K contains 0.40 mol dm−3 SO2(g), 0.25 mol dm−3 Cl2(g) and 0.70 mol dm−3 SO2Cl2(g). Deduce whether this mixture is at equilibrium and, if it is not, in which direction the net reaction will occur.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz i punktacja
Odpowiedź z klucza
(i) x-axis labelled reaction progress / reaction pathway / reaction coordinate AND y-axis labelled «potential» energy / enthalpy ✔
curve with one maximum, with the products at a lower energy than the reactants «exothermic» ✔
labelled arrow or line showing Ea from the reactants up to the top of the curve ✔
(ii) a greater proportion of molecules has energy equal to or greater than Ea ✔
the rate of successful collisions per unit time rises / a larger fraction of collisions is successful ✔
(iii) «provides» an alternative reaction pathway / mechanism ✔
with a lower Ea
OR
a larger fraction of collisions reaches the lowered energy requirement ✔
(iv) SO2: +4 AND SO2Cl2: +6 ✔
(v) ✔
(vi) «ΔS⊖ = 311.9 − (248.2 + 223.1) =» −159.4 «J K−1 mol−1» ✔
(vii) the number of moles of gas decreases «two moles of gaseous reactants give one mole of gas, so the products are more ordered» ✔
(viii) «ΔG⊖ = ΔH⊖ − TΔS⊖»
«= −67.2 − (400 × (−159.4 × 10−3)) =» −3.44 «kJ mol−1» ✔
(ix) «−3.44 × 103 = −8.31 × 400 × ln K»
« =» 1.03 ✔
«K =» 2.8 ✔
(x) « =» 7.0 ✔
«Q > K, so» not at equilibrium AND reaction proceeds in the reverse direction / towards SO2 and Cl2 ✔
Schemat punktowania
(i) "Reaction" or "time" alone is not enough for the x-axis label. A two-headed arrow for Ea is acceptable but an arrow pointing downward is not. Profiles that show an intermediate are also fine. (ii) "More collisions" on its own does not earn M2; it must be linked to collisions per unit time or to the chance that a collision succeeds. (iii) Accept a description of how the catalyst lowers Ea, such as reactants adsorbing on its surface and their bonds weakening, for M1. (iv) Signs written after the number (4+, 6+) are not accepted. (vii) ECF from (vi): no mark if the entropy change obtained in (vi) is positive. (viii) Award [1] for −7.2 «kJ mol−1» if −150 J K−1 mol−1 was used. (ix) Award [2] for a correct final answer in the range 2.7-2.9; award [2] for K = 4.5 if −5.00 kJ mol−1 was used. (x) M2 needs the value of Q to be shown; with K = 4.5 the conclusion is the same.
Komentarz
The sub-parts test eight different understandings; only the four that carry the most marks are tagged.