Zadanie 4e
Wzorowane na: Chemistry HL Paper 2, May 2026, TZ2, pytanie 4(e). Treść, dane i kontekst są zmienione; sprawdzane umiejętności, format i punktacja jak w oryginale.
Darmowe konto pozwoli wrócić do niego później.
Polecenie
Ethene reacts with water in the presence of an acid catalyst at room temperature.
C2H4(g) + H2O(l) → C2H5OH(l) ΔH⊖ = −44 kJ mol−1
(i) Calculate the change in entropy, ΔS⊖, for this reaction in J K−1 mol−1. Use section 13 of the data booklet.
(ii) Explain why the entropy change has this sign.
(iii) Calculate the change in Gibbs energy, ΔG⊖, in kJ mol−1, using sections 1 and 4 of the data booklet. If you were unable to get an answer to (i), use ΔS⊖ = −100 J K−1 mol−1, although this is not the correct value.
(iv) Explain why the formation of ethanol becomes less spontaneous as the temperature increases.
(v) Calculate the temperature, in K, above which the reaction becomes non-spontaneous. Use section 1 of the data booklet. If you were unable to get an answer to (i), use ΔS⊖ = −100 J K−1 mol−1, although this is not the correct value.
Sprawdź rozwiązanieUkryj rozwiązanieKlucz CKE i punktacja
Odpowiedź z klucza
(i) «ΔS⊖ = (+161) − (+220 + 70) =» −129 «J K−1 mol−1» ✔
(ii) 1 mol of gas «and 1 mol of liquid» changes to 1 mol of liquid OR the number of gas molecules / moles decreases ✔
(iii) «ΔS⊖ = −129 × 10−3 kJ K−1 mol−1 AND T = 298 K» ✔
«ΔG⊖ = −44 − (298 × (−129 × 10−3)) =» −5.6 «kJ mol−1» ✔
(iv) «ΔG = ΔH − TΔS» as T increases, −TΔS becomes more positive / TΔS becomes more negative ✔
ΔG becomes less negative / more positive ✔
(v) «0 = −44 − (T × (−129 × 10−3)), so » 341 «K» ✔
Schemat punktowania
(i) [1]. (ii) [1] must refer to the decrease in the number of gas molecules or moles. (iii) [2] M1 units converted and 298 K used; M2 value; award [2] for a correct final answer; award [2] for −14.2 «kJ mol-1» if −100 J K-1 mol-1 is used. (iv) [2] M1 the effect of increasing T on −TΔS; M2 ΔG less negative. (v) [1] accept 340 K; award [1] for 440 K if −100 J K-1 mol-1 is used.
Komentarz
In (iii) and (v) the entropy must be converted from J to kJ before it is combined with ΔH; leaving it in J gives a ΔG of about 38 000 kJ.