Question

Consider the endothermic reaction C2H4(g) + H2O(g) → C2H5OH(g). When would you expect this reaction to be spontaneous?

(A) At all temperatures (B) Above a certain temperature (C) below a certain temperature (D) At no temperature (E) Not enough information given

Answer is D but i have no idea why. Please explain in detail.

Answer 1

for a reaction to be spontaneous free energy change dG must be\positive
or dG < 0. And we know
dG = dH -TdS, depending on the sign of dH and dS we can say
whether dG positive or negative. and thus non spontaneous or spontaneous
it is given
endothermic reaction ;; means dH value will be positive.
C2H4(g) + H2O(g) --> C2H5OH(g).
number of mols of gas products formed[=1] is less than number of mols
of gas reactants [=2]. Thus entropy of reaction is reduced.
Hence dS is negetive

dH is positive
dS is negative
=> dG = dH - TdS will be positive at all temperatures ,because even though we change temperature TdS value remains

negative, which makes dG positive

dG > 0 means reaction is nonspontaneous

Answer D At no temperature

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