Consider a reaction where S rxn = -315 J/K and H rxn = -45.6 kJ. Which statement

Question

Consider a reaction where S rxn = -315 J/K and H rxn = -45.6 kJ. Which statement is true about the spontaneity of this reaction?

A) This reaction is nonspontaneous at 25°C but will become spontaneous if the temperature is decreased to -130°C. B) This reaction is spontaneous at 25°C, but will become nonspontaneous if the temperature is decreased to -130°C. C) Decreasing the temperature will have no effect on the spontaneity of the reaction. It is nonspontaneous at 25°C and will become even less spontaneous as the temperature decreases. D) This reaction is nonspontaneous at 25°C but will become spontaneous if the temperature is increased to 145°C.

Explanation / Answer

G = H-TS = -45600 - T*-315

A)

at 25C = 298K --> G =  -45600 + 298*315 = 48270; this is non-spontanous

further analysis,

note that

H < TS

H/S < T

(-45600/-315 < T

T > 144.7 K = -128.8 C (in order to be spontanous)

therefore; OPTION B

must be true

This reaction is spontaneous at 25°C, but will become nonspontaneous if the temperature is decreased to -130°C.

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