The equilibrium constant for the reaction given below is 2.0×10−7 at 300 K. Calculate the standard entropy change if △H∘=28.40kJmol−1 for the reaction: PCl5(g)⇌PCl3(g)+Cl2(g)
A
−33.6Jmol−1K−1
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B
33.6Jmol−1K−1
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C
−43.6Jmol−1K−1
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D
43.6Jmol−1K−1
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Solution
The correct option is A−33.6Jmol−1K−1 K=2.0×10−7T=300K△H∘=28.4kJmol−1△G∘=−2.303RTlogk=−2.303×8.314×10−3×300×log(2×10−7)=−5.744(log2−7)=−5.744(0.3010−7)=38.48kJ
From △G∘=△H∘−T△S∘⇒T△H∘−△G∘△S∘=△H∘−△G∘T=28.4−38.48300=0.0336kJmol−1
or △S=−33.6Jmol−1K−1