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Solutions — Chemistry STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 ScienceChemistrySolutions2 Marks
Question
At absolute zero, an exothermic reaction is always spontaneous but at temperatures above absolute zero, we have to consider both enthalpy and entropy before we can predict spontaneity. Why?
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Answer

As ∆G = ∆H –T∆S
The process is spontaneous when  ∆G is –ve. Since T = 0  so T∆S = 0, i.e ∆G = ∆H.  For an exhothermic process, ∆H is –ve,  therefore at absolute zero ∆G will always be –ve and hence a spontaneous process.
At temp. above absolute zero, ∆S is not zero. It may be +ve or –ve. Hence we have to consider both the ∆H and  T∆S for deciding ∆G and the spontaneity.

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