Question

For a spontaneous reaction, which of the following condition is correct, at all temperatures?

• A. $ \Delta H<0,\,\Delta S<0 $
• B. $ \Delta H>0,\,\Delta S<0 $
• C. $ \Delta H<0,\,\Delta S>0 $
• D. $ \Delta H>0,\,\Delta S>0 $

Answer 1

Answer: (C)

For a reaction, to be spontaneous, $ \Delta G $ (free energy change) must be negative. $ \Delta G $ is given by Gibbs-Helmoltz equation, which is $ \Delta G=\Delta H-T\cdot \Delta S $ Hence, from this equation it follows that $ \Delta H $ will be negative at all temperature if, $ \Delta H=-\,ve $ and $ \Delta S=+ve $ i.e., $ \Delta H<0 $ and $ \Delta S>0 $