Question
Why standard entropy of an elementary substance is not zero whereas standard enthalpy of formation is taken as zero?
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Answer
A substance has a perfectly ordered arrangement of its constituent particles only at absolute zero. Hence, entropy is zero only at absolute zero. Enthalpy of formation is the heat change involed in the formation of one mole of the substance from its elements. An element formed from itself means no heat change, i.e. $\Delta_\text{f}\text{H}^\circ=0.$
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