Why standard entropy of an elementary substance is not zero whereas standard enthalpy of formation is taken as zero?
Answers
Entropy is zero only at absolute zero. Enthalpy of formation is the heat change involved in the formation of one mole of the substance from its elements. An element formed from itself means no heat change, i.e. ΔfH0 = 0.
A perfectly ordered arrangement without any imperfections is possible only at the temperature of absolute zero i.e. 0 Kelvin.
At this temperature, there is no disorder as the particles stop moving. Hence entropy is taken as zero.
It is practically impossible to reach this temperature and hence standard entropy of an elementary substance is not zero.
When one mole of a substance is formed from its elements, the heat change involved is called enthalpy of formation. The standard enthalpy for formation of an element in its most stable state is taken as zero. This is the reference state.