Question

why the standard enthalpy of elementary substance is not zero while standard enthalpy of formation is taken as zero??​

Answer 1

Answer:

At this temperature, there is no disorder as the particles stop moving. ... 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.

Answer 2

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.