Question

why standard enthalpy of formation of diamond is non zero although it is an element

Answer 1

Out of the various allotropic forms of an element, only the standard heat of formation of the most stable allotropic form is taken as zero. Since graphite and not diamond is the most stable form of carbon therefore, the enthalpy of formation of diamond is not zero.

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Answer 2

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diamond is made up of carbon ,
when carbon forms the four covalent bonds with different carbon atoms then the structure of diamond is formed . Although diamond is made up of single element but every carbon atom is connected with 4 different carbons which causes it's enthalpy of formation non zero.


(see the attachment)

_________&___________
Elements in their standard state are not formed, they just are. So, ΔH°f for C (s, graphite) is zero, but the ΔH°f for C (s, diamond) is 2 kJ/mol. That is because graphite is the standard state for carbon, not diamond.

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