Question

Thermodynamically the most stable form of carbon is

A) Diamond

B) Graphite

C) Fullerenes

D) Coal

Answer 1

Hint: Carbon is known to exist in many allotropes which may be crystalline or amorphous. Allotropes are the structurally different forms of an element having different physical properties but identical chemical properties. Crystalline allotropic forms of carbon are diamond, graphite, fullerenes and carbon nanotubes whereas amorphous allotropic forms of carbon are coal, charcoal and coke.

Complete answer:

Graphite is thermodynamically the most stable form of carbon. Its standard enthalpy of formation is thus taken as zero. Let us see why it is more stable than all the other given allotropes of carbon.

Graphite is more stable than diamond and fullerene because its enthalpy of formation $ \Delta H_{f}^{o} $  is less than that of both diamond (1.98 kJ $ \text{mo}{{\text{l}}^{\text{-1}}} $ ) and fullerene (38.1 kJ $ \text{mo}{{\text{l}}^{\text{-1}}} $ ) at room temperature and atmospheric pressure. Also graphite has a layered structure in which all the carbon atoms are  $ s{{p}^{2}} $  hybridized and bonded to the other three carbon atoms forming hexagonal rings. Since only three electrons are used, the fourth valence electron is free to move. Thus it has a delocalized -electron cloud which is responsible for its high electrical conductivity. This delocalization adds up to its thermal stability.

Diamond has a three dimensional rigid structure in which carbon is  $ s{{p}^{3}} $ hybridized. Each carbon is attached to four carbon atoms through  $ C-C $ sigma bonds. Because of its compact structure, it has high density and is the hardest substance known.

Coal is an amorphous allotropic form of carbon. It does not have a defined structure or geometry and hence is less stable than graphite.

Structures of diamond and graphite are shown below:

Hence correct option is (B).

Additional Information: 

Diamond is the purest form of carbon. The conversion of diamond into graphite is thermodynamically feasible but does not occur in nature due to the requirement of very high activation energy. Diamond has the highest thermal conductivity but has no electrical conductivity. Fullerenes form a class of carbon allotropes having spheroidal structures and composition  $ {{C}_{2n}} $  (n≥30). Of all the fullerenes, Buckminsterfullerene  $ {{C}_{60}} $ is the most stable.

Note:

Students are likely to get confused between diamond and graphite for thermal stability. Diamond has a rigid and compact structure due to which it takes more energy and time to convert into graphite and therefore, it is kinetically more stable but thermodynamically less stable than graphite.