Account for the following: On the basis of enthalpy of formation, graphite is more stable than diamond, yet diamond does not change into graphite for years.
Answers
Becauese diamonds are hardest and kinetically stable than graphite.
Diamond is created deep underground under conditions of extreme pressure and temperature. Under these conditions diamond is actually the more stable of the two forms of carbon, and so over a period of millions of years carbonaceous deposits slowly crystallise into single crystal diamond gemstones.
Graphite is sp2 hybridised which contains free election hence here congugation occurs. But diamonds are sp3. So here no free electron no congugation.
If we look into the phase diagram of graphite and diamonds we can understand that The relationship between diamond and graphite is a thermodynamic and kinetic one, as can be seen in the phase diagram for carbon, above. At normal temperatures and pressures, graphite is only a few eV more stable than diamond, and the fact that diamond exists at all is due to the very large activation barrier for conversion between the two. There is no easy mechanism for this conversion and so transforming diamond into graphite, or vice versa, requires almost as much energy as destroying the entire lattice and rebuilding it. Once diamond is formed, therefore, it cannot reconvert back to graphite because the barrier is too high. So diamond is said to be metastable, since it is kinetically stable, not thermodynamically stable. The relationship between diamond and graphite is a thermodynamic and kinetic one, as can be seen in the phase diagram for carbon, above. At normal temperatures and pressures, graphite is only a few eV more stable than diamond, and the fact that diamond exists at all is due to the very large activation barrier for conversion between the two. There is no easy mechanism for this conversion and so transforming diamond into graphite, or vice versa, requires almost as much energy as destroying the entire lattice and rebuilding it. Once diamond is formed, therefore, it cannot reconvert back to graphite because the barrier is too high. So diamond is said to be metastable, since it is kinetically stable, not thermodynamically stable.