The standard state Gibbs free energies of formation of C(graphite) and C(diamond) at T = 298 K are
ΔfG∘[C(graphite)]=0 kJmol−1
ΔfG∘[C(diamond)]=2.9 kJmol−1
The standard state means that the pressure should be 1 bar, and substance should be pure at a given temperature. The conversion of graphite [C(graphite)] to diamond [C(diamond)] reduces its volume by 2×10−6m3mol−1. If C(graphite) is converted to C(diamond) isothermally at T = 298 K, the pressure at which C(graphite) is in equilibrium with C(diamond) is:
[Useful information: 1 J=1 kg m2s−2; 1 Pa=1 kg m−1s−2; 1 bar=105 Pa]