Elemental carbon usually exists in one of two forms: graphite or diamond. It is
ID: 1016123 • Letter: E
Question
Elemental carbon usually exists in one of two forms: graphite or diamond. It is generally believed that diamonds last forever. The table shows the standard enthalpy of formation (?H?f) and the standard molar entropy (S?) values for diamond and graphite.
Part A
What is the standard Gibbs free energy for the transformation of diamond to graphite at 298 K?
Cdiamond?Cgraphite
Express your answer to three significant figures and include the appropriate units.
Substance ?H?f (kJ/mol) S? (J/mol?K) Cgraphite 0 5.740 Cdiamond 1.897 2.38Explanation / Answer
Given:
Hof(C(diamond)) = 0.0 KJ/mol
Hof(C(graphite)) = 1.897 KJ/mol
Balanced chemical equation is:
C(diamond) ---> C(graphite)
?Ho rxn = 1*Hof(C(graphite)) - 1*Hof( C(diamond))
?Ho rxn = 1*(1.897) - 1*(0.0)
?Ho rxn = 1.897 KJ
Given:
Sof(C(diamond)) = 5.74 J/mol.K
Sof(C(graphite)) = 2.38 J/mol.K
Balanced chemical equation is:
C(diamond) ---> C(graphite)
?So rxn = 1*Sof(C(graphite)) - 1*Sof( C(diamond))
?So rxn = 1*(2.38) - 1*(5.74)
?So rxn = -3.36 J/K
?Ho = 1.897 KJ/mol
?So = -3.36 J/mol.K
= -0.00336 KJ/mol.K
T= 25.0 oC
= (25.0+273) K
= 298 K
use:
?Go = ?Ho - T*?So
?Go = 1.897 - 298.0 * -0.0034
?Go = 2.8983 KJ/mol
Answer: 2.90 KJ/mol
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