chemistry help?

Question

Calculate the standard enthalpy of formation for diamond, given that

C (graphite) + O2 (g) ---> CO2 (g) dH = -393.5 kJ/mol
C (diamond) + O2 (g) ---> CO2 (g) dH = -395.4 kJ/mol

Answer 1

Reverse the second equation and change the sign of the dH:
CO2 (g) --> C (diamond) + O2 dH = +395.4 kJ/mol
Adding the two equations after first cancelling what is the same on both sides gives:
C (graphite) --> C (diamond)
so add the two dH's to get -393.5 + 395.4 = +1.9 kJ/mol
So, the standard enthalpy for the formation of diamond is +1.9 kJ/mol.

Answer 2

Very interesting.