If the change in free energy of formation is zero, is change in enthalpy of formation always zero as well?

1 Answer
Truong-Son N.
Apr 6, 2018

Yes. The reference states of both #DeltaG_f^@# and #DeltaH_f^@# are the element in its standard state (#25^@ "C"#, #"1 bar"#). So if you have

#"O"_2(g) -> "O"_2(g)#

then #DeltaH_f^@# is obviously #"0 kJ/mol"# because nothing needs to happen to form #"O"_2(g)# from #"O"_2(g)#. And we can also tell that #DeltaG_f^@ = "0 kJ/mol"# since no reaction is going on to form #"O"_2(g)# from #"O"_2(g)#.

The entropy however must be questioned, and is pretty much never zero because it has a different reference state (not #25^@ "C"#!). The reference state of #S^@# is chosen to be #S("0 K") = "0 J/mol"cdot"K"#, so really we mean

#DeltaS^@ -= S^@ - S(0) = S^@#

when we say #S^@#.

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