Does anything have a positive enthalpy of formation?

1 Answer
Truong-Son N.
Feb 11, 2018

Plenty. But all elements in their elemental standard state have zero enthalpies of formation. You can only have positive values for non-elemental states.

http://stahonorschemistry.weebly.com/

I spot #"AgCN"(s)#, #"C"_2"H"_2(g)#, #"C"_2"H"_4(g)#, #"CS"_2(l)#, #"H"(g)#, #"HCN"(aq)#, and more...

and these reactions are...

#"Ag"(s) + "C"(gr) + 1/2"N"_2(g) -> "AgCN"(s)#

#2"C"(gr) + "H"_2(g) -> "C"_2"H"_2(g)#

#2"C"(gr) + 2"H"_2(g) -> "C"_2"H"_4(g)#

#"C"(gr) + 1/4"S"_8("orthorhombic") -> "CS"_2(l)#

#1/2"H"_2(g) -> "H"(g)#

#1/2"H"_2(g) + "C"(gr) + 1/2"N"_2(g) -> "HCN"(aq)#

You should be able to explain why the formation of #"H"(g)# is endothermic with no math and no tables.

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