Why is enthalpy of formation negative?

1 Answer
Truong-Son N.
Oct 28, 2015

It's not always negative. Sometimes it's positive.

A negative #DeltaH_f^o# indicates that the formation of a compound is exothermic---the amount of energy it takes to break bonds is less than the amount of energy that is released when making the bonds.

Some examples of this are:

#DeltaH_(f, "H"_2"O"(l))^o = -"285.8 kJ/mol"#
#DeltaH_(f, "C"_3"H"_8(g))^o = -"103.85 kJ/mol"#
#DeltaH_(f, "C"_6"H"_12"O"_6(s))^o = -"1273 kJ/mol"#

A positive #DeltaH_f^o# indicates that the formation of a compound is endothermic---the amount of energy it takes to break bonds is greater than the amount of energy that is released when making the bonds.

Some examples of this are:

#DeltaH_(f, "HI"(g))^o = "25.9 kJ/mol"#
#DeltaH_(f, "C"_6"H"_6(l))^o = "49.0 kJ/mol"#
#DeltaH_(f, "C"_2"H"_2(g))^o = "226.7 kJ/mol"#

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