Question

Why is the the heat of formation of hydrogen fluoride gas written like 1/2H2+F2-->HF
and not H2+F2-->2HF

Answer 1

The definition says it is related to the forming of ONE mole of the compound.
You should have written:

\[\frac{1}{2}H _{2}+\frac{1}{2}F _{2} \rightarrow HF\]

Answer 2

the definition of heat of formation?

Answer 3

Have a look at: http://en.wikipedia.org/wiki/Enthalpy_of_formation