[xangelofxdeathx]

The enthalpy change for which reaction represents the standard enthalpy of formation for hydrogen cyanide, HCN?

A) H_(g) + C_(graphite) + N_(g) ----> HCN_(g)
B) 1/2 H_2(g) + C_(graphite) + 1/2 N_2(g) ----> HCN_(g)
C) HCN_(g) ----> 1/2 H_2(g) + C_(graphite) + 1/2 N_2(g)
D) H_2(g) + 2C_(graphite) + N_2(g) ----> 2 HCN_(g)

The answer is D. My question is how do I tell the difference between choice B and choice D? In other words, how would I know that D is the right answer and B isn't?

[Sam (NG)]

As far as i am aware, B is the right answer, because the standard enthalpy of formation is: "the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states"

[xangelofxdeathx]

Oh I see, I guess I accidentally forgot about the definition of standard enthalpy of formation    Thanks.