Question

Identify the compound with the standard free energy of formation equal to zero. A) NaCl(s) B) N2(g) C) NO(g) D) O3(g) E) It is hard to determine.

I know the answer is B,but don't understand why. Please explain!

Answer 1

dG is based on:

- elemental states

- most likely phase at STP

- No complex species; must be only the SAME molecule

so... knowing this

a)

NaCl = has Na+ and Cl- species, therefore dG = not zero

b)

N2(g) , this is the natural state of nitrogen gas at STP, this is likely to have dG = 0

c)

NO(g) is a complex gas, has N and O as well, they can be reacted from N2 and O2

d)

O3 = this is not likely to be found at STP, it is a reaciton from O2 + O2 = O3 (not balanced)

E) not hard, therefore choose:

N2(g) only