Question

Which of the following has a standard enthalpy of formation value of zero at 25 degree C?
a. O2 (s)
b. O (g)
c. O2 (g)
d. O3 (g)
e. O2 (l)

I know it is O2 (l) but really don't understand why, please explain from the simplest concepts! So confused. Thanks!

Answer 1

All elements in their standard states (oxygen gas, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.
ther for the answer would be O (g)

Answer 2

enthalpy of an element in its standard state is zero.

Answer 3

Hmm, okay, but how do I know what their standard state is? Why is O2 in gas form its standard state? I can't seem to figure that out anywhere.

Answer 4

noo all elements will be in their standard form :) that you should know by basic knowledge in periodic table they mention all the elements standard state :)
O2 is molecule hence its not an element it cant be zero:)

Answer 5

Hmm, I guess I'm still kind of confused. I guess I don't get why O2 (g) is correct when O2 (l) isn't, or even why all of these wouldn't be correct since they are all elements & not compounds? Haha sorry, I'm just not getting this concept!

Answer 6

O(g) is correct and not O2(L) or O2 (g)
O is element and O2 is molecule like a compound

Answer 7

Hmm, my instructor marked that O2 (g) is the correct answer, but your explanation makes more sense. \: Maybe he made a mistake.
So if there were another question like this, it would be the pure element then?

Answer 8

yaaa :)

Answer 9

Alright, thank you very much! (: Makes sense.

Answer 10

you are welcome :)