Write a balanced formation equation at standard conditions for the compound HI.

I know the delta H for H is 218, and H2 is 0. What number do I use?

Question

Write a balanced formation equation at standard conditions for the compound HI.

I know the delta H for H is 218, and H2 is 0. What number do I use?

jfraser · Answer

when we do formation reactions, we always use the elements as they appear in nature, in their standard states. Since we find hydrogen as a diatomic molecule, then the formation reaction will use $H_2$, not just H. Iodine also is diatomic under standard state conditions, so the same rule applies

anonymous · Answer

How do you know if an element is diatomic? Do periodic tables usually clarify that in some way, shape or form?

jfraser · Answer

there are 7 elements that are diatomic in nature. I teach HOFBrINCl when I do it

Hydrogen $H_2$
Oxygen $O_2$
Fluorine $F_2$
Bromine $Br_2$
Iodine $I_2$
Nitrogen $N_2$
Chlorine $Cl_2$

H, O, Br, I, N, Cl are all the diatomics

anonymous · Answer

So the formation equation for CaCl2 would be:
1 Ca + 1 Cl2 --> CaCl2?

anonymous · Answer

I just checked and that is the correct answer. Thank you for your help!

jfraser · Answer

YVW

anonymous · Answer

I have another question. I know that the formation equation for O3 is:
3/2 O2 --> 1 O3 but how would I go about finding 3/2?

jfraser · Answer

the same way you'd double the delta H if you produced 2 moles, but 3/2 is the same as 1.5