Calculating Enthalpy for H3O: Investigating the Dissolution of Ammonium Nitrate

[Jeann25]

I need to find the enthalpy for H3O. I can't find it in my table in my book, and I'm not having much luck finding it online. Is there one for this molecule?

 

[siddharth]

What do you mean by enthalpy? Do you mean the standard enthalpy of formation?

Remember that we can only measure the change in enthalpy of a system.

 

[Jeann25]

I do mean the standard enthalpy of formation..
The reaction is NH4+ in water to form NH3 + H3O+

I can find NH4 and NH3, but not H3O

 

[Bystander]

This is a "trick" question: the "trick" being that your text source has to inform you that ΔH_f(H₃O⁺) is "Zero" by convention, your instructor has to point this out, and you have to notice the special case.

 

[Jeann25]

I'm really confused then. I'm supposed to show how there is a positive ΔH and a negative ΔH for the dissolution of ammonium nitrate. I did:

NH4NO3 --> NH4+ + NO3-
This is a ΔH of 28.1 kJ

I thought maybe the negative ΔH was the Qh2o of the surrounding, but he said no, that the water does participate in the reaction. So I tried protonation..

NH4+ + H2O --> NH3 + H3O+
The ΔH is 372.2

I'm not understanding how to show a negative ΔH here...

 

[Bystander]

(snip) I'm supposed to show how there is a positive ΔH and a negative ΔH for the dissolution of ammonium nitrate. (snip)

This statement makes absolutely NO sense: please give us an exact copy of the problem statement.