Question about standard enthalpy of formation of ions

[Ewokinarmor]

Hey everyone, this isn't strictly a homework problem but I'm curious about electrochemistry, went a bit ahead in my book, and I found something that doesn't seem to add up. Could you help me please?

 

Okay, so I was looking at the enthalpy change from the reaction of the Daniell Cell and found that its around -210 kJ, a value that agrees with the net ionic equation (when I looked up standard enthalpies of formation for Cu2+ and Zn2+):

 

Cu2+(aq) + Zn(s) -> Zn2+(aq) + Cu(s)

 

But here's my problem: the first and second ionization energies of Copper add up to 2702kJ/mol. If it takes 2702kJ to ionize one mole of Copper into Cu2+ (that is, remove the first two e-) then shouldn't that be the standard enthalpy of formation for Cu2+? If I apply that logic to Zn2+ then its standard enthalpy of formation should be 2639kJ/mol and the enthalpy for the reaction should be 2639 - 2702 = -63 kJ.

 

Neither of the two enthalpies of formation that I predicted are true and neither is my predicted enthalpy of the reaction. What did I do wrong? Does it have more to do with electron affinity than ionization energy? What DOES determine the enthalpy of formation of ions?

Edited December 29, 2015 by Ewokinarmor