Lattice Energy - Hard question - I keep getting wrong answer

AI Thread Summary
The discussion centers on calculating the lattice energy of potassium iodide (KI) using Born-Haber's law. The user initially arrives at a lattice energy of -567.4 kJ/mol, which is incorrect as it should be a positive value. Confusion arises regarding the correct application of the equation, particularly the treatment of electron affinity and the variable "V," which represents the enthalpy of vaporization for metal atoms. Clarification is sought on whether to subtract or add certain values in the equation. Understanding these variables and their correct application is crucial for accurately determining the lattice energy.
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Homework Statement



What is the lattice energy of KI? Given the information below.

Heat of formation for KI = -328 kJ/mol

Heat of sublimation for K = 89.20 kJ/mol

Ionization energy for K = 419 kJ/mol

Bond dissociation energy for I2 = 149 kJ/mol

Electron affinity for I = - 295 kJ/mol

Heat of sublimation for I2 = 48.30 kJ/mol

I used Born-Haber's law: Delta H of formation = .5*Delta Dissociation + Delta H of sublimation + Ionization Energy + Electron Affinity + Lattice Energy

Homework Equations



Born-Haber's law = Delta H of formation = .5*Delta Dissociation + Delta H of sublimation + Ionization Energy + Electron Affinity + Lattice Energy

The Attempt at a Solution



I used Born-Haber's law: Delta H of formation = .5*Delta Dissociation + Delta H of sublimation + Ionization Energy + Electron Affinity + Lattice Energy

-328 = (89.20 - 48.30) + .5 (149) + 419 - 295 + Lattice Energy

Lattice Energy = -567.4 (Which is wrong; it should be a positive answer)
 
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I plugged in +567.4 and it was wrong. What is going on? What does that variable V equal to in relation to my problem in the equation that you gave me from wiki?
 
also, it says you should subtract the electron affinity, there for making it positive in this problem. if that's the right equation you're trying to use.
 
mg0stisha said:
also, it says you should subtract the electron affinity, there for making it positive in this problem. if that's the right equation you're trying to use.

Ok but what is the variable "V" in my problem in relation to the equation from wikipedia:

Delta H = V + .5 B + IE - EA - U
 
It explains all of the variables underneath, but..

V is the enthalpy of vaporization for metal atoms.
 
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