# Lattice Energy - Hard question - I keep getting wrong answer

#### stacker

1. The problem statement, all variables and given/known data

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

2. Relevant equations

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

3. 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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#### stacker

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?

#### mg0stisha

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.

#### stacker

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

#### mg0stisha

It explains all of the variables underneath, but..

V is the enthalpy of vaporization for metal atoms.

"Lattice Energy - Hard question - I keep getting wrong answer"

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