Calculating lattice energy on ionic compound

  • Thread starter flemonster
  • Start date
  • #1
11
0

Homework Statement



Given the following thermodynamic data, calculate the lattice energy of CaBr2(s) caculate the lattice enegy:

(A) Δ°Hf CaBr2(s) = -675 kJ/mol
(B) Δ°Hf Ca(g) = 179 kJ/mol
(C) Δ°Hf Br(g) = 112 kJ/mol
(D) 1st ionization energy of Ca = 590 kJ/mol
(E) 2nd ionization energy of Ca = 1145 kJ/mol
(F) Electron affinity of Br = -325 kJ/mol


Homework Equations



Heats of Formation + ionization energies + electron affinity - lattice energy = heat of formation



The Attempt at a Solution



(B) + (C) + (D) + (E) + (F) + Lattice Energy = -675

solving for lattice energy:

[179 + 112 + 590 + 1145 + -325] + Lattice Energy = -675

Lattice energy = -675 - 1701

Lattice energy = -2376 kJ/mol

I think I am going wrong with something in the calculations of the energy bromine contributes to the equation. I know bromine is diatomic but I'm not sure how to incorporate that into the equation. I don't want you to give me the answer because I really want to be able to do this on my own but anything you can point out that can get me in the right direction are appreciated.
 

Answers and Replies

  • #2
Borek
Mentor
28,473
2,871
You are not following stoichiometry - you need to deal with two Br atoms all the time. 112 kJ/mol for Δ°Hf Br(g) is a dissociation enthalpy - but it produces just one Br atom, and you need two.

It should became obvious if you will try to write a "reaction equation" - after all, Hess law is just a way of dealing with the energy conservation. Mass conservation and charge conservation are used to balance reaction equation, same thinking can be applied to the energy conservation.
 

Related Threads on Calculating lattice energy on ionic compound

Replies
2
Views
1K
  • Last Post
Replies
1
Views
394
  • Last Post
Replies
3
Views
455
  • Last Post
Replies
1
Views
14K
  • Last Post
Replies
21
Views
2K
  • Last Post
Replies
5
Views
4K
  • Last Post
Replies
2
Views
11K
  • Last Post
Replies
9
Views
9K
Replies
1
Views
2K
  • Last Post
Replies
5
Views
18K
Top