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Topic: lattice energy  (Read 10277 times)

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studygirl

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lattice energy
« on: June 28, 2005, 03:30:03 AM »
hey,
can someone please check my work, and make sure my numbers are where they should be, thank you.
the question reads:
calculate the lattice energy for a solid CaBr2 from the following information:
-the heat sublimation of Ca is 121 kj/mol
the first ionization energy for Ca is 590KJ/MOL
The second ionization energy for Ca is 1145 KJ/MOL
The heat vapourization for liquid Br2 is 315KJ/MOL
The bond dissociation energy for gaseous Br2 is 193KJ/MOL
The electron affinity for gaseous bromine atoms is -324KJ/MOL
The overall heat of formatio of solid CaBr2 is -675KJ/MOL

okay so this is what i have:
1)Ca(s)---->Ca(s)   (121kj/mol)
2)Br2(s)--->2Br(s)   (193KJ/MOL)
3)Ca(g)--->Ca+(g)+e-  (590kj/mol)
   Ca+(g)--->Ca2+(g) + e- (1145kj/mol)
4)2[Br(s) + e- --->Br- (s)]   (-324kj/mol)
5)Ca2+ (g) + 2Br-(g) ---> CaBr2  (315kj/mol)
------------------------------------------------------
   Ca(s)+Br2(s)---> CaBr2(s)      (-675kj/mol)

so delta H overall =delta H1 +delta H2+ delta H3 etc..
                         =(-675  -121-193-590-1145-(-324)-315=x

is this sort of the right the way????

Offline xiankai

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Re:lattice energy
« Reply #1 on: June 28, 2005, 06:14:17 AM »
lattice energy = electrostatic energy that is bonding Ca2+ and Br- together.

writing the correct states is important because different states have different energy

1) sublimation is (s) --> (g)

2) both are in gaseous state

3) seems fine to me

4) both are in gaseous state as again. u can exclude the 2[...] because we're dealing with the reaction empirically.

5) this should be the ionic equation, but the bond energy is wrong.

6) u need to add the heat vapourization for liquid Br2, which is Br2(l) ---> Br2(g). this will have the same bond energy as u wrote earlier for (5).

7) for the overall heat of formation of solid CaBr2,

first we have solid calcium and liquid bromine at rtp.

Calcium gets ionized to its ion (Ca2+)

use point 3, add both ionisation energies together to find the enthalpy change.

Bromine first gets heated up to a gas, then dissociates to form its negative ion (Br-)

use point 6, then point 2 and point 4, make sure u double the energy calculated in point 4 because we are converting 2 bromine moles.

this way u will get Br2(l) ---> 2Br-(g)

now with both energies calculated earlier, add them together and u'll get the enthalpy change for the reaction making CaBr2.

with the calculation obtained, add it with the overall heat of formation to get the lattice energy, because the calculation u obtained is the energy in bonding, while the  overall heat of formation is the energy released. u'll get the energy level of CaBr2.

so far i dont think my theories came out right(and thus my calculations). can someone explain in further detail :S
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