lattice enthalpy calculation

[yo_wazzup41]

Find the lattice enthalpy of MgF2(s)

 

Mg(s) + F2(g) -------- MgF2(s) -1102 kJmol-1

 

1/2F2(g) -------- F (g) + 79 kJmol-1

 

F(g) + e- -------- F-(g) -348 kJmol-1

 

Mg (s) -------- Mg(g) +150 kJmol-1

 

Mg (s) -------- Mg^2+ (g) + 2e- +2186 kJmol-1

 

 

My calculation is :

 

-1102 + 79*2 + 348 *2 - 150 -2186

and my answer is :- 2584

 

but the modal answer is -2821

 

can anyone please help me thankx

Edited January 23, 2009 by yo_wazzup41

[ChemSiddiqui]

Find the lattice enthalpy of MgF2(s)

 

Mg(s) + F2(g) -------- MgF2(s) -1102 kJmol-1

 

1/2F2(g) -------- F (g) + 79 kJmol-1

 

F(g) + e- -------- F-(g) -348 kJmol-1

 

Mg (s) -------- Mg(g) +150 kJmol-1

 

Mg (s) -------- Mg^2+ (g) + 2e- +2186 kJmol-1

 

 

My calculation is :

 

-1102 + 79*2 + 348 *2 - 150 -2186

and my answer is :- 2584

 

but the modal answer is -2821

 

can anyone please help me thankx

 

 

A good idea will be to draw a born cycle before making any calculation. What you do then is that use the forumula:

 

Enthalpy of formation = sum of all other entalphies(including Lattice enthalphy).

 

now try to do it yourself and see if the answer comes out right.

 

hope i have helped