
Answer:
ΔHreaction (KJ) =  - 78 KJ
The value may vary in relation to the source of the bond enthalpy data.
 Bond enthalpies used  (From a table of bond enthalpies )
Bond      KJ/ mol
C≡O      1080
C-Cl       330
C=O Â Â Â Â Â 741
Cl-Cl      243
Explanation:
             CO(g) + Cl2(g)  →   COCl2(g)
1) Lewis structures
Valence electrons
C Â Â 4
O Â Â 6
Cl   7
Reactants
CO(g)
In the case of CO, carbon has a negative charge and oxygen a positive charge.
Cl2(g)
Products
COCl2(g)
_______________________
ΔHreaction (KJ) = ∑ n* ΔH ( reactants) - ∑ n* ΔH (product)
ΔHreaction (KJ) =  (ΔHC≡O +  ΔHCl-Cl)  - ( ΔHC=O  + 2* ΔHC-Cl)
Replacing the values
ΔHreaction (KJ) =  (1080 +  243)  - ( 741 + 2* 330)
ΔHreaction (KJ) = 1323 -  1401
             =  - 78 KJ