Using the data given, calculate the value of
Hrxn, in kJ/mol, for the reaction:

2 Mg(s) + SiCl4(l)
Si(s) + 2 MgCl2(s)
Hrxn = ?

Data:

Si(s) + 2 Cl2(g)
SiCl4(l)
H= ー687 kJ

Mg (s) + Cl2(g)
MgCl2(s)
H= ー641 kJ