The integrated circuits in your cell phone and computer are made from the semiconductor silicon. The silicon is obtained from a really inexpensive starting material, sand, which is primarily SiO2. One step in the purification of silicon is to separate it from solid impurities by forming the gas silicon tetrachloride.
Given the following reactions, what is the overall enthalpy change in converting 1.00 mol of silicon dioxide into pure silicon?

SiO2(s) + 2C(s) → Si(impure s) + 2CO(g); ΔH = +690 kJ
Si(impure s) + 2Cl2(g) → SiCl4(g); ΔH= -657 kJ
SiCl4(g) + 2Mg(s) → 2MgCl2(s) + Si(s); ΔH = -625 kJ