The Integrated Circuits in Your Cell Phone and Computer Are

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? $$\begin{array} { l c } { \text { Reaction } } & \boldsymbol { \Delta } \boldsymbol { H } ^ { \circ } ( \mathbf { k J } ) \\\left. \mathrm { SiO } _ { 2 } ( s ) + 2 \mathrm { C } ( s ) \rightarrow \mathrm { Si } \text { (impure } s \right) + 2 \mathrm { CO } ( g ) & + 690 \\\mathrm { Si } ( \mathrm { impure } ) + 2 \mathrm { Cl } _ { 2 } ( g ) \rightarrow \mathrm { SiCl } _ { 4 } ( g ) & - 657 \\\mathrm { SiCl } _ { 4 } ( g ) + 2 \mathrm { Mg } ( s ) \rightarrow 2 \mathrm { MgCl } _ { 2 } ( s ) + \mathrm { Si } ( s ) & - 625\end{array}$$

A) +1,972 kJ
B) -1,972 kJ
C) -592 kJ
D) +592 kJ
E) -625 kJ