Chapter 8, Problem 34QAP

A reaction used to produce the silicon for semiconductors from sand (SiO₂), can be broken up into three steps:

${\text{SiO}}_2\left(s\right)+2\text{C}\left(s\right)\to \text{Si}\left(s\right)+2\text{CO}\left(g\right)\Delta H=689.9\text{kJ}$

$\text{Si}\left(s\right)+2{\text{Cl}}_2\left(s\right)\to {\text{SiCl}}_{\text{4}}\left(g\right)\Delta H=-657.0\text{kJ}$

${\text{SiCl}}_4\left(g\right)+2\text{Mg}\left(s\right)\to 2{\text{MgCl}}_2\left(g\right)+\text{Si}\left(\text{s}\right)\Delta H=-625.6\text{kJ}$

(a) Write a thermochemical equation for the overall reaction where silicon is obtained from silicon dioxide and CO and MgCl₂ are by-products.

(b) What is ΔH for the formation of one mole of silicon?

(c) Is the overall reaction exothermic?