Solution: Silicon used in computer chips must have an

Chapter 11, Problem 11.129

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Silicon used in computer chips must have an impurity level below 1029 (that is, fewer than one impurity atom for every 109 Si atoms). Silicon is prepared by the reduction of quartz (SiO2) with coke (a form of carbon made by the destructive distillation of coal) at about 2000C: SiO2(s) 1 2C(s) Si(l) 1 2CO(g) Next, solid silicon is separated from other solid impurities by treatment with hydrogen chloride at 350C to form gaseous trichlorosilane (SiCl3H): Si(s) 1 3HCl(g) SiCl3H(g) 1 H2(g) Finally, ultrapure Si can be obtained by reversing the above reaction at 1000C: SiCl3H1g2 1 H21g2 Si1s2 1 3HCl1g2 (a) Trichlorosilane has a vapor pressure of 0.258 atm at 22C. What is its normal boiling point? Is trichlorosilanes boiling point consistent with the type of intermolecular forces that exist among its molecules? (The molar heat of vaporization of trichlorosilane is 28.8 kJ/mol.) (b) What types of crystals do Si and SiO2 form? (c) Silicon has a diamond crystal structure (see Figure 11.28). Each cubic unit cell (edge length a 5 543 pm) contains eight Si atoms. If there are 1.0 3 1013 boron atoms per cubic centimeter in a sample of pure silicon, how many Si atoms are there for every B atom in the sample? Does this sample satisfy the 1029 purity requirement for the electronic grade silicon?