In industry, silicon is obtained from $\text{SiO}_2$ by reacting it with carbon at over $2000\,^{\circ}\text{C}$. reaction 1: $\text{SiO}_2(s) + 2\text{C}(s) \rightarrow \text{Si}(l) + 2\text{CO}(g)$ Explain why the entropy change, $\Delta S$, of reaction 1 is positive.
Reaction 1 is strongly endothermic. Suggest the effect of an increase in temperature on the feasibility of this reaction. Explain your answer.
Silicon is purified by first passing a stream of $\text{HCl}(g)$ over it to make $\text{SiHCl}_3$. The $\text{SiHCl}_3$ produced is then distilled to separate out other impurities. reaction 2: $\text{Si}(s) + 3\text{HCl}(g) \rightarrow \text{SiHCl}_3(g) + \text{H}_2(g)$ Table 2.1 contains some standard entropy data. Use the information in Table 2.1 to calculate $\Delta S^{\circ}$ for reaction 2.
Reaction 3 is the reverse of reaction 2 and is used to make pure silicon. reaction 3: $\text{SiHCl}_3(g) + \text{H}_2(g) \rightarrow \text{Si}(s) + 3\text{HCl}(g)$ $\Delta H = +219.3\,\text{kJ mol}^{-1}$ Use this information together with your answer to (b)(i) to calculate the temperature, in $\text{K}$, at which reaction 3 becomes feasible. Show your working. If you could not do (b)(i), use $\Delta S^{\circ} = -150\,\text{J K}^{-1}\text{mol}^{-1}$ for reaction 2. This is not the correct answer to (b)(i).
Silicon may also be made by electrolysis of $\text{SiO}_2$ dissolved in molten $\text{CaCl}_2$. The cathode half-equation is shown below. $\text{SiO}_2 + 4e^- \rightarrow \text{Si} + 2\text{O}^{2-}$ Calculate the time, in seconds, needed to produce $1.00\,\text{g}$ of $\text{Si}$ by this electrolysis, using a current of $6.00\,\text{A}$. Assume that no other substances are formed at the cathode.