5. Chemical energetics
A section of Chemistry, 9701
Listing 10 of 306 questions
Silicon is the second most abundant element by mass in the Earth’s crust. In industry, silicon is extracted from SiO2 by reaction with carbon at over 2000 °C. reaction 1 SiO2+ 2C→ Si+ 2COExplain why the entropy change, ∆S, of reaction1 is positive. Reaction1 is highly endothermic. Suggest the effect of an increase in temperature on the feasibility of this reaction. Explain your answer. Silicon is purified by first heating it in a stream of HCl to form SiHCl 3. The SiHCl 3 formed is then distilled to remove other impurities. reaction 2 Si+ 3HCl → SiHCl 3+ H2Table2.1 shows some standard entropy data. Table 2.1 compound standard entropy, S o / J K–1 mol–1 Si19 HCl 187 SiHCl 3314 H2131 Use the data in Table2.1 to calculate ∆S o for reaction2.  ∆S o = J K–1 mol–1 Reaction3 is the reverse of reaction2 and is used to obtain pure silicon. reaction 3 SiHCl 3+ H2→ Si+ 3HCl ∆H = +219.3 kJ mol–1 Use this information and your answer to to calculate the temperature, in K, at which reaction3 becomes feasible. Show your working. [If you were unable to answer , you should use ∆S o = –150 J K–1 mol–1 for reaction2. This is not the correct answer to .]  temperature = K Silicon can also be produced by electrolysis of SiO2 dissolved in molten CaCl 2. The relevant half-equation for the cathode is shown. SiO2 + 4e– → Si + 2O2– Calculate the time, in seconds, required to produce 1.00 g of Si by this electrolysis, using a current of 6.00 A. Assume no other substances are produced at the cathode.  time = s 
9701_m22_qp_42
THEORY
2022
Paper 4, Variant 2
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