U5_Exam1-S10 POLLARD XXI-KEY

U5_Exam1-S10 POLLARD XXI-KEY - Exam I KEY CHEM XXI Spring...

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Exam I KEY CHEM XXI- Spring 2010 1. Silicon can react with oxygen in the atmosphere according to the following chemical equation: Si(s) + O 2 (g)  SiO 2 (s) a) (5) Predict the sign of H o rxn . Justify your answer based on the relative energetic stability of reactants and products. Negative. “A-A” bonds are being broken to form “A-B” bonds which are in general more stable. Therefore, heat is released. b) (5) Predict the sign of S o rxn . Justify your answer based on the relative entropic stability of reactants and products. Negative. The reaction is going from 1 mole of gas to no gas making the reactants more entropically stable. c) (5) Discuss whether this reaction is product-favored or reactant-favored at all temperatures, or whether temperature will affect its directionality. If process directionality is affected by T, explain how and justify your answer. The process will be reactant favored at higher temperatures and product favored at lower temperatures. As T increases, the negative sign on the entropy becomes more relevant on the overall directionality (ΔG = ΔH - TΔS). 2. Silicon dioxide SiO 2 (s) is used to produce pure silicon using carbon: SiO 2 (s) + 2 C(s) Si(s) + 2 CO(g) a) (10) Calculate H o rxn and S o rxn for this process: H o rxn = + 689.7 kJ S o rxn = +361.3 J/K
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U5_Exam1-S10 POLLARD XXI-KEY - Exam I KEY CHEM XXI Spring...

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