I Exam KEY CHEM XXI- Spring 2010 1. Silicon can react with oxygen in the atmosphere according to the following chemical equation: Si(s) + O2(g) SiO2(s) a) (5) Predict the sign of Horxn. 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 Sorxn. 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 (G = H-TS). 2. Silicon dioxide SiO2(s) is used to produce pure silicon using carbon: SiO2(s) + 2 C(s) Si(s) + 2 CO(g) a) (10) Calculate Horxn and Sorxn for this process: Horxn = + 689.7 kJ Sorxn = +361.3 J/K b) ((5) Should this reaction be performed at low or high temperature to favor the formation of Si(s)? Justify your answer. The reaction should be performed at high T as the favorability increases with increasing T. The + entropy becomes more relevant as T goes up according to G = H-TS.
c) (10) At what temperature can this reaction be expected to change directionality. Justify your answer. (give your answer in degrees K) G = H-TS = 0 H/S = T (crossover) = 689700/361.3 = 1908 K is when G goes from being positive to negative 3. The silicon produced via the previous process is at least 98% pure. However, for use in the electronics industry this silicon is transformed into a silicon compound that can be more easily purified by distillation, and then converted back into pure silicon. Trichlorosilane HSiCl3 is the silicon compound most commonly used as the intermediate compound. a) (10) When HSiCl3(g) is blown over silicon at high temperature, its decomposes to high-purity silicon. Calculate Gorxn for this process: 2HSiCl3 Si(s) + 2 HCl(g) + SiCl4(g) Horxn = +151 kJ Sorxn = +95.7 J/K G = H-TS = +122.5 kJ b) (5) Write the expression for the equilibrium constant Kp for this process: Kp = [HCl]2[SiCl4] / [HSiCl3]2
c) (10) This reaction is traditionally carried out at 1150 C. Calculate the value of Kp at this
o
temperature. Based on your discuss results, whether it would be convenient to increase or decrease the temperature of the process to favor Si(s) formation.
o o H rxn S rxn ) + RT R
K = exp(
= e(-151000/(8.314x1423) + 95.7/8.314) = 0.286 at 1423 K
Best to increase T to favor Si production since the extent will increase with T as the entropic change of the reaction becomes more relevant.
4. One of the products of the synthesis of pure silicon is silicon tetrachloride SiCl4. This compound can be further decomposed to generate pure silicon. Other silicon compounds, such as SiF4 and SiBr4 can also be used as intermediates in the synthesis of pure silicon. a) (5) Arrange SiCl4, SiF4 and SiBr4 in order of increasing energetic stability. Justify your answer.
SiF4 > SiCl4 > SiBr4 Si-F bonds are shorter, more polar and hence stronger than Si-Cl. The same is true for comparing Si-Cl and Si-Br bonds.
b) (5) Arrange SiCl4, SiF4 and SiBr4 in order of increasing entropic stability. Justify your answer. SiBr4 > SiCl4 > SiF4 This trend is based on mass. Larger mass equates to a higher molar entropy.
b) (5) Based on the previous results, predict which of these compounds can be expected to more easily decompose into pure silicon at low temperatures (SiX4 Si + 2 X2). SiBr4 should more easily decompose. The compound is less energetically stable and at low temperatures its entropic stability is less relevant.
5. In recent years, simpler methods to produce pure silicon have been based on the decomposition of silane, SiH4, on a hot metal surface: SiH4(g) Si(s) + 2 H2(g) a) (10) The activation energy for this process is 51.7 kJ/mol. Based on this information and the data provided on the table on page 1. Build a reaction coordinate diagram for this process. Your diagram should clearly show the relative energy of reactants, products, and the transition state Horxn = -34.3 kJ Transition state Ea=51.7 kJ E
H= -34.3 kJ
Reaction coordinate
b) (5) The kinetics of this reaction can be followed by measuring the pressure of SiH4(g) in the system as a function of time. The table shows experimental data for T = 700 K. What is the average rate of reaction at the beginning of the decomposition?
t(s) 0 120 240 360 480 600
PSiH4 (atm) 0.7 0.683 0.667 0.651 0.636 0.621
Rate = 1.42 x 10-4 PSiH4/s This is computed by taking the change in partial pressure over the first time interval (0 to 120s).
c) (5) If the reaction is first order as a function of PSiH4, Determine the half-life for SiH4 in this process. Take the ln of the first 2 values of PSiH4 and get 2.05 x 10-4 for k. Then t1/2= -ln0.5/2.05 x 10-4 = 3381 seconds