MATSCI 204 Thermodynamics and Phase Equilibria Winter 2013 ...
MATSCI 204 Thermodynamics and Phase Equilibria Winter 2013 Chapter #6 Practice problems Problem 1: (20 points) Consider the reaction SiHCl3(g) + H2(g)
Si(s) +3HCl (g)
a) Briefly explain how the following changes might shift an existing equilibrium of these 4 components (to products or reactants as written above): i) an increase in pressure ii) an increase in the amount of Si present. b) A gas mixture consisting of 98.9 mol % H2, 0.1 mol%SiHCl3 and 1 mol% HCl is introduced into a reaction chamber containing a Si single crystal wafer at 1 atm pressure. Will the reaction written above proceed to form products (Si deposition) or to form reactants (Si etching) at 800°C.? Show all work. DATA: SiHCl3 (g): ΔGof, 1073K = - 810.9 kJ/mol HCl (g): ΔGof, 1073K = -292.7 KJ/mol
1
Solution a) i. 3 moles of gas products, 2moles gas reactants; As P increases shift equilibrium to form reactants. ii. Si is a pure component; aSi = 1. Increasing amount of Si causes no change to equilibrium. b) What is the Q value relative to K? Assume ideal gas approximation: 3 3 PHCl a Si n! HCl 10 !6 Q= = = = 1.011 " 10 !3 PH 2 PSiHCl3 n! H 2 n! SiHCl3 (0.001)(0,989)
Next o * " #Grxn K = exp(( ) RT 0 #Grxn = 3#G 0f , HCl
' %%.......... from....#Grxn = 0.. @ equil. & " #G 0f , SiHCL 3 ...........T = 1073K
0 $ #Grxn = "67.2 KJ / mol
Hence.....K = 1.868 ! 10 3
Q
Si(s) +3HCl (g)
a) Briefly explain how the following changes might shift an existing equilibrium of these 4 components (to products or reactants as written above): i) an increase in pressure ii) an increase in the amount of Si present. b) A gas mixture consisting of 98.9 mol % H2, 0.1 mol%SiHCl3 and 1 mol% HCl is introduced into a reaction chamber containing a Si single crystal wafer at 1 atm pressure. Will the reaction written above proceed to form products (Si deposition) or to form reactants (Si etching) at 800°C.? Show all work. DATA: SiHCl3 (g): ΔGof, 1073K = - 810.9 kJ/mol HCl (g): ΔGof, 1073K = -292.7 KJ/mol
1
Solution a) i. 3 moles of gas products, 2moles gas reactants; As P increases shift equilibrium to form reactants. ii. Si is a pure component; aSi = 1. Increasing amount of Si causes no change to equilibrium. b) What is the Q value relative to K? Assume ideal gas approximation: 3 3 PHCl a Si n! HCl 10 !6 Q= = = = 1.011 " 10 !3 PH 2 PSiHCl3 n! H 2 n! SiHCl3 (0.001)(0,989)
Next o * " #Grxn K = exp(( ) RT 0 #Grxn = 3#G 0f , HCl
' %%.......... from....#Grxn = 0.. @ equil. & " #G 0f , SiHCL 3 ...........T = 1073K
0 $ #Grxn = "67.2 KJ / mol
Hence.....K = 1.868 ! 10 3
Q
