(6.5) Assume you are involved in a project to deposit ZnS and CdS films for infr
ID: 934612 • Letter: #
Question
(6.5) Assume you are involved in a project to deposit ZnS and CdS films for infrared optical coatings. Thermodynamic data reveal 1. H2S(g) + Zn(g) - > ZnS(s) + H2(g) Delta G degree = -76,400 + 82.1 T - 5.9 T In T (cal/mol) 2. H2S(g) + Cd(g) - > CdS(s) + H2(g) Delta G degree = -50,000 + 85.2 T - 6.64 T in T (cal/mol) (a) Are these reactions endothermic or exothermic? . (b) In practice, reactions 1 and 2 are carried out at 680 degree C and 600 degree C, respectively. From the vapor pressures of Zn and Cd at these temperatures, estimate the PH2/PH2S ratio for each reaction, assuming equilibrium conditions. (c) Recommend a reactor design to grow either ZnS or CdS, including a method for introducing reactants and heating substrates.Explanation / Answer
a) These reaction are exothermic as deltaG is negative for both the reactions.
b) Using the given equation for deltaG, we can calculate deltaG at 680C for ZnS formation.
deltaG = -76400 + (82.1*953) - (5.9*953*ln953) = -36728.26 cal/mol
deltaG = -RTlnKc => -36728.26 = -1.987*953*lnKc
lnKc = 20.03 => Kc = 1.06x1020
Kc = PH2/(PH2S)*(PZn)
Therefore, PH2/PH2S = 1.06x1020*PZn = 1.06x1024 (vapor pressure of zinc at 953K is close to 10 kPa)
similarly deltaG for the CdS formation reaction at 600C is
detalG = -50000 + (85.2*873) - (6.64*873*ln873) = -14875.41 cal/mol
lnKc = 14875.41/(1.987*873) = 8.58 => Kc = 3.76x108
Kc = PH2/(PH2S)*(PCd)
Therefore, PH2/PH2S = 3.76x108*PCd = 3.76x1012 (vapor pressure of cadmium at 873K is close to 10 kPa)
c) Since the ratio PH2/PH2S for both reactions are high, it requires effective removal of H2 gas and supply of excess H2S over the reservoir of Zn or Cd. It is better if the reactor is conical in shape with narrow opening at the bottom where the substrate is placed.
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