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Thermochemistry: Play With Reaction Synthlet


    
   or    Kelvin        
Reduction of iron(III) oxide with hydrogen
           +       3 2       +         3
Iron(III) oxide
+
Hydrogen
 
Reactants
 
Iron
+
Steam
 
Products
 
Products – Reactants
ΔfH
 
ΔfH
 
Σf-reactH
 
ΔfH
 
ΔfH
 
 
Σf-prodH
 
Σf-prodH - Σf-reactH = ΔrH
(-824.2 x 1)
+
(0.0 x 3)
=
-824.20
 
(0.0 x 2)
+
(-241.8 x 3)
=
-725.5
 
-725.5 - -824.2 = 98.7 kJmol-1
                             
S
 
S
 
ΣreactS
 
S
 
S
 
 
ΣprodS
 
ΣprodS - ΣreactS = ΔrS
(0.087 x 1)
+
(0.131 x 3)
=
0.479
 
(0.027 x 2)
+
(0.189 x 3)
=
0.621
 
0.621 - 0.479 = 0.142 kJK-1mol-1
Gibbs free energy at 298K (standard conditions and standard temperature):
 
ΔrG   = ΔrH – (T298 x ΔrS)   =   98.7– (298 x 0.142)  =
+56.5kJmol-1
ΔrH
=
+98.7 kJmol-1 Positive value: the reaction is endothermic
ΔrS
=
+0.142 kJK-1mol-1 Positive value: the system becomes more dispersed
ΔrG 298K
=
+56.5 kJmol-1 Positive value: the reaction is not feasible (spontaneous)
Keq 298K
=
1.23 x 10-10 Value is less than 1: the reaction proceeds to the left
ΔG° = 0.0, and the system is at equilibrium, at a temperature of 697K (424°C)

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List Thermochemical Reactions Build-a-Reaction Synthlet

© Mark R. Leach 1999 –


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