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Thermochemistry: Play With Reaction Synthlet


    
   or    Kelvin        
Cracking butane to ethylene and hydrogen
     2       +        
Butane
 
Reactants
 
Ethylene
+
Hydrogen
 
Products
 
Products – Reactants
ΔfH
 
 
Σf-reactH
 
ΔfH
 
ΔfH
 
 
Σf-prodH
 
Σf-prodH - Σf-reactH = ΔrH
(-126.2 x 1)
=
-126.15
 
(52.3 x 2)
+
(0.0 x 1)
=
104.5
 
104.5 - -126.2 = 230.7 kJmol-1
                             
S
 
 
ΣreactS
 
S
 
S
 
 
ΣprodS
 
ΣprodS - ΣreactS = ΔrS
(0.310 x 1)
=
0.310
 
(0.220 x 2)
+
(0.131 x 1)
=
0.570
 
0.570 - 0.310 = 0.260 kJK-1mol-1
Gibbs free energy at 298K (standard conditions and standard temperature):
 
ΔrG   = ΔrH – (T298 x ΔrS)   =   230.7– (298 x 0.260)  =
+153.3kJmol-1
ΔrH
=
+230.7 kJmol-1 Positive value: the reaction is endothermic
ΔrS
=
+0.260 kJK-1mol-1 Positive value: the system becomes more dispersed
ΔrG 298K
=
+153.3 kJmol-1 Positive value: the reaction is not feasible (spontaneous)
Keq 298K
=
1.34 x 10-27 Value is less than 1: the reaction proceeds to the left
ΔG° = 0.0, and the system is at equilibrium, at a temperature of 889K (616°C)

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List Thermochemical Reactions Build-a-Reaction Synthlet

© Mark R. Leach 1999 –


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