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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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