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Thermochemistry: Play With Reaction Synthlet


    
   or    Kelvin        
Decomposing butane to its elements
     4       +         5
Butane
 
Reactants
  
Carbon
 +
Hydrogen
 
Products
  
Products – Reactants
ΔfH⦵
  
 
Σf-reactH⦵
  
ΔfH⦵
  
ΔfH
 
 
Σf-prodH⦵
  
Σf-prodH⦵ - Σf-reactH⦵ = ΔrH⦵
(-126.2 x 1)
=
-126.15
 
(0.0 x 4)
+
(0.0 x 5)
=
0.0
 
0.0 - -126.2 = 126.2 kJmol-1
                             
S⦵
  
 
ΣreactS⦵
  
S⦵
  
S
 
 
ΣprodS⦵
  
ΣprodS⦵ - ΣreactS⦵ = ΔrS⦵
(0.310 x 1)
=
0.310
 
(0.006 x 4)
+
(0.131 x 5)
=
0.676
 
0.676 - 0.310 = 0.366 kJK-1mol-1
Gibbs free energy at 298K (standard conditions and standard temperature):
  
ΔrG⦵   = ΔrH⦵ – (T298 x ΔrS⦵)   =   126.2– (298 x 0.366)  =
+17.0kJmol-1
ΔrH⦵
=
 +126.2 kJmol-1 Positive value: the reaction is endothermic
ΔrS⦵
=
 +0.366 kJK-1mol-1 Positive value: the system becomes more dispersed
ΔrG⦵ 298K
=
 +17.0 kJmol-1 Positive value: the reaction is not feasible (spontaneous)
Keq 298K
=
 1.03 x 10-3 Value is less than 1: the reaction proceeds to the left
ΔG° = 0.0, and the system is at equilibrium, at a temperature of 345K (72°C)
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List Thermochemical Reactions Build-a-Reaction Synthlet

© Mark R. Leach 1999 –

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