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Solutions

Using the tables on the web, calculate DSo and DGo  for the following reactions using:
DSo  = SSoproducts - SSoreactants  and

DGorxn=SDGoproducts - SDGoreactants
 

1.   CH4 (g)   +    2 O2 (g)   --------->     CO2 (g)    +   2 H2O (l)

a)  DSo  = SSoproducts - SSoreactants
 

Soproducts Soreactants
(1 mol CO2)(213.7 J/Kmol) = 213.7 J/K
(1 mol CH4)(186.1 J/Kmol) = 186.1 J/K
(2 mol H2O)(69.940 J/Kmol) = 139.88 J/K
(2 mol O2)(205 J/Kmol) = 410 J/K
SSoproducts= 353.58 J/K
SSoreactants = 596.1 J/K
DSo= 353.58 J/K - 596.1 J/K = -242.52 J/K  

 

b)   DGorxn=SDGoproducts - SDGoreactants
 

DGoproducts DGoreactants
(1 mol CO2)(-394.4 kJ/mol) = -394.4 kJ
(1 mol CH4)(-50.81 kJ/mol) = -50.81 kJ
(2 mol H2O)(-237.192 J/Kmol) = -474.384 kJ
(2 mol O2)(0 kJ/mol) = 0 kJ
SDGoproducts = -868.784 kJ
SDGoreactants= -50.81 kJ
DGorxn= -868.784 kJ - (-50.81 kJ) = -817.974 kJ
spontaneous reaction

 

2.   2 MgO (s)  -------->      2 Mg (s)   +  O2 (g)

a)  DSo  = SSoproducts - SSoreactants
 

Soproducts Soreactants
(2 mol Mg)(32.69 J/Kmol)  = 65.38J/K
(2 mol MgO)(26.9 J/Kmol) = 53.8 J/K
(1 mol O2)(205 J/Kmol)  = 205 J/K
 
SSoproducts = 270.38 J/K
 SSoreactants = 53.8 J/K
DSo = 270.38 J/K - 53.8 J/K = 216.58 J/K  

 

b)   DGorxn=SDGoproducts - SDGoreactants
 

DGoproducts DGoreactants
(2 mol Mg)( 0 kJ/mol)  = 0 kJ
(2 mol MgO)(-569.0 kJ/mol) = -1138.0 kJ
(1 mol O2)( 0 kJ/mol)  = 0 kJ
 
  SDGoproducts = 0 kJ
SDGoreactants = -1138.0 kJ
   
DGorxn= 0 kJ - (-1138.0 kJ) = +1138.0 kJ
nonspontaneous reaction