Standard enthalpy of formation

Standard enthalpy of formation

1. 1.
a)   2 NO
(g)
  +  O
2(g)
  ----->  2 NO
2(g)
 
ΔH
o
rxn
 =ΔH
o
p
 -ΔH
o
r
 
            = [ 2 NO
2(g)
 ]  -  [ 2 NO
(g)
  +  O
2(g)
 ] 
            = [ 2 (+33.2 kJ/mole) ] - [ 2 (+90.2 kJ/mole) + 0 ] 
            =  +66.4 kJ/mole  - 180.4 kJ/mole  
            =  -114.0 kJ/mole 
The reaction is exothermic as written with a ΔH
o
rxn
 of -114.0 
kJ/mole.
Standard Heats of Formation
ΔH
o
rxn
=ΔH
o
p
-ΔH
o
r

2. 1 b)   NaOH
(s)
  +  HCl
(g)
   ------->   NaCl
(s)
   +   H
2
O
(g)
 
    ΔH
o
rxn 
= ΔH
o
p
 - ΔH
o
r 
                 = [NaCl
(s)
   +   H
2
O
(g)
] - [NaOH
(s)
  +  HCl
(g)
] 
                 = [(-411.2 kJ/mole) + (-241.8 kJ/mole)] - 
[(-425.6 kJ/mole) + (-92.3 kJ/mole)]  
                 = (-653.0 kJ/mole) - (-517.9 kJ/mole) 
                 = -135.1 kJ/mole 
The reaction is exothermic as written with 
a ΔH
o
rxn
 of -135.1 kJ/mole.

3. 2. a)  2 CO
(g)
 +  2 O
2(g)
   ------->   CO
2(g)
 
     ΔH
o
rxn
 = ΔH
o
p
 - ΔH
o
r
 
                  = [ CO
2(g)
 ] - [ 2 CO
(g)
  +  2 O
2(g)
 ] 
                  = [ 2 (-393.5 kJ/mole)] - [ 2 (-110.5 kJ/mole) + 2 (0) ] 
                  = -787.0 kJ/mole - ( -221.0 kJ/mole) 
                  = -566.0 kJ/mole 
The reaction is exothermic as written with a ΔH
o
rxn
 of -566.0 
kJ/mole.

4. 2 b) CH
4(g)
+ 2 O
2(g)
------> CO
2(g)
+ 2 H
2
O
(l)

ΔH
o
rxn
= ΔH
o
p
- ΔH
o
r

= [ CO
2(g)
+ 2 H
2
O
(l)
] - [ CH
4(g)
+ 2 O
2(g)
]
= [ (-393.5 kJ/mole) + 2 (-285.8 kJ/mole) ]
- [ (-74.8 kJ/mole) + 2 (0) ]
= -393.5 kJ/mole - 571.6 kJ/mole + 74.8
kJ/mole
= -890.3 kJ/mole
The reaction is exothermic as written with a ΔH
o
rxn

of -890.3 kJ/mole.

5. 2.c) 2 H
2
S
(g)
+ 3 O
2(g)
--------> 2 H
2
O
(l)
+ 2 SO
2(g)

ΔH
o
rxn
= ΔH
o
p
- ΔH
o
r
= [ 2 H
2
O
(l)
+ 2 SO
2(g)
] - [ 2 H
2
S
(g)
+ 3 O
2(g)
]
= [ 2 (-285.8 kJ/mole) + 2 (-296.8 kJ/mole) ] -
[ 2 (-20.6 kJ/mole) + 3 (0) ]
= -571.6 kJ/mole - 593.6 kJ/mole) + 41.2
kJ/mole
= -1124.0 kJ/mole
The reaction is exothermic as written with a ΔH
o
rxn
of
-1124.0 kJ/mole.

6. 3 a) SO
2(g)
+ ½O
2(g)
------> SO
3(g)

ΔH
o
rxn
= ΔH
o
p
- ΔH
o
r
= [ SO
3(g)
] - [ SO
2(g)
+ ½O
2(g)
]
= [ (-396.0 kJ/mole) ] - [ (-296.8 kJ/mole)
+ ½ (0) ]
= -396.0 kJ/mole + 296.8 kJ/mole
= -99.2 kJ/mole
The reaction is exothermic as written with a ΔH
o
rxn

of -99.2 kJ/mole.

7. 3b)   CaO
(s)
  +  H
2
O
(l) 
  --------->  Ca(OH)
2(s)
 
     ΔH
o
rxn
 = ΔH
o
p
 - ΔH
o
r
                  = [ Ca(OH)
2(s) 
] - [ CaO
(s)
  +  H
2
O
(l) 
] 
                  = [ -986.1 kJ/mole] - [ (-635.1 kJ/mole) + (-
285.8 kJ/mole) ] 
                  = -986.1 kJ/mole - (-920.9 kJ/mole) 
                  = -65.2 kJ/mole 
The reaction is exothermic as written with a ΔH
o
rxn
 of 
-65.2 kJ/mole.

8. 3c)     N
2(g)
  +   3 H
2(g)
   -------->   2 NH
3(g)
 
     ΔH
o
rxn
 = ΔH
o
p
 - ΔH
o
r
           = [ 2 NH
3(g) 
] - [  N
2(g)
  +   3 H
2(g)
 ] 
           = [ 2 (-46.1 kJ/mole) ] - [  0 +  3 (0) ] 
           = -92.2 kJ/mole - 0 kJ/mole 
           = -92.2 kJ/mole 
The reaction is exothermic as written with a ΔH
o
rxn
 of 
-92.2 kJ/mole.

9. 3d)   C
6
H
6(l)
  +  1½ O
2(l)
  ------>  6 C
(s)
  +  3  H
2
O
(l)
 
     ΔH
o
rxn
 = ΔH
o
p
 - ΔH
o
r
          = [ 6 C
(s)
  +  3  H
2
O
(l) 
] - [ C
6
H
6(l)
  +  1½ O
2(l)
 ] 
          = [ 6 (0) +  3 (-285.8 kJ/mole)
 
] - [ (+49.0 
kJ/mole) +  1½ (0) ] 
          = -857.4 kJ/mole)
 
 - (+49.0 kJ/mole)  
          =  -906.4 kJ/mole 
The reaction is exothermic as written with a ΔH
o
rxn
 of  
-906.4kJ/mole.

10. 3e)   NH
3(g)
  +  HCl
(g)
  ------->  NH
4
Cl
(s)
 
     ΔH
o
rxn
 = ΔH
o
p
 - ΔH
o
r
                  = [ NH
4
Cl
(s) 
] - [  NH
3(g)
  +  HCl
(g)
 ] 
                  = [ -314.4 kJ/mole ] - [ (-46.1 
kJ/mole) + (-92.3 kJ/mole) ] 
                  = -314.4 kJ/mole  -  (-46.1  kJ/mole  
+  -92.3 kJ/mole)  
                  = -314.4 kJ/mole  -  (-138.4 
kJ/mole)  
                  = -176.0 kJ/mole 
The reaction is exothermic as written with 
a ΔH
o
rxn
 of -176.0 kJ/mole.