Energy enthalpy and thermochemistry e sicl 4 l 2 h 2

ENERGY, ENTHALPY, AND THERMOCHEMISTRY
e. SiCl
4
(l) + 2 H
2
O(l)  SiO
2
(s) + 4 HCl(aq)
Because HCl(aq) is H
+
(aq) + Cl

(aq),
o
f
H Δ = 0 - 167 = -167 kJ/mol.
Δ H° =











 
 




 
mol
kJ 911
mol 1
mol
kJ 167
mol 4












 
 




 
mol
kJ 286
mol 2
mol
kJ 687
mol 1
Δ H° =  1579 kJ  (  1259 kJ) =  320. kJ
f. MgO(s) + H
2
O(l)  Mg(OH)
2
(s)
Δ H° =











 
mol
kJ 925
mol 1 











 






 
mol
kJ 286
mol 1
mol
kJ 602
mol 1
Δ H° =  925 kJ  (  888 kJ) =  37 kJ
68. a. 4 NH
3
(g) + 5 O
2
(g)  4 NO(g) + 6 H
2
O(g); Δ H° =

o
products , f p
H Δ n 

o
reactants f, r
Δ H n
Δ H° =











 












 







mol
kJ 46
mol 4
mol
kJ 242
mol 6
mol
kJ . 90
mol 4 =  908 kJ
2 NO(g) + O
2
(g)  2 NO
2
(g)
Δ H° =

























mol
kJ . 90
mol 2
mol
kJ 34
mol 2 =  112 kJ
3 NO
2
(g) + H
2
O(l)  2 HNO
3
(aq) + NO(g)
Δ H° =


















 
mol
kJ . 90
mol 1
mol
kJ 207
mol 2











 








mol
kJ 286
mol 1
mol
kJ 34
mol 3 =  140. kJ
b. 12 NH
3
(g) + 15 O
2
(g)  12 NO(g) + 18 H
2
O(g)
12 NO(g) + 6 O
2
(g)  12 NO
2
(g)
12 NO
2
(g) + 4 H
2
O(l)  8 HNO
3
(aq) + 4 NO(g)
4 H
2
O(g)  4 H
2
O(l)
_________________________________________________
12 NH
3
(g) + 21 O
2
(g)  8 HNO
3
(aq) + 4 NO(g) + 14 H
2
O(g)
The overall reaction must be exothermic because each step is exothermic.

CHAPTER 9 ENERGY, ENTHALPY, AND THERMOCHEMISTRY 381
69. 4 Na(s) + O
2
(g)  2 Na
2
O(s), Δ H° = 2 mol





 
mol
kJ 416
=  832 kJ
2 Na(s) + 2 H
2
O(l)  2 NaOH(aq) + H
2
(g)
Δ H° =











 












 
mol
kJ 286
mol 2
mol
kJ . 470
mol 2 =  368 kJ
2Na(s) + CO
2
(g)  Na
2
O(s) + CO(g)
Δ H° =











 












 






 
mol
kJ 5 . 393
mol 1
mol
kJ 5 . 110
mol 1
mol
kJ 416
mol 1 =  133 kJ
In reactions 2 and 3, sodium metal reacts with the "extinguishing agent." Both reactions are
exothermic and each reaction produces a flammable gas, H
2
and CO, respectively.
70. 3 Al(s) + 3 NH
4
ClO
4
(s)  Al
2
O
3
(s) + AlCl
3
(s) + 3 NO(g) + 6 H
2
O(g)
Δ H° =











 
 




 
 





 




 
mol
kJ 1676
mol 1
mol
kJ 704
mol 1
mol
kJ . 90
mol 3
mol
kJ 242
mol 6












 
mol
kJ 295
mol 3 =  2677 kJ
71. 5 N
2
O
4
(l) + 4 N
2
H
3
CH
3
(l)  12 H
2
O(g) + 9 N
2
(g) + 4 CO
2
(g)
Δ H° =











 






 
mol
kJ 5 . 393
mol 4
mol
kJ 242
mol 12














