Answers to Hess’s Law HW
1.
2Al(s) + 3/2O2(g)  Al2O3(s) + 1675.7 kJ keep this equation as is
2Fe(s) + 3/2O2(g)  Fe2O3(s) + 824.2 kJ flip this equation
Fe2O3(s) + 824.2 kJ  2Fe(s) + 3/2O2(g)
2Al(s) + 3/2O2(g)  Al2O3(s) + 1675.7 kJ________________
Fe2O3(s) + 2Al(s) + 824.2 kJ  Al2O3(s) + 2Fe(s) + 1675.7 kJ
Fe2O3(s) + 2Al(s)  Al2O3(s) + 2Fe(s) + 851.5 kJ
2.
2C(s) + O2(g)  2CO(g) + 221.0 kJ keep this equation as is and divide everything by 2
2H2(g) + O2(g)  2H2O(g) + 483.6 kJ flip this equation and divide everything by 2
C(s) + 1/2O2(g)  CO(g) + 110.5 kJ
241.8 kJ + H2O(g)  H2(g) + 1/2O2(g)_________________
C(s) + H2O(g) + 241.8 kJ  CO(g) + H2(g) + 110.5 kJ
C(s) + H2O(g) + 131.3 kJ  CO(g) + H2(g)
3.
2C(s) + O2(g)  2CO(g) + 221.0 kJ flip this equation and divide everything by 2
C(s) + O2(g)  CO2(g) + 393.5 kJ keep this equation as is
2H2(g) + O2(g)  2H2O(g) + 483.6 kJ keep equation as is but divide everything by 2
CO(g) + 110.5 kJ  C(s) + 1/2O2(g)
C(s) + O2(g)  CO2(g) + 393.5 kJ
H2(g) + 1/2O2(g)  H2O(g) + 241.8 kJ____________________________
CO(g) + H2(g) + O2(g) + 110.5 kJ  CO2(g) + H2O(g) + 635.3 kJ
CO(g) + H2(g) + O2(g)  CO2(g) + H2O(g) + 524.8 kJ
4.
C2H2(g) + 5/2O2(g)  2CO2(g) + H2O + 1299 kJ leave as is
H2(g) + 1/2O2(g)  H2O + 286kJ leave as is but multiply everything by 2
C2H6(g) + 7/2O2(g)  2CO2(g) + 3H2O + 1560 kJ reverse equation
C2H2(g) + 5/2O2(g)  2CO2(g) + H2O + 1299 kJ
1560 kJ + 2CO2(g) + 3H2O  C2H6(g) + 7/2O2(g)
2H2(g) + O2(g)  2H2O + 572 kJ_____________
C2H2(g) + 2H2(g) + 1560 kJ  C2H6(g) + 1871 kJ
C2H2(g)
+
m = 200 g
M = 26 g/mol
n = m/M
n = 200/26
n = 7.69 mol
2H2(g) 
C2H6(g) +
311 kJ
ΔH = -311 kJ
ΔH = Q\n
Q = (-311)(7.69)
Q = -2 392.3 kJ
5.
[2H2 + O2  2H2O + 483.6 kJ] leave equation as is but multiply everything by 3/2
2C + O2  2CO + 221 kJ reverse equation and divide everything by 2
CH4 + 2O2  CO2 + 2H2O + 802.7 kJ reverse equation
C + O2  CO2 + 393.5 kJ leave as is
3H2 + 3/2O2  3H2O + 725.4 kJ
CO + 110.5 kJ  C + 1/2O2
CO2 + 2H2O + 802.7 kJ  CH4 + 2O2
C + O2  CO2 + 393.5 kJ___________________
3H2 + CO + 913.2 kJ  H2O + CH4 + 1118.9 kJ
3H2 +
CO  H2O +
CH4 +
205.7 kJ
ΔH = -205.7 kJ
ΔH = Q\n
Q = (-205.7)(10.7)
Q = -2 203.9 kJ
m = 300 g
M = 28 g/mol
n = m/M
n = 300/28
n = 10.7 mol
6a)
C5H12 +
-173.5 kJ
8O2(g) 
0 kJ
5CO2(g)
-394 kJ
+
6H2O(l)
-286 kJ
+
3CO2(g)
-394 kJ
+
6H2O(l)
-286 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [5(-394) + 6(-286)] – [(-173.5) + 8(0)]
ΔH° = -3686 – (-173.5)
ΔH° = -3512.5 kJ
b)
Fe2O3(s)
-822.2 kJ
+
3CO(g) 
-110 kJ
2Fe(s)
0 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [2(0) + 3(-394)] – [(-822.2) + 3(-110)]
ΔH° = -1182 – (-1152.2)
ΔH° = -29.8 kJ
7.
C6H12(l) +
?
9O2(g) 
0 kJ
6CO2(g)
-394 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
-3824 = [6(-394) + 6(-286)] – [(x) + 9(0)]
-3824 = -4080 – x
x = -4080 + 3824
x = -256 kJ
8a)
CH4(g) +
-74.9 kJ
H2O(g) 
-242 kJ
CO(g) +
-110 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [3(0) + (-110)] – [(-74.9) + (-242)]
ΔH° = -110 – (-316.9)
ΔH° = +206.9 kJ
3H2(g)
0 kJ
b)
CO(g) +
-110 kJ
H2O(g) 
-242 kJ
CO2(g) +
-394 kJ
H2(g)
0 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [(-394) + (0)] – [(-110) + (-242)]
ΔH° = -394 – (-352)
ΔH° = -42 kJ
c)
N2(g)
0
+

3H2(g)
0
2NH3(g)
-46 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [2(-46)] – [(0) + 3(0)]
ΔH° = -92 kJ
9a)
4NH3(g)
-46 kJ
+

5O2(g)
0
4NO(g) +
+90.4 kJ
6H2O(g)
-242 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [4(90.4) + 6(-242)] – [4(-46) + 5(0)]
ΔH° = -1090.4 – (-184)
ΔH° = -906.4 kJ
b)
2NO(g) +
+90.4 kJ
O2(g)
0 kJ

2NO2(g)
+34 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [2(34)] – [2(90.4) + (0)]
ΔH° = 68 – (180.8)
ΔH° = -112.8 kJ
c)
3NO2(g)
+34 kJ
+
H2O(l) 
-286 kJ
2HNO3(l)
-174.1 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [2(-174.1) + (90.4)] – [3(34) + (-286)]
ΔH° = -257.8 – (-184)
ΔH° = -73.8 kJ
10a)
NH3(g) +
-46 kJ
HNO3(l)
-174.1 kJ

ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [(-365.6)] – [(-46) + (-174.1)]
ΔH° = -365.6 – (-220.1)
ΔH° = -145.5 kJ
NH4NO3(s)
-365.6 kJ
+
NO(g)
+90.4 kJ
b)
NH3(g) + HNO3(l)
Ep
NH4NO3(s)
Rx Coord.
c)
NH3(g) +
HNO3(l)
11.
2 C52H16O
-396.4 kJ
+

111 O2 
0 kJ
NH4NO3(s)
+
m = 5*107 g
M = 80 g/mol
n = m/M
n = 5*107/80
n = 625 000 mol
104 CO2 +
-394 kJ
145.5 kJ
ΔH = -145.5 kJ
ΔH = Q\n
Q = (-145.5)(625000)
Q = -90 937 500 kJ
16 H2O
-242 kJ
ΔH° = ΣΔHf°products - ΣΔHf°reactants
ΔH° = [104(-394) + 16(-242)] – [2(-396.4) + 111(0)]
ΔH° = -44848 – (-792.8)
ΔH° = -44055.2 kJ
2 C52H16O
m = 100000 g
M = 656 g/mol
n = m/M
n = 100000/656
n = 152.4 mol
+
111 O2 
104 CO2 +
16 H2O +
44055.2 kJ
ΔH = -44055.2 kJ
ΔH = Q\n
Q = (-44055.2)(152.4)
Q = -6714012.48 kJ