Enthalpy Worksheet With Answers

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Chapter 6 - Lecture Worksheet 3
Using standard heats of formation, calculate the standard enthalpy change for the following reaction.
CO
(g) + 4H
(g) -----> CH
(g) + 2H
O(g)
2
2
4
2
0
0
0
0
0
(CH
,g) + 2"H
(H
O,g) # "H
(CO
,g) # 4"H
(H
,g)
"H
= "H
rxn
f
f
f
f
4
2
2
2
= –75 + 2(–242) – (–394) – 4(0) kJ = – 165 kJ
!
(1) -711 kJ
(2) -165 kJ
(3) -15 kJ
(4) 15 kJ
(5) 165 kJ
(6) 229 kJ
(7) 711 kJ
Using average bond enthalpies estimate the standard enthalpy change for the same reaction.
CO
(g) + 4H
(g) -----> CH
(g) + 2H
O(g)
2
2
4
2
0
# 2BE(C = O) + 4BE(H $ H) $ 4BE(C $ H) $ 4BE(O $ H)
"H
rxn
≈ 2(728) + 4(436) – 4(413) – 4(463) kJ = – 304 kJ
!
Which is more accurate ? Why ?
ΔH° calculated from ΔH°
is more accurate because ΔH° calculated from bond enthalpies uses
f
bond enthalpies obtained from a number of different compounds in which the bonds arise and
AVERAGE
are not specific to the compound in question.
StandardEnthalpy of formation (kJ/mol)
Average Bond Enthalpies (kJ/mol)
CH
(g)
–75
H–H
436
4
CO
(g)
–394
N–H
391
2
C
H
(g)
+227
N–N
161
2
2
H
O(g)
–242
N=N
418
2
H
O(l)
–286
N≡N
945
2
NH
(g)
–46
O-H
463
3
HCl(g)
–92
C-O
351
HCl(aq)
-167
C=O
728
CaCO
(s)
-1208
C≡O
1072
3
Ca(OH)
(aq)
-1003
Cl-Cl
242
2
Ca(OH)
(s)
-986
Cl-H
432
2
CaO(s)
-635
Br-Br
193
SO
(g)
-297
Br-H
366
2
SO
(g)
-396
C-H
413
3

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