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Thermochemistry: enthalpies of formation, combustion, bond enthalpy, Hess's law
NEET level
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Difficulty 1 of 3
td077
For which of the following gas-phase reactions (ideal gases) is
Δ
H
=
Δ
U
?
A
PCl
X
5
(
g
)
PCl
X
3
(
g
)
+
Cl
X
2
(
g
)
B
N
X
2
(
g
)
+
O
X
2
(
g
)
2
NO
(
g
)
C
2
SO
X
2
(
g
)
+
O
X
2
(
g
)
2
SO
X
3
(
g
)
D
N
X
2
(
g
)
+
3
H
X
2
(
g
)
2
NH
X
3
(
g
)
Answer: B
Δ
H
=
Δ
U
+
Δ
n
g
RT
. Only the first reaction has
Δ
n
g
=
2
−
2
=
0
. The others have
Δ
n
g
=
−
2
,
−
1
and
+
1
respectively.
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