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UPSEE Solved Model Paper 3
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© examsnet.com
Question : 67
Total: 150
Direction : The bond dissociation energy of a diatomic molecule is also called bond energy. Bond energy is also called, the heat of formation of the bond from the gaseous atoms constituting the bond with reverse sign.
Example : H (g) + Cl (g) → H - Cl (g) ,
Δ
H
f
=
−
431
k
J
mol
−
1
or bond energy of H - Cl =
−
(
Δ
H
f
)
= - (431) = + 431 kJ
mol
−
1
. When a compound shows resonance there occurs a fair agreement between the calculated values of heat of formation obtained from bond enthalpies and any other method. However deviation occur insane of compounds having alternate double bonds.
Example :
C
6
H
6
(
g
)
→
6
C
(
g
)
+
6
H
(
g
)
Resonance energy = experimental heat of formation - calculated heat of formation
Estimate the average SF bond energy in
S
F
6
. The standard heat of formation values of
S
F
6
(
g
)
,
S
(
g
)
and F (g) are -1100, 275 and 80 kJ
mol
−
1
respectively.
309.17 kJ
206 kJ
109.2 kJ
275.8 kJ
Validate
Solution:
Given,
S (s) +
3
F
2
(g) →
S
F
6
; ΔH = - 1100 kJ ... (i)
1
2
F
2
(g) → F (g) ; ΔH = 80 kJ ... (ii)
S (s) → S (g) ; ΔH = + 275 kJ ... (iii)
Subtracting eq (iii) from eq (ii) with 6 gives
S (g) +
3
F
2
β
→
6
F
6
; ΔH = - 1100 - 275 = - 1375 kJ ... (iv)
Subtracting eq (ii) with 6 gives
3
F
2
(g) → 6F (g) ; ΔH = 6 × 80 kJ = 480 kJ ... (v)
Subtracting eq (v) from eq (iv)
S (g) + 6 F (g) →
S
F
6
; ΔH = - 1375 - 480
= - 1855 kJ or
S
F
6
→ S (g) + 6F (g) ; ΔH = + 1855 kJ
Average BE =
1855
6
= 309.17 kJ
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