Solved Example: Thermodynamics | Chemistry for JEE Main & Advanced PDF Download

 Page 1


JEE Solved Example on Thermodynamic 
JEE Mains 
Q1: Calculate the enthalpy of formation of ????
-
ions from the following thermochemical data: 
?? ?? ?? (?? )
? ?? (???? )
+
+ ????
-
(???? ) ; ?? ?? ?????? ?? °
= +???? . ???? ????
?? ?? ( ?? )
+
?? ?? ?? ?? (?? )
? ?? ?? ?? (?? )
; ?? ?? ?????? ?? °
= -?????? . ???? ????
 
(a) -?????? . ???? ???? 
(b) -?????? . ???? ???? 
(c) +?????? . ?? ???? 
(d) -?????? . ?? ???? 
Ans: (a) 
?H
°
= 57.32 = ? ?H
f
°
 (products ) - ? ?H
f
°
 (reactants )
57.32 = ?H
f
°
(OH
-
) + 285.83? H
f
°
(OH
-
) = -?????? . ???? ????
 
Q2: Using bond energy data, calculate heat of formation of isoprene  Given bond 
energy of C–H = 98.8 kcal, H–H = 104 kcal, C–C = 83 kcal, C=C = 147 kcal and ?H of C(s) ? C(g) = 171 
kcal 
(a) ???? . ?????????? ?????? -?? 
(b) ???? . ?????????? ?????? -?? 
(c) ?????????????? ?????? -?? 
(d) ?????????????? ?????? -?? 
Ans:  
?H = 5(171) + 4(104) - [(147 × 2) + (83 × 2) + (8 × 98.8)]
 = ???? . ?????????? 
 
 
 
 
 
 
Page 2


JEE Solved Example on Thermodynamic 
JEE Mains 
Q1: Calculate the enthalpy of formation of ????
-
ions from the following thermochemical data: 
?? ?? ?? (?? )
? ?? (???? )
+
+ ????
-
(???? ) ; ?? ?? ?????? ?? °
= +???? . ???? ????
?? ?? ( ?? )
+
?? ?? ?? ?? (?? )
? ?? ?? ?? (?? )
; ?? ?? ?????? ?? °
= -?????? . ???? ????
 
(a) -?????? . ???? ???? 
(b) -?????? . ???? ???? 
(c) +?????? . ?? ???? 
(d) -?????? . ?? ???? 
Ans: (a) 
?H
°
= 57.32 = ? ?H
f
°
 (products ) - ? ?H
f
°
 (reactants )
57.32 = ?H
f
°
(OH
-
) + 285.83? H
f
°
(OH
-
) = -?????? . ???? ????
 
Q2: Using bond energy data, calculate heat of formation of isoprene  Given bond 
energy of C–H = 98.8 kcal, H–H = 104 kcal, C–C = 83 kcal, C=C = 147 kcal and ?H of C(s) ? C(g) = 171 
kcal 
(a) ???? . ?????????? ?????? -?? 
(b) ???? . ?????????? ?????? -?? 
(c) ?????????????? ?????? -?? 
(d) ?????????????? ?????? -?? 
Ans:  
?H = 5(171) + 4(104) - [(147 × 2) + (83 × 2) + (8 × 98.8)]
 = ???? . ?????????? 
 
 
 
 
 
 
 
Q3: One mole of magnesium in the vapour state absorbed 1300 ???? of energy. If the first and 
second ionization energies of ???? are 750 and ???????? ???? ?????? -?? respectively, the final composition 
of the mixture is 
(a) ???? %????
+
and ???? %????
+?? 
(b) ???? %????
+
and ???? %????
+?? 
(c) ???? %????
+
and ???? %????
+?? 
(d) ???? %????
+
and ???? %????
+?? 
Ans: (c) 
Mg ? Mg
+
+ e
-
 
To ionise one mole of Mg completely 750 kJ is spent and energy left = 550 kJ. 
1450 kJ ionises 1 mole of Mg
+1
 
550 kJ ionises 0.38 mole of Mg
+1
 
Mg
+
? Mg
+2
+ e
-
1
1 - 0.38 0.38
% of Mg
+
= ???? % and that of Mg
+2
= ???? %
 
Q4:  
 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? … . . . (i) 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? ?? ?? + ?? . . . (ii) 
?? ?? ?? ? ?? ????
?? ( ?? ); ?? ?? ?? = -?????? ?? … . . (iii) 
 
?? ?? °
 for the following reaction ???????? + ?? ?? ?? ? ???????? + ?? ????
?? would be 
(a) +???????? ?? 
(b) -???????? ?? 
(c) +?????? ?? 
(d) -?????? ?? 
Ans: (d) 
?G
o
= 2? G
ZnO 
°
+ 2? G
SO
2
°
- 2? G
ZnS 
°
= [-616- 408] - [-293] = - 
731 J. 
Q5: The enthalpies of combustion of ?? (graphite) 
 and ?? (diamond) 
 are 393.5 and -?????? . ?? ???? /?????? 
respectively. The enthalpy of conversion of ?? (graphite) 
 to ?? (diamond) 
 in ???? /?????? is 
(a) -2.9 
(b) -888.9 
(c) 788.9 
(d) 1.9 
Ans: (d) 
Page 3


JEE Solved Example on Thermodynamic 
JEE Mains 
Q1: Calculate the enthalpy of formation of ????
-
ions from the following thermochemical data: 
?? ?? ?? (?? )
? ?? (???? )
+
+ ????
-
(???? ) ; ?? ?? ?????? ?? °
= +???? . ???? ????
?? ?? ( ?? )
+
?? ?? ?? ?? (?? )
? ?? ?? ?? (?? )
; ?? ?? ?????? ?? °
= -?????? . ???? ????
 
(a) -?????? . ???? ???? 
(b) -?????? . ???? ???? 
(c) +?????? . ?? ???? 
(d) -?????? . ?? ???? 
Ans: (a) 
?H
°
= 57.32 = ? ?H
f
°
 (products ) - ? ?H
f
°
 (reactants )
57.32 = ?H
f
°
(OH
-
) + 285.83? H
f
°
(OH
-
) = -?????? . ???? ????
 
Q2: Using bond energy data, calculate heat of formation of isoprene  Given bond 
energy of C–H = 98.8 kcal, H–H = 104 kcal, C–C = 83 kcal, C=C = 147 kcal and ?H of C(s) ? C(g) = 171 
kcal 
(a) ???? . ?????????? ?????? -?? 
(b) ???? . ?????????? ?????? -?? 
(c) ?????????????? ?????? -?? 
(d) ?????????????? ?????? -?? 
Ans:  
?H = 5(171) + 4(104) - [(147 × 2) + (83 × 2) + (8 × 98.8)]
 = ???? . ?????????? 
 
 
 
 
 
 
 
Q3: One mole of magnesium in the vapour state absorbed 1300 ???? of energy. If the first and 
second ionization energies of ???? are 750 and ???????? ???? ?????? -?? respectively, the final composition 
of the mixture is 
(a) ???? %????
+
and ???? %????
+?? 
(b) ???? %????
+
and ???? %????
+?? 
(c) ???? %????
+
and ???? %????
+?? 
(d) ???? %????
+
and ???? %????
+?? 
Ans: (c) 
Mg ? Mg
+
+ e
-
 
To ionise one mole of Mg completely 750 kJ is spent and energy left = 550 kJ. 
1450 kJ ionises 1 mole of Mg
+1
 
550 kJ ionises 0.38 mole of Mg
+1
 
Mg
+
? Mg
+2
+ e
-
1
1 - 0.38 0.38
% of Mg
+
= ???? % and that of Mg
+2
= ???? %
 
Q4:  
 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? … . . . (i) 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? ?? ?? + ?? . . . (ii) 
?? ?? ?? ? ?? ????
?? ( ?? ); ?? ?? ?? = -?????? ?? … . . (iii) 
 
?? ?? °
 for the following reaction ???????? + ?? ?? ?? ? ???????? + ?? ????
?? would be 
(a) +???????? ?? 
(b) -???????? ?? 
(c) +?????? ?? 
(d) -?????? ?? 
Ans: (d) 
?G
o
= 2? G
ZnO 
°
+ 2? G
SO
2
°
- 2? G
ZnS 
°
= [-616- 408] - [-293] = - 
731 J. 
Q5: The enthalpies of combustion of ?? (graphite) 
 and ?? (diamond) 
 are 393.5 and -?????? . ?? ???? /?????? 
respectively. The enthalpy of conversion of ?? (graphite) 
 to ?? (diamond) 
 in ???? /?????? is 
(a) -2.9 
(b) -888.9 
(c) 788.9 
(d) 1.9 
Ans: (d) 
C
graphite 
+ O
2
? CO
2
; ?H = -393.5
C
diamond 
+ O
2
? CO
2
; ?H = -395.4
 ? C
graphite 
C
diamond 
= -393.5 + 395.4 = ?? . ?? 
Q6: Which one of the following statement is false? 
(a) work is a state function. 
(b) temperature is a state function. 
(c) work appears at the boundary of the system. 
(d) change in the state is completely defined when the initial and final states are specifi ed. 
Ans: (a) 
Q7: One mole of ice is converted into water at ?????? ?? . The  
?? ?? -?? respectively. The enthalpy 
change for the conversion is 
(a) ???? . ???? ?? ?????? -?? 
(b) ???????? ?? ???????? -?? 
(c) ?????? . ?? ?? ???????? -?? 
(d) ?????? . ?? ?? ???????? -?? 
Ans: (b) 
?G = ?H - T?S. At equilibrium ?G = 0, so ?H = T?S 
?H = 273 × (60.01 - 38.20) = 5954.13 J mole
-1
. 
Q8: The difference between heats of reaction at constant pressure and constant volume of the 
following reaction would be 
?? ?? ?? ?? ?? (?? ) + ???? ?? ?? ( ?? ) ? ???? ????
?? ( ?? ) + ?? ?? ?? ?? (?? )  at ????
°
?? 
in ???? ?????? -?? is 
(a) -7.43 
(b) +4.72 
(c) -5.72 
(d) +8.43 
Ans:  (a) 
?H = ?U + ?n
g
RT
 ? ?H - ?U = ?n
g
RT =
-3 × 8.314× 298
1000
= -?? . ???????? ???? ?????? -1
. 
 
Q9:  For which of the following change ?? ?? ? ?? ?? ? 
(A) ?? ?? ( ?? ) + ?? ?? ( ?? ) ? ?????? (?? ) 
(B) ?????? (???? ) + ???????? (???? ) ? ???????? (???? ) + ?? ?? ?? (?? ) 
(C) ?? (?? ) + ?? ?? ( ?? ) ? ????
?? ( ?? ) 
(D) ?? ?? ( ?? ) + ?? ?? ?? ( ?? ) ? ?? ????
?? ( ?? ) 
Ans: (D) ??? = ??? + ??? ?? ?
 
Page 4


JEE Solved Example on Thermodynamic 
JEE Mains 
Q1: Calculate the enthalpy of formation of ????
-
ions from the following thermochemical data: 
?? ?? ?? (?? )
? ?? (???? )
+
+ ????
-
(???? ) ; ?? ?? ?????? ?? °
= +???? . ???? ????
?? ?? ( ?? )
+
?? ?? ?? ?? (?? )
? ?? ?? ?? (?? )
; ?? ?? ?????? ?? °
= -?????? . ???? ????
 
(a) -?????? . ???? ???? 
(b) -?????? . ???? ???? 
(c) +?????? . ?? ???? 
(d) -?????? . ?? ???? 
Ans: (a) 
?H
°
= 57.32 = ? ?H
f
°
 (products ) - ? ?H
f
°
 (reactants )
57.32 = ?H
f
°
(OH
-
) + 285.83? H
f
°
(OH
-
) = -?????? . ???? ????
 
Q2: Using bond energy data, calculate heat of formation of isoprene  Given bond 
energy of C–H = 98.8 kcal, H–H = 104 kcal, C–C = 83 kcal, C=C = 147 kcal and ?H of C(s) ? C(g) = 171 
kcal 
(a) ???? . ?????????? ?????? -?? 
(b) ???? . ?????????? ?????? -?? 
(c) ?????????????? ?????? -?? 
(d) ?????????????? ?????? -?? 
Ans:  
?H = 5(171) + 4(104) - [(147 × 2) + (83 × 2) + (8 × 98.8)]
 = ???? . ?????????? 
 
 
 
 
 
 
 
Q3: One mole of magnesium in the vapour state absorbed 1300 ???? of energy. If the first and 
second ionization energies of ???? are 750 and ???????? ???? ?????? -?? respectively, the final composition 
of the mixture is 
(a) ???? %????
+
and ???? %????
+?? 
(b) ???? %????
+
and ???? %????
+?? 
(c) ???? %????
+
and ???? %????
+?? 
(d) ???? %????
+
and ???? %????
+?? 
Ans: (c) 
Mg ? Mg
+
+ e
-
 
To ionise one mole of Mg completely 750 kJ is spent and energy left = 550 kJ. 
1450 kJ ionises 1 mole of Mg
+1
 
550 kJ ionises 0.38 mole of Mg
+1
 
Mg
+
? Mg
+2
+ e
-
1
1 - 0.38 0.38
% of Mg
+
= ???? % and that of Mg
+2
= ???? %
 
Q4:  
 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? … . . . (i) 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? ?? ?? + ?? . . . (ii) 
?? ?? ?? ? ?? ????
?? ( ?? ); ?? ?? ?? = -?????? ?? … . . (iii) 
 
?? ?? °
 for the following reaction ???????? + ?? ?? ?? ? ???????? + ?? ????
?? would be 
(a) +???????? ?? 
(b) -???????? ?? 
(c) +?????? ?? 
(d) -?????? ?? 
Ans: (d) 
?G
o
= 2? G
ZnO 
°
+ 2? G
SO
2
°
- 2? G
ZnS 
°
= [-616- 408] - [-293] = - 
731 J. 
Q5: The enthalpies of combustion of ?? (graphite) 
 and ?? (diamond) 
 are 393.5 and -?????? . ?? ???? /?????? 
respectively. The enthalpy of conversion of ?? (graphite) 
 to ?? (diamond) 
 in ???? /?????? is 
(a) -2.9 
(b) -888.9 
(c) 788.9 
(d) 1.9 
Ans: (d) 
C
graphite 
+ O
2
? CO
2
; ?H = -393.5
C
diamond 
+ O
2
? CO
2
; ?H = -395.4
 ? C
graphite 
C
diamond 
= -393.5 + 395.4 = ?? . ?? 
Q6: Which one of the following statement is false? 
(a) work is a state function. 
(b) temperature is a state function. 
(c) work appears at the boundary of the system. 
(d) change in the state is completely defined when the initial and final states are specifi ed. 
Ans: (a) 
Q7: One mole of ice is converted into water at ?????? ?? . The  
?? ?? -?? respectively. The enthalpy 
change for the conversion is 
(a) ???? . ???? ?? ?????? -?? 
(b) ???????? ?? ???????? -?? 
(c) ?????? . ?? ?? ???????? -?? 
(d) ?????? . ?? ?? ???????? -?? 
Ans: (b) 
?G = ?H - T?S. At equilibrium ?G = 0, so ?H = T?S 
?H = 273 × (60.01 - 38.20) = 5954.13 J mole
-1
. 
Q8: The difference between heats of reaction at constant pressure and constant volume of the 
following reaction would be 
?? ?? ?? ?? ?? (?? ) + ???? ?? ?? ( ?? ) ? ???? ????
?? ( ?? ) + ?? ?? ?? ?? (?? )  at ????
°
?? 
in ???? ?????? -?? is 
(a) -7.43 
(b) +4.72 
(c) -5.72 
(d) +8.43 
Ans:  (a) 
?H = ?U + ?n
g
RT
 ? ?H - ?U = ?n
g
RT =
-3 × 8.314× 298
1000
= -?? . ???????? ???? ?????? -1
. 
 
Q9:  For which of the following change ?? ?? ? ?? ?? ? 
(A) ?? ?? ( ?? ) + ?? ?? ( ?? ) ? ?????? (?? ) 
(B) ?????? (???? ) + ???????? (???? ) ? ???????? (???? ) + ?? ?? ?? (?? ) 
(C) ?? (?? ) + ?? ?? ( ?? ) ? ????
?? ( ?? ) 
(D) ?? ?? ( ?? ) + ?? ?? ?? ( ?? ) ? ?? ????
?? ( ?? ) 
Ans: (D) ??? = ??? + ??? ?? ?
 
 Case-I = ??? 9
= 2 - 1 - 1 = 0
 Case-II = ??? 9
= 0 - 0 = 0
 Case-III = ??? 9
= 1 - 1 = 0
 Case-IV = ??? 9
= 2 - 3 - 1 = -2 ? 0
 ? In option (?)??? = ??? 
 
 
 
 
Q10:  The reactions 
????
?? ( ?? ) + ????
?? ( ?? ) ? ????
?? ???? (?? ) + ?????? (?? ).
 has ???? = -???????????? Bond  Bond Energy ???????? ?? ?? -????
? ????
?? ?????????? ? ??????
?? ?????? ? ?? ?? ???? ?????
?? ?? : ?? = ?? : ?? 
From the given data, what is the bond enthalpy of ???? - ???? bond 
(A) ???????????? 
(B) ???????????? 
(C) ???? . ???????????? 
(D) ???? . ???????????? 
Ans:  (D) CH
4
+ Cl
2
 ? ? CH
3
Cl+ HCl 
?H = -25
 -25 = 4 × C - H + 4Cl
 -Cl- 3 × CH- 1C- Cl- 1H- Cl
 
 
Q11:  If ?? ?? , ?? ?? and ?? ?? are enthalpies of ?? - ?? , ?? = ?? and ?? - ?? bonds respectively, and ?? ?? is the 
enthalpy of vaporization of water, estimate the standard enthalpy of combustion of hydrogen 
(A) ?? ?? +
?? ?? ?? - ?? ?? ?? + ?? ?? 
(B) ?? ?? +
?? ?? ?? - ?? ?? ?? - ?? ?? 
(C) ?? ?? +
?? ?? ?? - ?? ?? + ?? ?? 
(D) ?? ?? ?? - ?? ?? -
?? ?? ?? - ?? ?? 
Ans: B 
Page 5


JEE Solved Example on Thermodynamic 
JEE Mains 
Q1: Calculate the enthalpy of formation of ????
-
ions from the following thermochemical data: 
?? ?? ?? (?? )
? ?? (???? )
+
+ ????
-
(???? ) ; ?? ?? ?????? ?? °
= +???? . ???? ????
?? ?? ( ?? )
+
?? ?? ?? ?? (?? )
? ?? ?? ?? (?? )
; ?? ?? ?????? ?? °
= -?????? . ???? ????
 
(a) -?????? . ???? ???? 
(b) -?????? . ???? ???? 
(c) +?????? . ?? ???? 
(d) -?????? . ?? ???? 
Ans: (a) 
?H
°
= 57.32 = ? ?H
f
°
 (products ) - ? ?H
f
°
 (reactants )
57.32 = ?H
f
°
(OH
-
) + 285.83? H
f
°
(OH
-
) = -?????? . ???? ????
 
Q2: Using bond energy data, calculate heat of formation of isoprene  Given bond 
energy of C–H = 98.8 kcal, H–H = 104 kcal, C–C = 83 kcal, C=C = 147 kcal and ?H of C(s) ? C(g) = 171 
kcal 
(a) ???? . ?????????? ?????? -?? 
(b) ???? . ?????????? ?????? -?? 
(c) ?????????????? ?????? -?? 
(d) ?????????????? ?????? -?? 
Ans:  
?H = 5(171) + 4(104) - [(147 × 2) + (83 × 2) + (8 × 98.8)]
 = ???? . ?????????? 
 
 
 
 
 
 
 
Q3: One mole of magnesium in the vapour state absorbed 1300 ???? of energy. If the first and 
second ionization energies of ???? are 750 and ???????? ???? ?????? -?? respectively, the final composition 
of the mixture is 
(a) ???? %????
+
and ???? %????
+?? 
(b) ???? %????
+
and ???? %????
+?? 
(c) ???? %????
+
and ???? %????
+?? 
(d) ???? %????
+
and ???? %????
+?? 
Ans: (c) 
Mg ? Mg
+
+ e
-
 
To ionise one mole of Mg completely 750 kJ is spent and energy left = 550 kJ. 
1450 kJ ionises 1 mole of Mg
+1
 
550 kJ ionises 0.38 mole of Mg
+1
 
Mg
+
? Mg
+2
+ e
-
1
1 - 0.38 0.38
% of Mg
+
= ???? % and that of Mg
+2
= ???? %
 
Q4:  
 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? … . . . (i) 
?????? + ?? ?? ? ???????? ; ?? ?? ?? = -?????? ?? ?? ?? + ?? . . . (ii) 
?? ?? ?? ? ?? ????
?? ( ?? ); ?? ?? ?? = -?????? ?? … . . (iii) 
 
?? ?? °
 for the following reaction ???????? + ?? ?? ?? ? ???????? + ?? ????
?? would be 
(a) +???????? ?? 
(b) -???????? ?? 
(c) +?????? ?? 
(d) -?????? ?? 
Ans: (d) 
?G
o
= 2? G
ZnO 
°
+ 2? G
SO
2
°
- 2? G
ZnS 
°
= [-616- 408] - [-293] = - 
731 J. 
Q5: The enthalpies of combustion of ?? (graphite) 
 and ?? (diamond) 
 are 393.5 and -?????? . ?? ???? /?????? 
respectively. The enthalpy of conversion of ?? (graphite) 
 to ?? (diamond) 
 in ???? /?????? is 
(a) -2.9 
(b) -888.9 
(c) 788.9 
(d) 1.9 
Ans: (d) 
C
graphite 
+ O
2
? CO
2
; ?H = -393.5
C
diamond 
+ O
2
? CO
2
; ?H = -395.4
 ? C
graphite 
C
diamond 
= -393.5 + 395.4 = ?? . ?? 
Q6: Which one of the following statement is false? 
(a) work is a state function. 
(b) temperature is a state function. 
(c) work appears at the boundary of the system. 
(d) change in the state is completely defined when the initial and final states are specifi ed. 
Ans: (a) 
Q7: One mole of ice is converted into water at ?????? ?? . The  
?? ?? -?? respectively. The enthalpy 
change for the conversion is 
(a) ???? . ???? ?? ?????? -?? 
(b) ???????? ?? ???????? -?? 
(c) ?????? . ?? ?? ???????? -?? 
(d) ?????? . ?? ?? ???????? -?? 
Ans: (b) 
?G = ?H - T?S. At equilibrium ?G = 0, so ?H = T?S 
?H = 273 × (60.01 - 38.20) = 5954.13 J mole
-1
. 
Q8: The difference between heats of reaction at constant pressure and constant volume of the 
following reaction would be 
?? ?? ?? ?? ?? (?? ) + ???? ?? ?? ( ?? ) ? ???? ????
?? ( ?? ) + ?? ?? ?? ?? (?? )  at ????
°
?? 
in ???? ?????? -?? is 
(a) -7.43 
(b) +4.72 
(c) -5.72 
(d) +8.43 
Ans:  (a) 
?H = ?U + ?n
g
RT
 ? ?H - ?U = ?n
g
RT =
-3 × 8.314× 298
1000
= -?? . ???????? ???? ?????? -1
. 
 
Q9:  For which of the following change ?? ?? ? ?? ?? ? 
(A) ?? ?? ( ?? ) + ?? ?? ( ?? ) ? ?????? (?? ) 
(B) ?????? (???? ) + ???????? (???? ) ? ???????? (???? ) + ?? ?? ?? (?? ) 
(C) ?? (?? ) + ?? ?? ( ?? ) ? ????
?? ( ?? ) 
(D) ?? ?? ( ?? ) + ?? ?? ?? ( ?? ) ? ?? ????
?? ( ?? ) 
Ans: (D) ??? = ??? + ??? ?? ?
 
 Case-I = ??? 9
= 2 - 1 - 1 = 0
 Case-II = ??? 9
= 0 - 0 = 0
 Case-III = ??? 9
= 1 - 1 = 0
 Case-IV = ??? 9
= 2 - 3 - 1 = -2 ? 0
 ? In option (?)??? = ??? 
 
 
 
 
Q10:  The reactions 
????
?? ( ?? ) + ????
?? ( ?? ) ? ????
?? ???? (?? ) + ?????? (?? ).
 has ???? = -???????????? Bond  Bond Energy ???????? ?? ?? -????
? ????
?? ?????????? ? ??????
?? ?????? ? ?? ?? ???? ?????
?? ?? : ?? = ?? : ?? 
From the given data, what is the bond enthalpy of ???? - ???? bond 
(A) ???????????? 
(B) ???????????? 
(C) ???? . ???????????? 
(D) ???? . ???????????? 
Ans:  (D) CH
4
+ Cl
2
 ? ? CH
3
Cl+ HCl 
?H = -25
 -25 = 4 × C - H + 4Cl
 -Cl- 3 × CH- 1C- Cl- 1H- Cl
 
 
Q11:  If ?? ?? , ?? ?? and ?? ?? are enthalpies of ?? - ?? , ?? = ?? and ?? - ?? bonds respectively, and ?? ?? is the 
enthalpy of vaporization of water, estimate the standard enthalpy of combustion of hydrogen 
(A) ?? ?? +
?? ?? ?? - ?? ?? ?? + ?? ?? 
(B) ?? ?? +
?? ?? ?? - ?? ?? ?? - ?? ?? 
(C) ?? ?? +
?? ?? ?? - ?? ?? + ?? ?? 
(D) ?? ?? ?? - ?? ?? -
?? ?? ?? - ?? ?? 
Ans: B 
 
??? comb 
= ???? (?? - 11) + 1/2?? · ?? (?? = ?? - 2 × ?? · ?? (???? ) 
B.E. (vap) 
= ?? 1
+ ?? /2 - 2 × ?? 3
- ?? 4
 
 
 
Q12: For the allotropic change represented by the equation ?? (graphite) ? ?? (diamond), 
?? = ?? . ?? ???? . If ?? ?? of diamond and ?? ?? of graphite are separately burnt to yield ????
?? , the enthalpy 
liberated in first case is 
(A) Less than in the second case by ?? . ?? ???? 
(B) More than in the second case by ???? . ?? ???? 
(C) More than in the second case by ?? . ???? ???? 
(D) Less than in second case by ???? . ?? ???? 
Ans:  (C)