NAVEEN’S CREATIVE ACADEMY THERMODYNAMICS
NCERT QUESTIONS AND EXERCISES
6.1. Choose the correct answer. A thermodynamic state function is a quantity
(i) used to determine the heat changes (ii) whose value is independent of path
(iii) used to determine pressure-volume work (iv) whose value depends on temperature only.
Ans. (ii) whose value is independent of path
Q. 6.2. For the process to occur under adiabatic conditions, the correct condition is:
(i) T = 0 (ii) p = 0 (iii) q = 0 (iv) w = 0
Ans. (iii) q = 0
Q. 6.3. The enthalpies of all the elements in their standard states are:
(i) unity (ii) zero (iii) < 0 (iv) different for each element.
Ans. (ii) zero
-1
Q. 6.4. of combustion of methane is - X kJ mol . The value of is :
(i) = (ii) > (iii) < (iv) = 0
Ans. The balanced equation for combustion of methane will be
( ) ( ) ( ) ()
Thus, ( )
Thus, Hence, (iii) is the correct answer.
-1 -1
Q. 6.5. The enthalpy of combustion of methane, graphite and dihydrogen at 298 K are – 890.3 kJ mol , - 393.5 kJ mol
-1
and - 285.8 kJ mol respectively. Enthalpy of formation of CH4 (g) will be:
-1 -1 -1 -1
(i) – 74.8 kJ mol (ii) – 52.27 kJ mol (iii) + 74.8 kJ mol (iv) +52.26 kJ mol
-1
Ans. Given: (i) ( ) ( ) ( ) () kJ mol
-1
(ii) ( ) ( ) ( ) kJ mol
-1
(iii) ( ) ( ) () kJ mol
Aim: ( ) ( ) ( )
Eqn. (ii) + 2 x Eqn. (iii) – Eqn. (i) gives the required equations with
( ) ( ) = 74.8
Hence, (i) is the correct answer.
Q. 6.6. A reaction, A+B C + D + q is found to have a positive entropy change. The reaction will be
(i) possible at high temperature (ii) possible only at low temperature
(iii) not possible at any temperature (iv) possible at any temperature.
Ans. Here. H = -ve and S = +ve. G = H - T S For the reaction to be spontaneous, G should be –ve which will be so at any
temperature, i.e. option (iv) is correct.
Q.6.7. In a process, 701 J of heat is absorbed by a system and 394 J of work is done by the system. What is the change in
internal energy of the process?
Ans. q =+701 J. w =- 394 J, U = ?
By first law of thermodynamics U = q + w = 701 J + (- 394 J) = + 307 J
i.e., internal energy of the system increases by 307 J.
Q. 6.8. The reaction of cynamide, NH2CN(s), with dioxygen was carried out in a bomb calorimeter, and U was found to be
– 742.7 at 298 K. Calculate the enthalpy change of the reaction f 298 K
NH2CN(s) + O2 (g) ( ) ( ) ( ).
Ans. ( )
( )( )( )
JEE MAINS & NEET M (89505-01600, 92552-33925) 1
, NAVEEN’S CREATIVE ACADEMY THERMODYNAMICS
Q. 6.9. Calculate the number of kJ necessary raise the temperature of 60.0 g of aluminium from 35 to 55 C. Molar heat
capacity of aluminium is 24 J .
Ans. ( )( )( )
Q. 6.10. Calculate the enthalpy change on freezing 1.0 mol of water at 10.0°C to ice at – 10.0 C,
[ ( )]
[ ( )]
Ans. Total =(1 mol water at 10 C→1 mol of water at 0 C) + (1 mol water at 0 C→1 mol ice at 0 C) + (1 mol ice at 0 C→1 mol
ice at 10 C)
[ ( )] ( )] ( )
( )( ) ( ) ( )( ) ( )
Note: Directly also, as in each step, heat is evolved, each step will have a negative sign with H.
Q. 6.11. Enthalpy of combustion of carbon to . Calculate the heat released upon formation of 35.2
g of from carbon and dioxygen gas.
Ans. ( ) ( )
1 mol = 44 g
Heat released when 44 g CO2 formed = 393.5 kJ
Heat released when 35.2 g CO2 is formed =
Q. 6.12. Enthalples of formation of ( ) ( ) ( ) ( ) are-110,-393, 81 and 9.7 kJ respectively.
Find the of for the reaction:
( ) ( ) ( ) ( )
Ans. ∑ ( ) ∑ ( )
[ ( ) ( )] [ ( ) ( )]
[ ( )] [ ( )]
Q. 6.13. Given : N₂ (g) + 3 H2 (g) → 2 NH3 (g); What is the standard enthalpy of formation of NH3
gas?
Ans. Reaction for the enthalpy of formation of NH3 (g) is: ( ) ( ) ( )
Q. 6.14. Calculate the standard enthalpy of formation of CH3OH( ) from the following data:
(i) () ( ) ()
(ii) ( ) ( ) ( )
(iii) ( ) ( ) ()
Ans. Aim: C(s) + 2 H2 (g) + ( ) ()
-1 -1
Eqn. (ii) + 2 x Eqn. (iii) - Eqn. (i) gives the required eqn. with ΔΗ = - 393 + 2 (- 286) - (- 726) kJ mol = - 239 kJ mol
Q. 6.15. Calculate the enthalpy change for the process
CCl4 (g) → C (g) + 4 Cl (g) and calculate bond enthalpy of C - Cl in CCl4 (g)
Given: ( ) ( )
( ) where is enthalpy of atomisation.
( )
Ans. The given data imply as under : ( ) () ( )
( ) ( ) ( ) ()
( ) ( ) ( )
( ) ( ) ( )
Eqn. (iii) + 2 x Eqn. (iv) - Eqn. (i) - Eqn. (ii) gives the required equation with
-1
Bond enthalpy of C-Cl in CCI, (average value) = 326 kJ mol
Q. 6.16. For an isolated system, = 0, what will be S?
JEE MAINS & NEET M (89505-01600, 92552-33925) 2
NCERT QUESTIONS AND EXERCISES
6.1. Choose the correct answer. A thermodynamic state function is a quantity
(i) used to determine the heat changes (ii) whose value is independent of path
(iii) used to determine pressure-volume work (iv) whose value depends on temperature only.
Ans. (ii) whose value is independent of path
Q. 6.2. For the process to occur under adiabatic conditions, the correct condition is:
(i) T = 0 (ii) p = 0 (iii) q = 0 (iv) w = 0
Ans. (iii) q = 0
Q. 6.3. The enthalpies of all the elements in their standard states are:
(i) unity (ii) zero (iii) < 0 (iv) different for each element.
Ans. (ii) zero
-1
Q. 6.4. of combustion of methane is - X kJ mol . The value of is :
(i) = (ii) > (iii) < (iv) = 0
Ans. The balanced equation for combustion of methane will be
( ) ( ) ( ) ()
Thus, ( )
Thus, Hence, (iii) is the correct answer.
-1 -1
Q. 6.5. The enthalpy of combustion of methane, graphite and dihydrogen at 298 K are – 890.3 kJ mol , - 393.5 kJ mol
-1
and - 285.8 kJ mol respectively. Enthalpy of formation of CH4 (g) will be:
-1 -1 -1 -1
(i) – 74.8 kJ mol (ii) – 52.27 kJ mol (iii) + 74.8 kJ mol (iv) +52.26 kJ mol
-1
Ans. Given: (i) ( ) ( ) ( ) () kJ mol
-1
(ii) ( ) ( ) ( ) kJ mol
-1
(iii) ( ) ( ) () kJ mol
Aim: ( ) ( ) ( )
Eqn. (ii) + 2 x Eqn. (iii) – Eqn. (i) gives the required equations with
( ) ( ) = 74.8
Hence, (i) is the correct answer.
Q. 6.6. A reaction, A+B C + D + q is found to have a positive entropy change. The reaction will be
(i) possible at high temperature (ii) possible only at low temperature
(iii) not possible at any temperature (iv) possible at any temperature.
Ans. Here. H = -ve and S = +ve. G = H - T S For the reaction to be spontaneous, G should be –ve which will be so at any
temperature, i.e. option (iv) is correct.
Q.6.7. In a process, 701 J of heat is absorbed by a system and 394 J of work is done by the system. What is the change in
internal energy of the process?
Ans. q =+701 J. w =- 394 J, U = ?
By first law of thermodynamics U = q + w = 701 J + (- 394 J) = + 307 J
i.e., internal energy of the system increases by 307 J.
Q. 6.8. The reaction of cynamide, NH2CN(s), with dioxygen was carried out in a bomb calorimeter, and U was found to be
– 742.7 at 298 K. Calculate the enthalpy change of the reaction f 298 K
NH2CN(s) + O2 (g) ( ) ( ) ( ).
Ans. ( )
( )( )( )
JEE MAINS & NEET M (89505-01600, 92552-33925) 1
, NAVEEN’S CREATIVE ACADEMY THERMODYNAMICS
Q. 6.9. Calculate the number of kJ necessary raise the temperature of 60.0 g of aluminium from 35 to 55 C. Molar heat
capacity of aluminium is 24 J .
Ans. ( )( )( )
Q. 6.10. Calculate the enthalpy change on freezing 1.0 mol of water at 10.0°C to ice at – 10.0 C,
[ ( )]
[ ( )]
Ans. Total =(1 mol water at 10 C→1 mol of water at 0 C) + (1 mol water at 0 C→1 mol ice at 0 C) + (1 mol ice at 0 C→1 mol
ice at 10 C)
[ ( )] ( )] ( )
( )( ) ( ) ( )( ) ( )
Note: Directly also, as in each step, heat is evolved, each step will have a negative sign with H.
Q. 6.11. Enthalpy of combustion of carbon to . Calculate the heat released upon formation of 35.2
g of from carbon and dioxygen gas.
Ans. ( ) ( )
1 mol = 44 g
Heat released when 44 g CO2 formed = 393.5 kJ
Heat released when 35.2 g CO2 is formed =
Q. 6.12. Enthalples of formation of ( ) ( ) ( ) ( ) are-110,-393, 81 and 9.7 kJ respectively.
Find the of for the reaction:
( ) ( ) ( ) ( )
Ans. ∑ ( ) ∑ ( )
[ ( ) ( )] [ ( ) ( )]
[ ( )] [ ( )]
Q. 6.13. Given : N₂ (g) + 3 H2 (g) → 2 NH3 (g); What is the standard enthalpy of formation of NH3
gas?
Ans. Reaction for the enthalpy of formation of NH3 (g) is: ( ) ( ) ( )
Q. 6.14. Calculate the standard enthalpy of formation of CH3OH( ) from the following data:
(i) () ( ) ()
(ii) ( ) ( ) ( )
(iii) ( ) ( ) ()
Ans. Aim: C(s) + 2 H2 (g) + ( ) ()
-1 -1
Eqn. (ii) + 2 x Eqn. (iii) - Eqn. (i) gives the required eqn. with ΔΗ = - 393 + 2 (- 286) - (- 726) kJ mol = - 239 kJ mol
Q. 6.15. Calculate the enthalpy change for the process
CCl4 (g) → C (g) + 4 Cl (g) and calculate bond enthalpy of C - Cl in CCl4 (g)
Given: ( ) ( )
( ) where is enthalpy of atomisation.
( )
Ans. The given data imply as under : ( ) () ( )
( ) ( ) ( ) ()
( ) ( ) ( )
( ) ( ) ( )
Eqn. (iii) + 2 x Eqn. (iv) - Eqn. (i) - Eqn. (ii) gives the required equation with
-1
Bond enthalpy of C-Cl in CCI, (average value) = 326 kJ mol
Q. 6.16. For an isolated system, = 0, what will be S?
JEE MAINS & NEET M (89505-01600, 92552-33925) 2
