Level 2 Objective Problems of Thermodynamics

Level – 2

1. Evaporation of water is a spontaneous process although it  

      (A) Is an exothermic reaction                            

      (B) is an endothermic reaction  

      (C) Is a photo chemical reaction                       

      (D) proceed without heat loss or heat gain    

2. For the two reactions given below  

      H₂(g) + 1/2 O₂(g) ——> H₂O(g) + X₁KJ   

      H₂(g) + 1/2 O₂(g) ——> H₂O(l) + X₂KJ  

Select the correct answer  

      (A) X₁ > X₂                                                           (B) X₁ < X₂  

      (C) X₁ = X₂                                                           (D) X₁ + X₂ = 0  

3. Which of the following is wrong?  

      (A) change in internal energy of an ideal gas on isothermal expansion is zero  

      (B) in a cyclic process w ≠ Q  

      (C) for an ideal gas [∂H/∂P]_T = 0  

      (D) all   

4. The heats of neutralization of four acids a, b c and d when neutralized against a common base are 13.7, 9.4, 11.2 and 12.4 Kcal respectively. The weakest among these acids is  

      (A) c                                                                      (B) b  

      (C) a                                                                      (D) d    

5. The bond energies of C = C, C —— H, H —— H and C = C are 198, 98, 103, 145 Kcal respectively. The enthalpy change of the reaction  

      CH = CH + H₂ ———> CH₂ = CH₂ is  

      (A) -152 Kcal                                                      (B) 96 Kcal  

      (C) 48 Kcal                                                           (D) -40 Kcal  

6.   Which plot represents for an exothermic reaction?  

7.  The molar heat capacity of water in equilibrium with ice at constant pressure is  

      (A) negative                                                         (B) zero  

      (C) infinity                                                            (D) 40.45 KJK^-1mol^-1  

8. Enthalpy of CH₄ + 1/2 O₂ ——> CH₃OH is negative. If enthalpy of combustion of CH₄ and CH₃OH are x and y respectively. Then which relation is correct?  

      (A) x > y                                                                (B) x < y  

      (C) x = y                                                                (D) x < y  

9. When 10 ml of a strong acid are added to 10 ml of an alkali, the temperature rises 5°C. If 100 ml of each liquids are mixed, the temperature rise would be  

      (A) 0.5°C                                                              (B) 10°C  

      (C) 7.5°C                                                              (D) same  

10. X is a metal that forms an oxide X₂O  

      1/2 X₂O ——> X + 1/4 O₂, ΔH = 120 Kcal  

When a sample of metal X reacts with one mole of oxygen, what will be the DH in that case?  

      (A) 480 kcal                                                         (B) -240 kcal  

      (C) -480 kcal                                                       (D) 240 kcal      

11. AB, A₂ and B₂ are diatomic molecules. If the bond enthalpies of A₂, AB & B₂ are in the ratio 1:1:0.5 and enthalpy of formation of AB from A₂ and B₂ – 100 kJ/mol^–1. What is the bond enthalpy of A₂?  

      (A) 400 kJ/mol                                                     (B) 200 kJ/mol  

      (C) 100 kJ/mol                                                     (D) 300 kJ/mol  

12. Which of the following corresponds to the definition of enthalpy of formation at 

298 K?  

      (A) C(graphite) + 2H₂(g) + 1/2 O₂(l) CH₃OH(g)  

      (B) C(diamond) + 2H₂(g) + 1/2 O₂(g) CH₃OH (l)  

      (C) 2C(graphite) + 4H₂(g) + O₂(g) 2CH₃OH (l)  

      (D) C(graphite) + 2H₂(g) + 1/2 O₂(g) CH₃OH(l)  

13. The heat of neutralisation of HCl by NaOH is –12.1kJ/mole, the energy of dissociation of HCl is     

      (A) –43.8 kJ                                                         (B) 43.8 kJ      

      (C) 68 kJ                                                               (D) –68 kJ  

14. The dissociation energy of CH₄ and C₂H₆ are respectively 360 & 620 k. cal/mole. The bond energy of C–C is  

      (A) 260 kcal/mole                                               (B) 180 kcal/mole  

      (C) 130 kcal/mole                                               (D) 80 kcal/mole  

15. Identify the intensive property from the following:  

      (A) Enthalpy                                                        (B) Temperature  

      (C) Volume                                                          (D) Refractive index  

16. Which of the following expression is not correct?  

17. For a reaction A(g) ——> B(g) at equilibrium, the partial pressure of B is found to be one fourth of the partial pressure of A. The value of ΔG⁰ of the reaction A —> B is  

      (A) RT ln4                                                            (B) -RT ln4  

      (C) RT log4                                                          (D) -RT log4  

18. For an irreversible isothermal expansion of an ideal gas  

      (A) ΔS_sys = ΔS_surr                          (B) ΔS_sys = ΔS_surr  

      (C) |ΔS_sys| > |ΔS_surr|                       (D) |ΔS_sys| < |ΔS_surr|  

19. H₂(g) ——> 2H(g)  

      (A) H atom has higher entropy                        (B) hydrogen molecule has entropy  

      (C) both have same entropy                             (D) none

20. An endothermic reaction is spontaneous only if  

      (A) the entropy of the surrounding increases     (B) entropy of the system increases  

      (C) total entropy decreases                               (D) none

Level – II  

            1.         B                       2.         A                       3.         B          

            4.         B                       5.         D                       6.         A          

            7.         C                       8.         B                       9.         D          

            10.       C                       11.       A                       12.       d          

            13.       C                       14.       D                       15.       B & D  

            16.       C                       17.       A                       18.       C  

            19.       A                       20.       B  

ANSWERS TO ASSIGNMENT PROBLEMS  

Subjective:  

Level - O  

1. Since sublimation involves the process

                 ΔH_s

      Solid ———> Vapour ………………………….. (1)

                     ΔH_f                 ΔH_v

      or Solid ———> Liquid ———> Vapour  ...... (2)  

      By (1) and (2), ΔH_s  ——> ΔH_s + ΔH_v  

2. At all temperature conditions,  

      ΔH < 0 and ΔS > 0, so that ΔHG < 0.  

3. It is so because only H⁺ and OH^– react in every case and it is the enthalpy of formation of water.  

4.   (i)   When number of molecules of products are more than number of molecules in the reactants.  

      (ii) When solid state changes to liquid state.  

      (iii) When liquid state changes to gaseous state.  

      (iv) When solid changes to gaseous state.  

5. Solute molecules also set free to move in solution and thus disorder increases. 

6. The system: ice water at m. pt., is in equilibrium and thus ΔG = 0.  

7. ΔG = ΔH – TΔS; since the reaction proceeds to completion that is irreversible or spontaneous and thus ΔG = –ve. Therefore, since ΔH = +ve... ΔH – TΔS = –ve orΔS. should be +ve.  

8. The standard state chosen for carbon is graphite and not diamond.  

9. No, because it is the sum of different types of energies and some of which can not be determined?  

10. Heat evolved will be different. This is because they have different crystal structure.  

11. Because all chemical reactions are accompanied by bond rearrangements. The total bond energy of reactants is not equal to the total bond energy of products hence heat is either evolved or absorbed.

12. Sublimation is a process which is assumed to take place in two step:  

First a solid converts into liquid and second a liquid converts into vapours hence the sum of enthalpy of fusion and enthalpy of vapourisation is equal to enthalpy of sublimation.  

13. In an ideal gas, there are no intermolecular forces of attraction. Hence no energy is required to overcome these forces. Moreover, when a gas expands against vacuum, work done is zero (because P_ext = 0). Hence internal energy of the system does not change i.e. there is no absorption or evolution of heat.  

14.-212.76 kcal                                                        

15.-29.00 kcal  

16.Chemical energy  

18. Zero  

19. 2.4 × 10²  

20. At equilibrium products are much more in abundance than the reactants (k > > 1).  

Objective:  

Level – I  

            1.         D                       2.         C                        3.         A          

            4.         D                       5.         A                        6.         D          

            7.         B                       8.         C                        9.         B          

            10.       C                       11.       d                        12.       D          

            13.       C                       14.       D                        15.       d  

            16.       D                       17.       D                        18.       D  

            19.       B                       20.       C                         21.       B  

            22.       D                      23.       B                          24.       C          

            25.       A  

Level – II  

            1.         B                       2.         A                       3.         B          

            4.         B                       5.         D                       6.         A          

            7.         C                       8.         B                       9.         D          

            10.       C                       11.       A                       12.       d          

            13.       C                       14.       D                       15.       B & D  

            16.       C                       17.       A                       18.       C  

            19.       A                       20.       B

To read more, Buy study materials of Thermodynamics comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Chemistry here.