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?  

249_exothermic reaction.JPG

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?  

356_equations.JPG   

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) ΔSsys = ΔSsurr                          (B) ΔSsys = ΔSsurr  

      (C) |ΔSsys| > |ΔSsurr|                       (D) |ΔSsys| < |ΔSsurr|  

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

                 ΔHs

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

                     ΔHf                 ΔHv

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

      By (1) and (2), ΔHs  ——> ΔHs + ΔHv  

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 Pext = 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

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