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National Open University Exam Past Question CHM 301 – PHYSICAL CHEMISTRY III (2014_2)

CHM 301 – PHYSICAL CHEMISTRY III (3 units)                

Answer any five questions

 

  1. (a) (i) What is a state property? (2 marks)

(ii) Give four examples. (½ mark each = 2)

(b) State the third law of thermodynamics (2 marks)

(c) Describe briefly the types of system that exist in chemical thermodynamics. (6 marks) (d) A gas expands from 10 m3 to 12 m3 against a constant pressure of 1 bar (1 x105 N m-2) at 298 K. What is the work done on the gas? (3 marks)

 

  1. (a) Describe briefly what you understand by the term “Absolute zero (temperature)” (4

marks)

(b) Answer these questions: (1 mark each = 5 marks)

(i) The total energy of the universe is constant. This is a statement of the                        

law of thermodynamics

(ii) The change in entropy for any process is not dependent upon the pathway by which the  process  occurs.  In  order  words,  the  change  in  entropy  for  any  process  is  a

                           function

(iii) Refractive index is an intensive property. True or false

(iv) The energy required to cook your meals is an extensive property. True or false

(v) In a closed system, heat neither leaves nor enters. True or false

 

(c) Initially, a mixture of 0.100 M NO, 0.050 M H2, 0.100 M H2O was allowed to reach equilibrium (initially there was no N2).  At equilibrium the concentration of NO was found to be 0.062 M.  Determine the value of the equilibrium constant, Kc , for the reaction:  (5 marks)

 

  1. (a) Given the following information: (1½ each = 3 marks) A + B ®   C  + D      DH  = 80 kJ    (Eq 1) C  + D   ®   E               DH  = -12 kJ    (Eq 2)
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DH DS Temperature Reaction
positive Any temperature Spontaneous
positive negative Any temperature

 

Calculate DH for the reactions:   (i) A + B  ®  E     (ii) A + B + E  ®  2C + 2D (b) Copy and complete the following table: (6 marks)

 

positive positive Spontaneous
positive positive Non-spontaneous
negative negative Low temperature
negative negative High temperature

 

(c) Calculate the standard free energy of formation of HI(g) at 25 oC from its standard entropy and its standard enthalpy of formation, given the following information:

DHo  (HI, g) = 26.48 kJ;  DHo  (H , g) = 0;  DHo  (I , g) = 0.

f                                                          f       2                                f    2

So (HI, g) = 206.6 J K-1;  So (H2, g) = 130.7 J K-1;  So (I2, g) = 116.1 J K-1. (5 marks)

 

  1. (a) (i) What are colligative properties? (3 marks)

(ii) Give two examples of colligative properties  (2 marks)

(b) What is the total volume of the solution, when 3.80 mol of water is mixed with 0.500 mol of ethanol? The partial molar volumes of water and ethanol at this composition are

1.80 x105 m3 mol-1 and 5.34 × 10-5 m3 mol-1, respectively. (4 marks)

(c) Calculate the molecular mass of 9.21g non-volatile organic compound, dissolved in

50g of pure water at 25oC, which depresses the vapour pressure of the water from 3.16 x

103 to 3.10 x 103 N m-2. (5 marks)

 

  1. (a) State the first law of thermodynamics (3 marks)

(b) Calculate the change in the entropy of 50 g of water when it freezes at 0 oC in a refrigerator ice tray. So (ice) = 43.2 J K-1 mol-1and So (water) = 65.2 J K-1 mol-1.  (4 marks) (c) The enthalpy of vaporization of a substance is 72 J mol-1 and its boiling point is -73 oC.

Calculate the entropy of vaporization. (3 marks)

(d) Predict the sign of the entropy change for these processes: (1 mark each = 4 marks)

(i) SF4(g) =  F2(g)  ®  SF6(g)                               (ii) H2S(g)  +  NH3(g) ®  NH4HS(s) (iii) O2(g)  + (at 400 K)  ®  O2(g) (at 800 K)        (iv) CH3OH(l) ®  CH3OH(g)

 

  1. (a) Define the term ‘standard enthalpy of reaction’. (2 marks)

(b) The combustion of propane is described by the equation (3 marks) C3H8(g) + 5O2(g)   ®  3CO2(g) + 4H2O(l)        DH  = -2220 kJ.

Calculate the mass of propane that must be burned to obtain 350 kJ of heat.

(c) Calculate DH for the reaction:   N2     +   3H2     ®   2NH3; given the bond dissociation energies: N-N = 163 kJ mol-1; H-H = 436 kJ mol-1; N-H = 391 kJ mol-1 (5 marks)

(d) True or False? (1 mark each = 4 mark)

  1. i) water flows down the hill spontaneously
  2. ii) a gas cannot expand spontaneously into vacuum

 

  1. (a) Distinguish between and ‘extensive variable’ and an ‘intensive variable’. (4 marks)

(b) Write short notes on each of the followings: (1½ marks each = 7½ marks) (i)         Isothermal process

(ii)    Adiabatic process (iii)   Isobaric process (iv)    Isochoric process (v)     Cyclic process

 

(c) Calculate the work that must be done at S.T.P to make room for the products of octane combustion. G.M.V at S.T.P = 22.4 dm3mol-1. Standard pressure = 1 x105 Nm-2. (3 marks)

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