CY 102: Physical Chemistry End Semester May 3, 2004

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1 CY 102: Physical Chemistry End Semester May 3, 2004 Answer All Questions R = (8.314 J = atm dm 3 ) K -1 mol -1 Trouton s constant = 88 J K -1 mol -1 F = 96,500 Cmol -1. Question 1: Fill in the blanks (a) For free expansion of a gas, S surr = Zero and S system = Positive (Choose from: positive, negative, zero) (b) The enthalpy of vaporization of water is kj mol -1. Its entropy of vaporization is 500 J mol -1 K -1. (c) For a one-component system the molar volume of the liquid phase is larger than the molar volume of the solid phase. Its pressure versus Temperature phase diagram is P solid liquid vapor T (d) For the reversible compression of an isolated ideal gas at 298K to a volume half of its initial volume, the change in molar entropy (in joule units) is Zero. (e) Substance A and B form an eutectic mixture at C with a molar composition A:B = 3:2. The schematic cooling curve of an intimate mixture of 5 moles of A and 2 moles of B is Temperature Cooling of A + B liquid mixture Separation of A Separation of Eutectic Cooling of solid Time

2 (f) For a third order reaction 3A products, the time taken for 75% change in the concentration of A is 5 times that for 50% change. (g) The rate law, d[d] /dt for the mechanism A B; B + C D, using SSA to the intermediate, is d[d]/dt = k 1 k 2 [A][C] / k -1 + k 2 [C]. (h) The emf of the cell SCE // H + (aq)/q, H 2 Q / Pt is 0.16V at 298K. Given E SCE = 0.242V and E 0 (H 2 Q /Q) = 0.699V, the ph of the aqueous solution is 5. (i) Consider the cell reaction Cu(s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag(s) at 298K. If the concentration for Cu 2+ is 1.0M and the emf of the cell is zero, the concentration of Ag + is 1.65 x 10-8 M. (Given E 0 (Cu 2+ /Cu) = 0.34V and E 0 (Ag + /Ag = 0.80V) (j) Reaction A + B P is first order with respect to each reactant. Given [A] 0 = 0.10M, [B] 0 = 6.0M and k = 5.0 x 10-3 M -1 s -1, the concentration of a remaining at t = 100s is 4.98 x 10-3 M. Question 2: (a) Ammonium hydrosulphide decomposes according to the reaction, NH 4 HS(s) NH 3 (g) + H 2 S(g) G 0 for this reaction at 300K is 6198 J mol -1. Some solid NH 4 HS is placed in an evacuated vessel at 300K. What is the pressure of NH 3 (g) at equilibrium? Ans: G 0 = -RT ln K p ; K p = e - G0/RT = e -(6198/(8..314x300)) = K p = P NH3. P H2s = p 2 = P NH3 = = atm. (b) The boiling point of a normal liquid is C at 1 atm. Estimate the vapour pressure of the liquid (in atm unit) at 27 0 C and 1 atm total pressure. Ans: H 0 ν = Troutons constant x Boiling temperature = 88 ( ) = Jmol -1 ln P 2 /P 1 = - H 0 ν/r (1/T 2 1/T 1 ) (Clausius Clayperon) ln P 2 = /8.314 (1/300 1/400) = P 2 = atm. (c) A reversible heat engine absorbs 40 kj of heat at 500K, performs 10 kj of work and rejects the remaining heat to the sink at 300 K. Calculate the entropy change of the universe. Ans: S (source) = /500 = -80 JK -1 S (sink) = 30000/300 = 100 JK -1 S(univ) = ( ) JK -1 = 20 JK -1.

3 (d) Using the data given, calculate the standard Gibbs free energy of formation of urea, CO(NH 2 ) 2. CO 2 (g) + 2NH 3 (g) H 2 O(g) + CO(NH 2 ) 2 G 0 1 = J 2H 2 O(g) 2H 2 (g) + O 2 (g) G 0 2 = J C(graphite) + O 2 (g) CO 2 (g) G 0 3 = J N 2 (g) + 3H 2 (g) 2NH 3 (g) G 0 4 = 32.4 J Ans: Equation for formation of urea: C (graphite) + ½ O 2 (g) + N 2 (g) + 2H 2 (g) = CO(NH 2 ) 2 (s) Accordingly from the above equations: G 0 (formation) = G G 0 2/2 + G G 0 4 = Jmol -1. (e) State True (T) or False (F) in the space provided: (i) The efficiency of Carnot heat engine depends on the heat capacity of the working substance. [F] (ii) For an ideal gas at constant pressure, the graph of H versus T and G versus T will always have opposite slopes [T) (iii) For a substance with different allotropic forms at constant pressure, an increase in temperature favours the stability of the allotrope with higher entropy [T]. (iv) ( U/ V) s = P [F] Question 3: (a) Estimate the solubility product of PbSO 4 at 298 K. Given E 0 (Pb 2+ /Pb) = V and E 0 of the half-cell SO 4 2- (aq) / PbSO 4 (s) / Pb(s) is 0.356V. Ans: PbSO 4 (s) Pb 2+ + SO 4 2- E 0 cell = (-0.125) = V n = 2 log 10 K sp = ne 0 cell/ = 2(-0.231)/ = K sp = 1.52 x 10-8 (b) For the cell reaction Zn + Cu 2+ (1M) Zn 2+ (1M) + Cu, H 0 = kj mol -1 at 298K. Given E 0 (Zn 2+ /Zn) = V and E 0 (Cu 2+ /Cu) = 0.34V, calculate the temperature coefficient, ( E 0 / T) p of the cell. Ans: G 0 = -nfe 0 = -2 x x 1.10 = KJ mol -1. H 0 = kj mol -1 (Given) S 0 = H 0 - G 0 /T = JK - mol -1 S 0 = nf ( E 0 / T) p ( E/ T) p = x 10-4 VK -1.

4 (c) The activation energy for the decomposition of HL is 184 KJ mol -1. By what factor the number of effective collisions increases if the temperature is increased from 500K to 700K? Ans: log e k 2 /k 1 = Ea/R [T 2 T 1 /T 1 T 2 ] = 184 x 10 3 /8.314 (200 / 500 x 700) = 12.6 k 2 /k 1 = 3.1 x (d) An enzyme has a turnover number of 1 x 10 4 min -1. If 6.67 (or 20/3) moles of substrate is turned-over per hour per gram of the enzyme when the substrate concentration is twice the Michaelis constant, calculate the molar mass of the enzyme. Ans: # = k 2 [E] 0 [S] 0 /k m + [S] 0 = 2/3 k 2 [E] 0 k m = ½ [S] 0 20/3 x 1/60 = 2/3 x 1 x 10 4 x 1/M M = 6 x mol -1. (e) For a parallel first order reaction B A C, derive the expression for the concentration of B at any time t. Ans: d[b]/dt = k 1 [A] = k 1 [A] 0 e -k t d[b] = k 1 [A] 0 e -k t [B] = -k 1 [A] 0 /k 1 e -k t + constant At t = 0, [B] = 0; Constant = +k 1 [A] 0 /k 1 [1 e -k t ] (where k = k 1 + k 2 ) Question 4: Calculate the entropy change of the universe when an aluminum block of 500 gm mass and total heat capacity at constant pressure of J K -1 at C, is placed in a lake of water temperature 10 0 C. Ans: Heat rejected to lake = (100 10) J S lake = x 90 / = JK -1 S block = nc p ln T 2 /T 1 = ln / = JK -1 S univ = ( ) JK -1 = JK -1.; (Note: The mass of block is not used in calculation. It is simply to imply that the lake is too large and its temperature will not rise because of the block). Question 5: The equilibrium constant for the reaction 2HL H 2 + L 2 is 1.7 x 10-2 at C and 2.2 x 10-2 at C. If H 0 remains constant during this temperature interval, calculate S 0 at C.

5 Ans: ln K 2 /K 1 = - H 0 /R (1/T 1 1/T 2 ) (Van t Hoff s eqn.) ln (2.2x10-2 /1.7x10-2 ) = - H 0 /8.314 (1/773 1/673) H 0 = J mol -1. G 0 (400 0 C) = -RT ln (1.7 x 10-2 ) = J mol -1 G 0 = H 0 - T S 0 so, S 0 (400 0 C) = H 0 - G 0 / T = / 673 = J K -1 mol -1. Question 6: The melting point of bismuth (Atomic mass 209) is C. At this temperature, the density of solid bismuth is kg dm -3 and liquid bismuth is 10.0 kg dm -3 respectively. The melting point falls by degree when the pressure is increased by 1 atm. calculate the latent heat of fusion of bismuth. Ans: dp/dt = S/ V = H/T V V = V l V s = (1/10 1/9.673) dm 3 kg -1 = x 10-3 dm 3 kg -1 = {( x 10-3 ) x 209}dm 3 mol -1 = dm 3 mol -1. H = dp/dt. T V = 1/( ) x.554 x ( ) = kj mol -1. Question 7: The solubility of methyl amine at 298K in water is 8.5 times higher than in chloroform. Calculate the percentage of the base that remains in 1000 ml of chloroform if it is extracted (i) four times, always with 200 ml of water and (ii) twice with 400 ml of water. Ans: (i) x 4 = 100 (K D V 0 /V 1 +K 0 V 0 ) 4.% K D = 1/8.5, V 0 = 1000 ml, V 1 = 200 ml, x 4 = 1.9% (ii) V 1 = 400 ml X 2 = 100 ((1/8.5) (1000) / (.1/8.5) (1000)) = 5.2% Question 8: A mole of helium initially at 300K & 10 atm was heated to 600K and the final pressure of the gas was adjusted to 100 atm. Calculate G for the process assuming ideal behaviour of He. Given S 0 298K = 126 JK -1 mol -1. Ans: C p (He) = 5/2 R, n = 1 G = G, (300K, 10atm 300K, 1atm) + G 2 (300K, 1atm 600K, 1atm) + G 3 (600K, 1atm 600K, 100atm) G 1 = nrt ln 1/10 = J

6 G 2 = H (T 2 S 2 T 1 S 1 ) = C p T 600 (S 0 + C p ln 600/298) (S 0 + C p ln 300/298) = = J G 3 = RT ln 100/1 = J G total = = J. Question 9: A solution of A is mixed with an equal volume of a solution of B containing the same number of moles and the reaction A + B C occurs. At the end of 1 h, A is 75% reacted. How much of A will be left unreacted at the end of 2 h, if the reaction is (a) first order in A and zero order in B (b) first order in both A and B and (c) zero order in both A and B? Ans: (a) 1 st order in A zero order in B: %/1h 25 75%/1h 6.25 (or) t 3/4 = 2 t 1/2 = 1h t 1/2 = 30 min In 2h, 4 t 1/2, amt. remaining = (1/2) 4 = 1/16 i.e., 6.25%. (b) R = K[A] [B] / K = 1/t. x/a(a-x) = 75 / 100x25 = 3/100 M -1 h -1. x / a x = K at = 3/100 x 100 x 2 = 6 (a x) = 14.29% (c) R = K 0% remaining. Question 10: A certain unimolecular reaction proceeds by a direct path (i) as well as by a catalysed path (ii) E a for path (i) exceeds that of path (ii) by 20 KJ mol -1. The rate of path (ii) is 36 times more than that of path (i) at 300 K. How the entropy of activation is modified in the presence of the catalyst? Ans: k cat / k uncat = e ( S# c - S# uc )/R. e (E uc E c )/RT 36 = e x 20x103 / x 300. e (x ) = e x = x = S # c - S # uc / R = S # c - S # uc = JK -1 mol -1. S # decreases by JK -1 mol -1. Question 11: Given: HNO 2 H + + NO 2 - ; pka = 3.3 H 2 O H + + OH - ; pkw = H + + NO e HNO 2 + H 2 O ; E 0 = 0.94V

7 Compute the half-cell potential for the reaction NO H 2 O + 2e NO OH - at 298K. Ans: G 0 = G0 + 2 G0 + G0 = RT pk a + 2 (2.303 RT pkw) 2F(0.94) = RT (pka + 2pKw) = x x 298 x = = J mol -1. E 0 = - G 0 /nf = / 2x96500 = Question 12: 25.0ml of 0.24 M Cu + is titrated with 0.05M Cr 2 O 2- solution in a cell SCE// redox mixture / Pt. Calculate the emf of the cell, when (i) 10.0 ml of Cr 2 O 7 2- solution is added and (ii) 20.0 ml of Cr 2 O 7 2- is added. Assume [H + ] = 1.0M. Given E 0 (Cu 2+ /Cu + ) = 0.153V; E 0 (Cr 2 O 7 2- / Cr 3+ ) = 1.33V; E SCE = V. Ans: (i) Normality of Cu + = 0.24; Cr 2 O 7 2- = 0.30 N M equivalents of Cu + = 6; milliequivalents of Cr 2 O 7 2- = 3 This corresponds to 50% of equivalence point E 0 Cu 2+ / Cu + = V E cell = = V. (iii) milliequivalents of Cr 2 O 7 2- = 6 This corresponds to equivalence point E eq = n 1 E n 2 E 0 2 / n 1 + n 2 = 1(0.153) + 6(1.33) = V E cell = = 0.92 V. Question 13: The proposed mechanism for the decomposition of ethanal catalysed by iodine is given by l 2 2l L + CH 3 CHO Hl + CH 3 CO CH 3 CO CH 3 + CO CH 3 + l 2 CH 3 l + l CH 3 + Hl CH 4 + l CH 3 l + Hl CH 4 + l 2 Derive an expression for the rate of formation of CO. Ans: (i) d[co]/dt = k 3 [CH 3 CO] (ii) d[ch 3 CO]/dt = k 2 [I] [CH 3 CHO] k 3 [CH 3 CO] =0 [CH 3 CO] = k 2 [I] [CH 3 CHO] / k 3 (iii) d[i] / dt = 2k 1 [I 2 ] 2k -1 [I] 2 k 2 [I] [CH 3 CHO] + k 4 [CH 3 ] [I 2 ] + k 5 [CH 3 ] [HI] = 0

8 (iv) d[ch 3 ] / dt = k 3 [CH 3 CO] k 4 [CH 3 ][I 2 ] k 5 [CH 3 ] [HI] = 0 Adding (ii) + (iii) + (iv) 2k 1 [I 2 ] = 2k -1 [I] 2 (v) [I] = (k 1 [I 2 ]/k -1 ) 1/2 (vi) From (ii) k 3 [CH 3 CO] = k 2 [I] [CH 3 CHO] d[co] / dt = k 2 [CH 3 CHO] [I] = k 2 [CH 3 CHO] (k 1 [I 2 ]/k -1 ) 1/2 = k 2 (k 1 /k -1 ) 1/2 [CH 3 CHO] [I 2 ] 1/2

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