School of Chemistry, UNIVERSITY OF KWAZULU-NATAL, WESTVILLE NOVEMBER 2007 EXAMINATION CHEM230: PHYSICAL CHEMISTRY Page 2

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1 Page 2 QUESTION 1 a) The usefulness of radiocarbon dating is limited to objects no older than years. Given that the half-life of decay of carbon-14 is 5.73 x 10 3 years, (i) (ii) estimate the value of the decay constant. What percentage of carbon-14, originally present in the sample, remains after years? (6) b) In a certain experiment, the rate of hydrogen peroxide decomposition, H 2 O 2 H 2 O + ½O 2, is followed by titration of H 2 O 2 against a potassium permanganate solution. At regular time intervals, an equal volume of H 2 O 2 was withdrawn to give the following data: Time/min Vol. KMnO 4 used/cm Confirm by a graphical method that the reaction follows first-order kinetics, and calculate the value of the rate constant. (9) c) Derive an integrated rate equation for the reaction, 2A Products.

2 Page 3 QUESTION 2 a) The second-order rate constants for the reaction of oxygen atoms with aromatic hydrocarbons have been measured. In the reaction of oxygen atoms with benzene the rate constants are 1.44 x 10 7 dm 3 mol -1 s -1 at K and 3.03 x 10 7 dm 3 mol -1 s -1 at K. Determine the preexponential factor and activation energy of the reaction. (7) b) Briefly describe two methods for determining the partial order with respect to reactants. (3) c) One mole of an ideal gas is compressed at constant pressure (1 atm) from 15.5 dm 3 to a final volume of 2.6 dm 3. Given C p = J K -1 mol -1, calculate w, U and H for a reversible adiabatic compression. (8) d) Estimate the work done on the system, if the process in 2(c) is done isothermally. (2)

3 Page 4 QUESTION 3 a) Calculate the third law entropy of 1 kg of liquid aluminium by making use of the following data: Absolute entropy of aluminium at 298 K = J K -1 mol -1 C P (s) = x 10-4 (T 273) kj K -1 kg -1 Melting point = K Enthalpy of fusion = kj mol -1 (10) b) For the reaction, Br 2 (g) 2Br(g), given K P at 1200 K = 2.11 x 10-3 and H o = kj mol -1, calculate at 1000 K: (i) K, (ii) G o, (iii) S o, and (iv) the degree of dissociation. (10)

4 Page 5 QUESTION 4 a) A constant volume bomb calorimeter required a current of 0.60 A from a 12.0 V supply passed for 420 s to increase the temperature by 3.0 degrees from K. When mg of phenol, C 6 H 5 OH(s), is burned in the calorimeter under the same conditions, the temperature increased from K to K. Calculate the molar enthalpy of combustion, H o f, for phenol. (9) b) (i) Show thermodynamically, why absolute zero is not attainable in a finite number of steps. (ii) Describe the ways by which low temperatures can be achieved. (6) c) Describe the basis of the Born-Haber cycle by making use of the determination of the lattice energy of sodium bromide as an example. Give appropriate chemical reactions, but no enthalpy values are needed.

5 Page 6 QUESTION 5 a) A conductivity cell has a resistance of 250 Ω when filled with 0.02 mol dm -3 KCl at 25 C, and a resistance of 1 x 10 5 Ω when filled with 6 x 10-5 mol dm -3 aqueous ammonia solution. The specific conductivity of 0.02 mol dm -3 KCl is S m -1, and the limiting molar conductivities for NH + 4 and OH are 7.34 x 10-3 and S m 2 mol -1, respectively. Calculate the cell constant and the degree of dissociation of the ammonia in the 6 x 10-5 mol dm -3 solution. (8) b) The specific conductivity is 3.82 S m -1 for a solution of 0.1 mol dm -3 in KCl and 0.2 mol dm -3 in the strong electrolyte, NCl. Calculate λ + for N + ; the limiting molar conductivity values for K + and Cl are 7.4 x 10-3 S m 2 mol -1 and 7.6 x 10-3 S m 2 mol -1 respectively. (7) c) Given that E o is V for the Ag*Ag + half cell and V for the Ag*AgBr(s) half-cell at 25 C, calculate the solubility product for AgBr.

6 Page 7 QUESTION 6 a) At 298 K, the limiting molar conductivity of a weak acid is found to be 388 S cm 2 mol -1. The molar conductivities of the acid at different dilutions were measured at 298 K and the data obtained is as follows: C/mol dm Λ/S cm 2 mol The Ostwald s dilution law for weak electrolytes is expressed as 1 Λ = 1 Λ 0 + ΛC Λ 2 0K a Prove that Oswald s dilution law is obeyed by the weak acid and estimate the dissociation constant of the weak acid by making use of all the data. (9) b) A Hittorf s method transference cell is filled with 0.2 mol dm -3 AgNO 3 solution, and has a copper anode and silver cathode. After passage of current, the anode compartment was found to contain 0.03 mol of copper nitrate and the cathode compartment lost 0.04 mol of silver nitrate. Write the electrode reaction and the gain and loss by transference for each compartment and calculate the transport number for the nitrate ion. (6) c) A 2.50 cm 3 aliquot of a solution that contains 3.8 ppm Fe 3+ is treated with an appropriate excess of KSCN and diluted to 50.0 cm 3. What is the absorbance of the resulting solution at 580 nm in a 2.0 cm cell? (The molar absorptivity of Fe(SCN) 2+ is 7.00 x 10 5 dm 3 mol -1 cm -1.)

7 Page 8 QUESTION 7 a) Sketch and label a diagram of a double-beam UV-Visible spectrophotometer. Describe the choices for various components that are used in the assembly. (8) b) Describe the advantages of the method of standard addition. (2) c) Palladium(II) and gold(iii) can be determined simultaneously by reaction with methiomeprazine (C 19 H 24 N 2 S 2 ). The absorption maximum for the Pd complex occurs at 480 nm, while that for the Au complex is at 635 nm. The molar absorptivity data at these wavelengths are as follows: Molar Absorptivity, ε/dm 3 mol -1 cm -1 At 480 nm At 635 nm Pd complex 3.55 x x 10 2 Au complex 2.96 x x 10 4 A 25.0 cm 3 sample was treated with an excess of methiomeprazine and subsequently diluted to 50.0 cm 3. Calculate the molar concentrations of Pd(II) and Au(III) in the sample, if the diluted solution had an absorbance of at 480 nm and at 635 nm, when measured in a 1.00 cm cell. (10)

N09/4/CHEMI/HP2/ENG/TZ0/XX+ CHEMISTRY HIGHER level. Tuesday 3 November 2009 (afternoon) Candidate session number. 2 hours 15 minutes

N09/4/CHEMI/HP2/ENG/TZ0/XX+ CHEMISTRY HIGHER level. Tuesday 3 November 2009 (afternoon) Candidate session number. 2 hours 15 minutes N09/4/CHEMI/HP/ENG/TZ0/XX+ 8809610 CHEMISTRY HIGHER level Paper Tuesday 3 November 009 (afternoon) hours 15 minutes 0 0 Candidate session number INSTRUCTIONS TO CANDIDATES Write your session number in