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 SCHOOL OF CHEMISTRY & PHYSICS
UNIVERSITY OF KWAZULU‐NATAL, PIETERMARITZBURG CAMPUS CHEM230: PHYSICAL CHEMISTRY NOVEMBER 2014 MAIN EXAMINATION DURATION: 3 HOURS TOTAL MARKS: 100
Moderating Board: Internal Examiners: Dr A Mambanda Dr D Reddy (Coordinator) Dr P Ndungu Prof I V Nikolayenko Dr B A Xulu Instructions: 
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This paper consists of nine (9) pages, including a data sheet and the periodic table of elements. Please see that you have them all. This paper is divided into three (3) sections. Please answer questions from EACH SECTION in a SEPARATE BOOKLET. Ensure that you list the section on the front cover of the answer booklet. Please ensure that you affix your barcode on the front cover of each submitted answer booklet. The use of non‐programmable calculators is permitted. Graph paper will be provided, where necessary. Answer a total of FIVE (5) questions. Students are requested, in their own interests, to write legibly Page 1 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry SECTION A – FUNDAMENTALS OF THERMODYNAMICS Question One (a)
One mole of an ideal monatomic gas, initially occupying a volume of 11.2 L at 2.00 atm, is compressed to a final pressure of 4.00 atm isothermally and reversibly. (i)
Explain briefly the meaning of the thermodynamic terms work, w, and heat, q, and calculate their values for the above process. (ii) (8) What are the values of U and H for the same process? (2) (iii) How much work would have been done, were the ideal gas compressed isothermally by a constant external pressure of 4.00 atm? (2) ―
O
(b) Given the standard enthalpies of formation of water, fH [H2O(ℓ)] = –285.8 kJ mol–1, and ―
O
steam, fH [H2O(g)] = –241.8 kJ mol–1, and the molar heat capacities (in J K–1 mol–1): CP,m [H2 g ] 28.8, CP,m [O2 g ] 29.3, CP,m [H2 O g ] 33.6, calculate: (i) ―
O
rH at 25 C for the reaction when 1.00 mol of H2(g) is combusted in 10.00 mol of O2(g). (2) (ii) ―
O
rU at 25 C. (3) ―
O
(iii) rH at 500 K. State any assumption made. (3) [20] Page 2 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry Question Two (a) Which of the following processes are spontaneous? (i) Perfume aroma from an open bottle filling the air. (ii) Outlining your chemistry notes. (iii) Sugar dissolving in hot tea. (iv) Raking leaves into a pile Do not include humans in the system! (4) (b) (i) Give the thermodynamic definition of entropy, S, and briefly explain its meaning. (3) (ii) Calculate H and Suni when two 10.0 kg copper blocks, one at 0.00 C and another at 100.00 C, are brought in contact in an insulated container (consider it as an isolated system). The specific heat capacity of copper is 0.385 J K–1 g–1 and practically does not change over the temperature range involved. Show your reasoning clearly. (7) (c) (i) State the Third Law thermodynamics and give a brief statistical interpretation of entropy. (3) (ii) ―
O
―
O
Explain the difference between Sm (298 K) and fS (298 K). (3) [20] Page 3 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry SECTION B – CHEMICAL EQUILIBRIUM & CHEMICAL KINETICS Question Three (a)
Consider the following reaction: 2CO(g) + O2(g) ⇌ 2CO2(g) (i)
―
O
Calculate rG (375 K) from the standard thermodynamic data: Species CO(g) CO2(g) fH (298 K) / kJ mol–1 ― O
–110.53 –393.51 ― O
–137.17 –394.36 fG (298 K) / kJ mol–1 (ii)
State any assumption made. Is the reaction spontaneous in the forward direction at 375 K? (8) (b) At 25.0 °C, the equilibrium 2NOBr(g) ⇌ 2NO(g) + Br2(g) is rapidly established. When 1.10 g of NOBr is placed into a 1.0 dm3 vessel at 25.0 °C and allowed to equilibrate, the measured pressure is 0.355 bar. (i)
Calculate the degree of dissociation, , for this system. (4) (ii)
Calculate KC and KP for this system. (3) (c) An equilibrium constant, KC, is increased by a factor of 3 when the temperature of the system ― O
is raised from 25.0 °C to 40.0 °C. Calculate the standard enthalpy change, rH . (3) (d) Consider the following equilibrium of two perfect gases in a container at 25 °C, 2A(g) ⇌ B(g) The pressure in the container is now increased by decreasing the volume of the container. Explain how the composition of A and B will change during this process and also state whether the equilibrium constant, KP, will increase, decrease or stay the same. (2) [20] Page 4 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry Question Four (a)
Describe briefly how concentration and temperature can influence the rate of chemical reaction. (3) (b) The half‐life of pyruvic acid in the presence of the aminotransferase enzyme was found to be 256 s. Determine if the time taken for the concentration of pyruvic acid to fall to 1/64th of its initial value in this first‐order reaction is less than 30 minutes. (4) (c)
The reaction A  products is known to be second‐order with respect to A. Derive the integrated rate expression: A
for this reaction. A
k2 t A
1
(4) (d)
Consider the reaction: A + B  products Describe, with the use of suitable equations, how the method of initial rates can be used to determine the individual orders for A and B. (5) (e) A sample of milk kept at 25 °C is found to sour 40 times as fast as when it is kept at 4 °C. Estimate the activation energy of the souring process. (4) [20] Page 5 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry SECTION C – ELECTROCHEMISTRY & SPECTROSCOPY Question Five (a)
Describe the difference(s) between a galvanic and an electrolytic cell. (b)
(2)
Define the liquid junction potential. (c)
(1) Consider the following electrochemical cell at 25 C: Pt(s) | Sn2+ (aq, 0.50 M), Sn4+ (aq, 0.50 M) || I–(aq, 0.15 M) | AgI(s) | Ag(s) Given the following standard reduction potentials: AgI(s) + e–  Ag(s) + I–(aq) E = –0.15 V Sn4+(aq) + 2e–  Sn2+(aq) E = +0.15 V (i)
Write down the balanced equation of the overall chemical equation taking place in the above cell. (2) (ii)
Calculate the emf, E, for the above cell. (4) (iii)
Calculate the equilibrium constant, KC, for the overall reaction. (2) (d)
What are electrolytes and how are they classified (give one example of each)? (e)
(3) A conducitivity cell when standardized with 0.01M KCl was found to have a resistance of 189 Ω. With a 0.01M ammonia solution, the resistance was 2460 Ω. Calculate the base dissociation constant of ammonia, given the following molar conductivities at these concentrations: λ(K+) = 73.5 Ω–1 cm2 mol–1, λ(Cl–) = 76.4 Ω–1 cm2 mol–1, λ(NH+4 ) = 73.4 Ω–1 cm2 mol–1 and λ(OH–) = 198.6 Ω–1 cm2 mol–1. (6) [20] Page 6 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry Question Six (a)
Describe briefly the function of the following components of a UV/Visible spectrometer. (i)
A deuterium lamp. (ii)
A photomultiplier. (iii)
A monochromator. (6) (b)
Briefly describe chromophores and give three examples. (c)
(4) Why should you never use absorption measurements greater than 2? (d)
(2) Titanium (Ti) reacts with hydrogen peroxide and 1 M sulfuric acid to form a coloured complex that has a maximum absorbance at 415 nm. A titanium standard with a concentration of 2.08 x 10–5 M had a percentage transmittance of 31.5% at 415 nm in a 1.00 cm cell. (i)
What is the molar absorptivity of the complex? (Include units in your answer). (3) (ii) A 1.0504 g sample of titanium ore was treated with hydrogen peroxide and 1 M sulfuric acid to form a coloured complex. The final volume of the solution was 100.00 mL. A 1.00 mL aliquot of this solution was diluted to 250.00 mL and the absorbance of this diluted solution was measured to be 0.891. What is the % composition of titanium in the ore? Assume that there is 1 mole of Ti per mole of complex. (5) [20] Page 7 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry University of KwaZulu‐Natal School of Chemistry & Physics (Pietermaritzburg) DATA SHEET Physical Constants
= 1.381  1023 J K1 = 6.626  1034 J s Boltzmann constant Planck constant kB h   h 2 = 1.055  1034 J s Elementary charge Speed of light in vacuum Avogadro constant Gas constant Molar volume of an ideal gas Faraday constant Atomic mass unit (amu) Rest mass of electron Rest mass of proton Rest mass of neutron Vacuum permittivity Standard acceleration of gravity e c L or NA R = kBL Vm F = eL u me mp mn = 1.602  1019 C = 2.998  108 m s1 = 6.022  1023 mol1 = 8.315 J K1 mol1 = 0.08206 L atm K1 mol1 = 22.41 L mol1 (at STP: 1 atm and 273.15 K) = 24.79 L mol1 (at SATP: 1 bar and 298.15 K) = 9.649  104 C mol1 = 1.661  1027 kg = 9.109  1031 kg = 1.673  1027 kg = 1.675  1027 kg = 8.854  1012 C2 J1 m1 = 9.807 m s2 o g Conversion Factors 1 micron () 1 Ångström (Å) 1 L 1 atm 1 bar 1 J 1 cal 1 eV 1 L atm 1 W 1 ppm = 1  106 m = 1 μm = 1  1010 m = 0.1 nm = 100 pm = 1  103 m3 = 1 dm3 = 1.013  105 N m‐2 = 1.013  105 Pa = 760 Torr = 1  105 Pa = 1 N m = 1 Pa m3 = 1 m2 kg s2 = 1 V C = 0.2390 cal = 4.184 J = 1.602  1019 J = 101.3 J = 1 J s1 = 1 μg g1 = mg kg1 1
= 1 mg L (dilute aqueous solutions only) Page 8 of 9 University of KwaZulu‐Natal, School of Chemistry and Physics, Pietermaritzburg Campus November 2014 Main Examination Session, CHEM230 Physical Chemistry Page 9 of 9