Final Exam of Physical Chemistry (2) Paper A

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1 Academic Year, Fall Semester Final Exam of Physical Chemistry (2) Paper A USEFUL CONSTANT Plank constant: h = Js; Light velocity in vacuum: c = ms -1 ; Avogadro constant: L = mol -1 ; Gravitational acceleration: g = 9.81 ms -2 Debye-Hückel limiting law: lg zz 1 I Attention: Please write all the answers on your ANSWER SHEET Part I Choose the best answers (3 points for each problem, totally 30 points) [1] Which of the following electrolytes obeys the Debye-Hückel limiting law in a widest concentration range? (A) MgCl 2 ; (B) MgSO 4 ; (C) NaCl; (D)AlCl 3 [2] For As 2 S 3 sol, if the precipitation value of SnCl 2 is x mol/l, the precipitation of MgSO 4 is (A) Equal to x mol/l (C) Larger than x mol/l; (B) Less than x mol/l; (D) Uncertain; [3] For an overall reaction A + B P, its rate law is experimentally determined to be r = k[a][b], what conclusion can you reach? (A) This is a simple reaction; (B) This is a bimolecular reaction; (C) The unit of rate constant k is s -1 ; (D) uncertain. [4] During formation of HBr, Br and H usually exist as active intermediates. For the following reaction, which is the most possible chain terminating step? (A) Br+ H HBr; (B) Br+Br Br 2 ; (C) H+H H 2 ; (D) H + HBr H 2 + Br. [5] Which of the following factors does not affect surface tension of a

2 liquid? (A) Temperature; (B) External pressure over pressure; (C) Contacting phase; (D) Surface area of the liquid. [6] For a firs-order reaction, the unit of its rate constant is: (A) s -1 ; (B)dm 3 mol -1 s -1 ; (C) mol dm -3 s -1 ; (D) dm -6 mol -2 s -1 ; [7] Activation energy can be of different nature. The activation energy defined by simple collision theory is (A) Potential energy; (B) vibrational energy; (C) Transitional energy; (D) photo energy. [8] The electrode potential of which electrode is the highest? (A) Ag(s)Ag 2 O(s)OH (m); (B) Ag(s)Ag + (m); (C) Ag(s)AgCl(s)Cl (m); (D) Ag(s)[Ag(NH 3 ) 2 ] + (m) [9] The quantum efficiency of photochemical formation of HCl is 10 6 times higher than that of HBr. What cause this great difference? (A) The primary photochemical process of HCl formation takes place much easier; (B) Unreactive decay in HBr formation is rather serious; (C) For HCl formation, the secondary photochemical process is chain reaction, but for HBr formation, it is not; (D) The life of Br is much longer. [10] Which of the following parameters of a colloidal particle can not be determined by ultramicroscopy? (A) The shape; (B) The molar mass; (C) The size; (D) The electrokinetic potential. Part II Fill in the blanks (totally 22 points) [1] (3 points) At 298 K, the solubility of AgCl in water is molkg -1. Its solubility in molkg -1 KNO 3 solution is molkg -1.

3 [2] (3 points) The cell notation of a reversible cell that can be used to measure the decomposition equilibrium constant of Ag 2 O is. [3] (2 points) List at least two ways to reduce the polarization of an electrode: ;. [4] (3 points) Sulfur, a gel which is always negatively charged due to adsorption of HS, can be prepared by purging SO 2 into H 2 S solution. Write the diagram of its colloidal structure. [5] (3 points) During formation of HCl, the newly formed HCl usually possess higher energy. If a light quantum passes through the reaction system, a laser radiation with wavelength of 760 nm can be emitted. The difference between the activated HCl and the HCl molecule at ground state is kjmol -1. [6] (3 points) If m NaCl solution is added into the CuSO 4 solution, the electrode potential of Cu(s)CuSO 4 (0.010 m) will (increase or decrease). [7] (2 points) Heating can destroy the stability of sol. Give at least two reasons to explain this fact. ;. [8] (3 points) Electrochemical reduction is commonly adopted for organic synthesis in aqueous solution. Why Pb or Pb alloy is usually used as the cathode material. Part III Answer the following questions (totally 28 points) [1] (12 points) For some reactions, their temperature-dependence of reaction rate can be expressed as: m - Ec k AT exp (Eq. 1) RT where E c is a threshold energy independent of temperature. The empirical equation proposed by vant Hoff in the middle 19 century is

4 d ln k dt B T 2 C (Eq. 2) (1) Please relate the empirical constant in Eq. 2, i.e., B and C, to the physical parameters in Eq. 1; (2) Why can Arrhenius neglect C in his empirical equation? (3) For deduction the relation in (1), we usually assume that A is independent of T. Is this assumption true? Which theory can explain this dependence? At elevated temperature, does A increase or decrease? [2] (16 points) At 298 K, reaction Ag (s) + 1 Hg 2 Cl 2 (s) AgCl(s) + Hg(l) 2 r H m = 7950 Jmol -1. The standard entropies of these substances are listed in the following table Substances Ag(s) AgCl(s) Hg(l) Hg 2 Cl 2 (s) S m / Jmol -1 K -1 (1) Write the notation of the cell in which this reaction takes place; (2) Write the electrode reactions; (3) Calculate the electromotive force of this reversible cell at 298 K; (4) Determine the temperature coefficient of electromotive force. Part IV calculation (totally 20 points) [1] (10 points) A glass capillary of 250 mm long with different diameters (R 1, R 2 ) as shown in the right figure is plugged vertically in water. This glass capillary can be completely wetted by water. The surface tension and density of water at 298 K is N/m and 1.00 g/ml respectively. If water can rise 200 mm high in the capillary, (1) Determine the largest diameter of the thick part (R 1 ); (2) Determine the diameter of the thin part (R 2 ). (3) If we turn the capillary upside

5 down, how high will water to rise? [2] (10 points) The bromination of acetone is acid catalyzed: H CH3 COCH3 Br2 CH3COCH 2Br H The rate of disappearance of bromine was measured at different experimental rounds: Rounds [CH 3 COCH 3 ] / moldm -3 [Br 2 ] /moldm -3 [H 3 O + ] / moldm -3 r / moldm -3 s (1) Determine the rate law for the reaction. (2) Determine the rate constant. (3) The following mechanism has been proposed for this reaction: Br A B B C C D Show whether or not this mechanism fit the rate law obtained in (1). (4) If E a, 1, E a, -1, E a, 2, E a, 3 stands for the activation energy of corresponding reaction, express the apparent activation energy (E a ) of the overall reaction by activation energy of the separate steps involved.

6 Academic Year, Fall Semester Final Exam of Physical Chemistry (2) Paper A ANSWER SHEET Part I Choose the best answers (3 points for each problem, totally 30 points) Problem [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] Answer Part II Fill in the blanks (totally 22 points) [1] molkg -1. [2]. [3] ;. [4]. [5] kjmol -1. [6]. [7] ;. [8].

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