PHYSICAL CHEMISTRY CHEM330

Transcription

1 PHYSICAL CHEMISTRY CHEM330 Duration: 3 hours Total Marks: 100 Internal Examiner: External Examiner: Professor B S Martincigh Professor J C Swarts University of the Free State INSTRUCTIONS: 1. Answer five of the seven questions. 2. Programmable calculators may be used but all working must be shown. 3. This question paper consists of 9 pages. Please make sure you have them all. 4. Graph paper is provided. 5. A periodic table and data sheet are attached at the end of this question paper. DATA: m( 1 H) = u m( 127 I) = u m( 14 N) = u m( 16 O) = u

2 QUESTION 1 Page 2 Give the reaction mechanism for a typical enzyme-catalysed reaction where there is a single substrate. (3) Use the reaction scheme proposed in to derive an equation for the rate of the enzyme-catalysed reaction. (7) The oxidation of ethanol (S) to ethanal (P) is catalysed in aqueous solution by alcohol dehydrogenase (E). The rate of reaction, v, at 298 K and fixed [E] 0, was found to vary with the concentration of substrate as follows: [S]/mol dm ν/10-3 mol dm -3 s Verify that these data are consistent with the above reaction scheme and determine the value of the Michaelis constant. (10) QUESTION 2 Draw the phase diagram for sulfur, showing how the equilibria between the solid, liquid and gaseous phases vary. Label the regions and intersections of the diagram. For each of these and the phase boundaries give the number of degrees of freedom. (Metastable equilibria need not be shown.) (10) Explain the significance of the Clapeyron equation and where it may be applied. (3) When benzene freezes at 5.5 C its density changes from g cm 3 to g cm 3. Its enthalpy of fusion is kj mol 1. Estimate the freezing point of benzene at 1000 atm. (7)

3 QUESTION 3 Page 3 For a mixture of substances, μ J G = n J p, T, n Show that this leads to the Fundamental Equation of Thermodynamics and hence show that additional (non-expansion) work can arise from the changing composition of a system. (5) Find Δ mix G, Δ mix V, Δ mix S and Δ mix H for mixing 500 g of hexane with 500 g of heptane at 30 C. Assume an ideal solution. (7) At 39.9 C the pure vapour pressure of ethanol is Torr, and that of isooctane is 43.9 Torr. An ethanol-isooctane solution at 39.9 C with x liquid (ethanol) = has a vapour pressure of Torr and x vapour (ethanol) = (i) (ii) Calculate the activities and activity coefficients of both components in this solution on the Raoult s law basis. What do the above values of the activity coefficients indicate to you about this solution? (8)

4 QUESTION 4 Page 4 Benzene and toluene form nearly ideal solutions. Consider a solution of 1.0 mol benzene and 3.0 mol toluene. At 20 C the vapour pressures of pure benzene and toluene are 9.9 kpa and 2.9 kpa, respectively. The solution is boiled by reducing the external pressure below the vapour pressure. Calculate: (i) (ii) (iii) the pressure when boiling begins, the composition of each component in the vapour, and the vapour pressure when only a few drops of liquid remain. Assume that the rate of vaporization is low enough for the temperature to remain constant at 20 C. (10) Magnesium melts at 651 C and zinc at 419 C, and these elements form a compound MgZn 2 which melts at 595 C. The eutectic mixture of zinc and the compound contains 3% of magnesium and melts at 368 C, while the other eutectic contains 49% of magnesium and melts at 347 C. (Atomic masses: Mg = 24.31, Zn = 65.39) (i) (ii) Sketch the phase diagram for magnesium and zinc. Sketch the cooling curve which you would expect to get with a mixture of 20% magnesium and 80% zinc, and describe the changes which take place when this mixture cools. (10)

5 QUESTION 5 Page 5 (d) Briefly state the assumptions in classical physics which failed to account for the observed behaviour of very small particles and give reasons for their failure. (6) What is meant by the photoelectric effect and what evidence does it provide? (2) A particle undergoes harmonic motion if it is subjected to a Hooke s-law restoring force. (i) Give an equation that describes Hooke s law and explain all symbols used. (ii) How does the potential energy of a particle subjected to this force vary? Give an equation and draw an appropriate sketch. (4) The vibrational motion of the bond in the hydrogen iodide molecule can be modelled as a harmonic oscillator of mass m m H = x kg (since the large iodine atom is almost stationary) and force constant k = N m -1. (i) Calculate the zero-point energy for the hydrogen iodide molecule. (ii) (iii) Calculate the separation between the energy levels. What is the wavelength of light that would induce a transition between neighbouring levels? (8)

6 QUESTION 6 Page 6 Compare the appearance of the pure rotational spectrum of a diatomic molecule obtained by infrared and Raman spectroscopy. (5) The equilibrium internuclear distance of the molecule 1 H 127 I is pm. Calculate the rotational constant of the molecule in cm -1. The force constant of the molecule 14 N 16 O is 1550 N m -1. Calculate the wavenumber of the fundamental transition, v = 0 v = 1. (6) (5) (d) The symmetric vibration for carbon dioxide is infrared inactive but Raman active. The converse is true for the antisymmetric vibration. Explain this statement. (4)

7 QUESTION 7 Page 7 Give the equation that defines the Langmuir adsorption isotherm and define all symbols. (2) State the assumptions on which the Langmuir adsorption model is based. Why are these assumptions too restrictive and how is this reflected in the enthalpy of adsorption? (4) (3) (d) The data below are for the adsorption of hydrogen on copper powder at 298 K. p/atm V/cm Confirm that they fit the Langmuir isotherm. (6) (e) (f) (g) Evaluate the volume of hydrogen required to completely cover the surface of the copper adsorbent. (2) How can the value obtained in (e) be used to calculate the specific surface area of the copper powder? (1) Name two experimental techniques that could be used to study the nature of the surface of the copper powder. (2)

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