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 8 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( 2 H) = 2.0141 u m( 35 Cl) = 34.9689 u
QUESTION 1 Page 2 What are the indications that a reaction is occurring by a composite mechanism? (3) Explain what is meant by the steady-state approximation in chemical kinetics. Why is it useful, and under what conditions is it valid? (4) An oversimplified version of the CH 3 CHO decomposition mechanism is CH 3 CHO CH 3 + CHO CH 3 + CH 3 CHO CH 4 + CH 3 CO CH 3 CO CO + CH 3 2CH 3 C 2 H 6 (The CHO reacts to form minor amounts of various species.) (i) (ii) (iii) Identify the initiation, propagation and termination steps. What is the overall reaction (neglecting minor products formed in initiation and termination steps)? Show that rate = k[ch 3 CHO] 3/2, where k = k 2 (k 1 /2k 4 ) 1/2. (13)
QUESTION 2 Page 3 How does the molar Gibbs energy of a pure substance vary with temperature? (1) (d) The standard molar entropies of water ice, liquid, and vapour are 37.99, 69.91 and 188.83 J K -1 mol -1, respectively. Sketch and appropriately label a graph that shows how the Gibbs energies of each of these phases varies with temperature. (4) The molar volume of a certain solid is 142.0 cm 3 mol 1 at 1.00 atm and 427.15 K, its melting temperature. The molar volume of the liquid at this temperature and pressure is 152.6 cm 3 mol 1. At 1.2 x 10 6 Pa the melting temperature changes to 429.26 K. Calculate the enthalpy and entropy of fusion of the solid. (7) Draw the phase diagram for carbon dioxide, showing how the equilibria between solid, liquid and gas vary. Label the regions and intersections of the diagram. For each of these and the phase boundaries give the number of degrees of freedom. (8) QUESTION 3 Define the partial molar volume V i of substance i in a solution. (2) For a solution of ethanol and water at 20 C that has 0.2 mole fraction ethanol, the partial molar volume of water is 17.9 cm 3 mol -1 and the partial molar volume of ethanol is 55.0 cm 3 mol -1. What volumes of pure ethanol and water are required to make 1 dm 3 of this solution? At 20 C the density of ethanol is 0.789 g cm -3 and the density of water is 0.998 g cm -3. (8) A dilute solution of chloroform in acetone behaves as an ideal-dilute solution. The pure vapour pressure of acetone is 45.9 kpa at 35.2 C. The Henry s law constant when the concentration of chloroform is expressed as a mole fraction is 19.3 kpa. (i) (ii) (iii) What is meant by an ideal-dilute solution? Calculate the vapour pressure of each component, the total pressure, and the composition of the vapour phase when the mole fraction of chloroform is 0.2, on the assumption that the conditions of the ideal-dilute solution are satisfied. Calculate ΔG of mixing for this solution. (10)
QUESTION 4 Page 4 (i) What is an azeotrope? (ii) How can an azeotrope be distinguished from a pure compound? (iii) Give an example of a useful application of an azeotrope. (iv) Plot the following boiling point data for benzene-ethanol solutions at 1.013 bar and estimate the azeotropic composition. Boiling point/ C 78 75 70 70 75 80 x liquid (benzene) 0 0.04 0.21 0.86 0.96 1.00 y vapour (benzene) 0 0.18 0.42 0.66 0.83 1.00 (v) State the range of mole fractions of benzene for which pure benzene could be obtained by fractional distillation at 1.013 bar. (11) Redraw the figure shown below in your answer book and then answer the questions which follow. (i) (ii) (iii) (iv) (v) (vi) Distinguish between congruent and incongruent melting. On your figure label clearly the feature that denotes congruent melting and that which denotes incongruent melting. On your figure mark the eutectic and the monotectic points. What is the composition of the eutectic mixture and at what temperature does it melt? What is the composition of the monotectic mixture and at what temperature does it melt? Sketch the cooling curves for the isopleths a and b shown in the figure. (9)
QUESTION 5 Page 5 What is meant by the photoelectric effect? What are the characteristics of the photoelectric effect? Explain how this effect provides strong evidence for the existence of photons. (1) (6) (7) (d) The work function for metallic rubidium is 2.09 ev. Calculate the kinetic energy and the speed of the electrons ejected by light of wavelength 195 nm. (6) QUESTION 6 Calculate the separation between the rotational lines of the spectrum of gaseous 2 H 35 Cl, given that the internuclear distance is 127.5 pm. Discuss the advantages of microwave spectroscopy in the study of pure rotational spectra. (9) (5) The vibration frequency of the N 2 molecule corresponds to a wavenumber of 2360 cm -1. Calculate the zero-point energy and the energy corresponding to v = 1. (6)
QUESTION 7 Page 6 The principal features of chemisorption are expected to be well described by the Langmuir adsorption isotherm Kp θ = 1 + Kp where p is the pressure of the gas and K is independent of pressure. Derive this equation, stating clearly the assumptions on which it is based. Why does the Langmuir isotherm fail to describe the characteristics of physisorption? (9) Carbon monoxide is found to adsorb on the surface of a particular transition metal crystal. N, the number of CO molecules adsorbed per square metre, is measured as a function of CO gas pressure, p CO, at 400 K. The results are presented below: p CO /10-4 Pa 0.44 1.7 4.0 9.3 N/10 18 m -2 0.80 2.4 4.0 5.6 Show that the data conform to the Langmuir isotherm equation given in. Calculate the values for K and for N m, the maximum surface concentration of CO which can be adsorbed. (11)