School of Chemistry UNIVERSITY OF KWAZULU-NATAL, PIETERMARITZBURG MAY 2011 EXAMINATION CHEM163 : CHEMISTRY & SOCIETY I

Transcription

1 School of Chemistry UNIVERSITY OF KWAZULU-NATAL, PIETERMARITZBURG CHEM163 : CHEMISTRY & SOCIETY I DURATION: 2 HOURS TOTAL MARKS : 100 External Examiner: Professor OQ Munro University of KwaZulu-Natal, Pietermaritzburg Internal Examiner: Mrs K-L Barry University of KwaZulu-Natal, Pietermaritzburg Notes: This paper consists of 23 pages including the cover page, periodic table and a data sheet. Please ensure that you have them all. The use of non-programmable electronic calculators is permitted. STUDENTS ARE REQUESTED, IN THEIR OWN INTERESTS, TO WRITE LEGIBLY STUDENT NUMBER: SEAT NO.: Question Total Marks Marks Internal Examiner Marks External Examiner ANSWER ALL QUESTIONS DIRECTLY ON THE PAPER AND WHERE NECESSARY OVER THE PAGE 1

2 QUESTION ONE 1.1 Carry out the following calculation and express the answer to the correct number of significant figures. [(285.3 x 10 5 ) (1.200 x 10 7 )] x (2) 1.2 What is the mass in grams of one chlorine atom? (2) 2

3 1.3 A submicroscopic particle suspended in a solution has a volume of 2.50 µm 3. What is this volume in litres? Quote your answer to the correct number of significant figures. (2) 1.4 Define the difference between an equivalence point and an end point in a titration. (3) 1.5 Draw a titration curve for the titration of M HCl against M NaOH. Consider NaOH to be the titrant. Indicate the ph of the equivalence point. (2) 3

4 1.6 A solution contains 5.0 g of toluene (C 7 H 8 ) dissolved in 225 g of benzene and has a density g cm -3. of a) Calculate the molarity of the solution. Show all working. (4) b) Calculate the molality of the solution. Show all working. (2) 4

5 QUESTION TWO [17] 2.1 Consider the combustion of ethylene, C 2 H 4 (g) + 3O 2 (g) 2CO 2 (g) + 2H 2 O(g) If the concentration of C 2 H 4 is decreasing at a rate of 0.37 M s -1, what is the rate of change in concentration of H 2 O? (2) 2.2 A reaction A + B C obeys the following rate law: Rate = k[a] 2 [B] a) If [A] is doubled how will the rate change? (1) b) What are the units of the rate constant? Note: The concentration in the rate law is a molar concentration. (2) 5

6 2.3 Which of the following two curves represents the higher temperature and which the lower temperature? Explain your answer. (2) 2.4 At 23 C, the second-order decomposition of NO 2 has a rate constant of 0.54 M -1 s -1. If the initial concentration of nitrogen dioxide was M what is the half life for this reaction in minutes? 2NO 2 2NO + O 2 (3) 6

7 2.5 Urea (NH 2 CONH 2 ) is the end product in protein metabolism in animals. The decomposition of urea in 0.1 M HCl occurs according to the following reaction: NH 2 CONH 2 (aq) + H + (aq) + 2H 2 O(l) 2NH + 4 (aq) + HCO 3 (aq). The reaction is first order in urea and first order overall. What is the concentration of urea in this solution after 1.00 hour if the starting concentration of urea is M at 61 C. The rate constant for this reaction at 61 C is 4.28 x 10-4 s -1. (3) 7

8 QUESTION THREE [13] 3.1 Consider the following equilibrium for which K p = 1.48 x 10 4 at 184 C. 2NO(g) + O 2 (g) 2NO 2 (g) a) What is the value of K c for the above reaction? (3) b) What is the value of K p for the reaction NO(g) + ½O 2 (g) NO 2 (g)? (2) 8

9 3.2 At 1258 C K c = 1.04 x 10 3 for the following reaction. Br 2 (g) 2Br(g) A L vessel containing an equilibrium mixture of the gases has g Br 2 (g) in it. What is the mass in grams of Br(g) in the vessel? (5) 9

10 3.3 Should the temperature be increased or decreased to favour product formation for the following reaction? Explain your answer. Co(H 2 O) 2+ 6 (aq) + 4Cl - (aq) CoCl 2-4 (aq) + 6H 2 O(l) H > 0 (3) 3.4 Calculate the molar solubility of Ag 2 S at 25 C. K sp = 6 x at 25 C. (4) 10

11 QUESTION FOUR [17] 4.1 What is the conjugate base of the Brønsted-Lowry acid: H 2 PO 4? (1) 4.2 Calculate the ph of a 0.75 M ethylamine (C 2 H 5 NH 2 ) solution. K a (C 2 H 5 NH 3 + ) = 1.56 x K w = 1.00 x (4) 11

12 4.3 A sample of 7.5 L of NH 3 gas at 22 ºC and 735 torr is bubbled into a 0.50 L solution of 0.40 M HCl. Assuming that all the NH 3 dissolves and that the volume of the solution remains 0.50 L, calculate the ph of the resulting solution after the reaction between NH 3 and HCl has occurred. K a (NH + 4 ) = 5.56 x (6) 12

13 4.4 Explain why a mixture of HCl and KCl does not function as a buffer, whereas a mixture of CH 3 COOH and CH 3 COONa does when a small amount of strong acid is added to the system. Include suitable equilibria/reaction equations in your answer. (4) 13

14 QUESTION FIVE [15] 5.1 Although water and carbon tetrachloride, CCl 4 (l), do not mix, their vapours form homogeneous mixtures. Explain. (2) 5.2 Why do bubbles form on the inside wall of a cooking pot when water is heated on the stove, even though the temperature is well below the boiling point of water? (1) 14

15 5.3 Gaseous iodine pentaflouride IF 5, can be prepared by the reaction of solid iodine and gaseous fluorine: I 2 (s) + 5F 2 (g) 2IF 5 (g) A 5.00 L flask containing 10.0 g I 2 is charged with 10.0 g F 2 and the reaction proceeds until one of the reagents is completely consumed. After the reaction is complete, the temperature in the flask is 125 ºC. What is the mole fraction of IF 5 (g) in the flask? (5) [8] 15

16 QUESTION SIX 6.1 Calculate the final temperature of the mixture when 455 g of stainless steel at 79.8 ºC is placed in a calorimeter containing 101 g of H 2 O at 17.8 ºC. The specific heat capacity of liquid water is 4.18 J g -1 K -1 and of stainless steel is 0.51 J g -1 K -1. (4) 6.2 At one time, a common means of forming small quantities of O 2 gas in the laboratory was to heat KClO 3 : 2KClO 3 (s) 2KCl(s) + 3O 2 (g) H = 89.4 kj Calculate H for the formation of g of KCl. (3) 16

17 6.3 Ammonia (NH 3 ) burns in the presence of a platinum catalyst to produce nitric oxide (NO) according to the following reaction: 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) Using the thermochemical equations given below, calculate the heat of this reaction at constant pressure. N 2 + O 2 (g) 2NO(g) N 2 (s) + 3H 2 (g) 2NH 3 (g) 2H 2 (g) + O 2 (g) 2H 2 O(g) Hº = kj Hº = 91.8 kj Hº = kj (4) 6.4 Write the balanced chemical equation for the standard enthalpy of formation of naphthalene (C 10 H 8 (s)). (2) [13] 17

18 QUESTION SEVEN 7.1 What is the oxidation number of S in S 2 O 3 2-? (1) 7.2 Name the following compound: MnCl 2. (1) 7.3 Balance the following redox reaction in acidic media. NO 2 - (aq) + Cr 2 O 7 2- (aq) Cr 3+ (aq) + NO 3 - (aq) (5) 18

19 7.4 A voltaic cell is constructed that uses the following reaction and operates at 298 K: Zn(s) + Ni 2+ (aq) Zn 2+ (aq) + Ni(s) What is the emf of this cell when [Ni 2+ ] = 3.00 M and [Zn 2+ ] = M? Half-Reaction E Ni 2+ (aq) + 2e - Ni(s) 0.28 V Zn 2+ (aq) + 2e - Zn(s) V (4) 19

20 7.5 Which of the following metal/s are suitable for use as sacrificial anode/s to protect against corrosion of an underground iron tank? Explain your answer. Silver Copper Lead Zinc You are provided with the following information: Half-Reaction E Ag + (aq) + e - Ag(s) V Cu 2+ (aq) + 2e - Cu(s) V Fe 2+ (aq) + 2e - Fe(s) V Pb 2+ (aq) + 2e - Pb(s) V Zn 2+ (aq) + 2e - Zn(s) V (3) 20

21 7.6 Magnesium is produced at a large facility by electrolysis of molten MgCl 2. What is the total energy requirement (in J) to produce 3.30 x 10 2 kg of Mg for this electrolysis if the applied emf is 4.20 V. (3) [17] [100] 21

22 H Periodic Table He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba * La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra (226) 89** Ac (227) 104 Rf (267) 105 Db (268) 106 Sg (266) 107 Bh (270) 108 Hs (277) 109 Mt (276) 110 Ds (281) 111 Rg (280) * Lanthanide Series 58 Ce Pr Nd Pm (147) 62 Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ** Actinide Series 90 Th (232) 91 Pa (231) 92 U (238) 93 Np (237) 94 Pu (239) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (252) 99 Es (252) 100 Fm (257) 101 Md (256) 102 No (259) 103 Lr (260)

23 DATA SHEET Physical Constants Conversion Factors Boltzmann constant k = x J K -1 Planck constant h = x J s Elementary charge e = x C Speed of light in vacuum c = x 10 8 m s -1 = x cm s -1 Avogadro constant L or NA = x mol -1 Gas constant R = kl = J K -1 mol -1 Molar volume of an ideal gas = L kpa K -1 mol -1 = L atm K -1 mol -1 o V m = L mol -1 (at atm and K) Vm = L mol -1 (at kpa and K) Faraday constant F = el = x 10 4 C mol -1 Atomic mass unit (amu) u = x kg Rest mass of electron me = x kg Rest mass of proton m p = x kg Rest mass of neutron mn = x kg Vacuum permittivity ε υ = x J -1 C 2 m - 1 Standard acceleration of free fall g = m s - 2 Rydberg constant for the H atom RH = cm -1 1micron (µ) = 10-6 m = 1 µm 1 Ångström (Å) = 1 x m = 0.1 nm = 100 pm 1 L = 10-3 m 3 = 1 dm 3 1 atm = x 10 5 N m -2 = x 10 5 Pa = 760 mmhg = 760 torr 1 bar = x 10 5 Pa 1J = cal = 1 Pa m 3 = 1 m 2 kg s -2 1 cal = J 1 ev = x J 1 L atm = J 1 W = 1 J s -1 1 ppm = 1 µg g -1 = mg kg -1 = 1 mg L -1 (dilute aqueous solutions only) 1 tonne = 1000 kg Prefixes to Units P T G M k d c m µ n p f peta tera giga mega kilo deci centi milli micro nano pico femto