Download 2013 Final Exam Answers

 BCIT Fall 2013 Chem 3615 Final Exam Name: ___________________ Attempt all questions in this exam. Read each question carefully and give a complete answer in the space provided. Part marks given for wrong answers with partially correct reasoning/calculations. A constant and formula sheet, Periodic Table and Tables with Electronegativity values and Standard Reduction Potentials are attached at the back. Total points = 80 Good Luck! 1 of 19
Section I: (27 points total, 1 point each) Choose the BEST answer to the following questions. 1. Use the kinetic molecular theory of gases to predict what would happen to a closed sample of a gas whose temperature increased while its volume decreased. a) Its pressure would decrease. b) Its pressure would increase. c) Its pressure would hold constant. d) The number of moles of the gas would decrease. e) The average kinetic energy of the molecules of the gas would decrease. 2. Which of the following effects will make PV/nRT less than 1 for a real gas? a) The gas molecules are large enough to occupy a substantial amount of space. b) A large number of molecules have speeds greater than the average speed. c) The gas molecules have a very low molar mass. d) The gas molecules attract one another. e) none of these 3. A 1.0 L and 10.0 L contain the same number of gaseous He atoms. The flasks are at 25°C. Which of the following statements is false? a) The average kinetic energy of the atoms in the two flasks is the same. b) The average speed of the atoms in the two flasks is the same. c) The average force of a collision between an atom and the container’s wall is the same for the two flasks. d) The frequency with which the atoms collide with the container’s walls is the same for the two flasks. e) The pressure in the two flasks is different. 4. Which of the following properties of a real gas is related to the b coefficient in the van der Waals equation? a) Real gases consist of molecules or atoms that have volume. b) The average speed of the molecules of a real gas increases with temperature. c) There are attractive forces between atoms or molecules of a real gas. d) Real gases expand when heated. e) None of the above. 2 of 19
5. If a1 and a2 are constants, 1(x) and 2(x) are functions, and  is a Hermitian operator that satisfy the equation, Â1(x) = a11(x) and Â2(x) = a22(x). Which of the following statements is false? a) 1 (x) is an eigenfunction of  with eigenvalue a1. b) a1 is a real number. c) A Ψ x
d) Â1(cx) = a1c 1(x) where c is a constant. e) Âc1(x) = cÂ1(x) where c is a constant. 6. Ψ x
AΨ x
AΨ x The valence bonding molecular orbital of a hydrogen chloride, HCl, molecule may be described as the linear combinations of the hydrogen 1s and chlorine 3p atomic orbitals HCl = 0.64H 1s + 0.77Cl 3p What is the probability of finding the electron near the chlorine atom. a) 13% b) 41% c) 59% d) 64% e) 77% 7. How many electrons in an atom can have both the quantum numbers n = 3 and  = 2? a) 2 b) 5 c) 6 d) 10 e) 18 8. Which quantum number determines the shape of a hydrogen atomic orbital? a) n b)  c) m d) ms e) more information is needed 9. Atoms having greatly differing electronegativities are expected to form: a) no bonds b) polar covalent bonds c) pure covalent bonds d) ionic bonds e) metallic bonds 3 of 19
10. The electron pair in a C‐F bond could be considered a) closer to C because carbon has a larger radius and thus exerts greater control over the shared electron pair. b) closer to F because fluorine has a higher electronegativity than carbon. c) closer to C because carbon has a lower electronegativity than fluorine. d) an inadequate model since the bond is ionic. e) centrally located directly between the C and F. 11. According to the VSEPR model, the arrangement of electron pairs around NH3 and CH4 are a) different because in each case there are a different number of atoms around the central atom. b) different because in each case there are a different number of electron pairs around the central atom. c) the same because both nitrogen and carbon are both in the second period. d) the same because in each case there are the same number of electron pairs around the central atom. e) different or the same, depending on the conditions leading to maximum repulsion. 12. Which of the following exhibits resonance? a) CH4 b) PCl5 c) H2O d) NO2ˉ e) At least two of the above molecules exhibit resonance. 13. The boiling point of methanol, CH3OH, is much higher than that of ethane, CH3CH3. This is primarily due to a) the difference in molar masses of methanol and ethane. b) the hydrogen bonding in methanol. c) the significant molecular size difference between methanol and ethane. d) the carbon oxygen double bond in the methanol. e) none of these. 14. Which of the following substance will have the largest band gap? a) Ge b) Sn c) GaAs d) GaP e) ZnS 4 of 19
15. In which case will a reaction at constant pressure not be spontaneous at any temperatures? a) ΔH is positive, ΔS is positive. b) ΔH is positive, ΔS is negative. c) ΔH is negative, ΔS is positive. d) ΔH is negative, ΔS is negative. e) none of these 16. Which of the following is true about chemical equilibrium? a) It is microscopically and macroscopically static. b) It is microscopically and macroscopically dynamic. c) It is microscopically static and macroscopically dynamic. d) It is microscopically dynamic and macroscopically static. e) None of these are true about chemical equilibrium. 17‐18. The following two equations represent equilibria that lie far to the right. HA(aq) + H2O() ⇌ H3O+(aq) + Aˉ(aq) H2O() + Bˉ(aq) ⇌ HB(aq) + OHˉ(aq) 17. Identify the strongest acid. a) HA b) HB c) H3O+ d) Aˉ e) Bˉ 18. Identify the strongest base. a) HA b) HB c) OHˉ d) Aˉ e) Bˉ 19. Given the reaction A(g) + B(g) ⇌ C(g) + D(g). The system is at equilibrium. Upon adding gas A, the value of K: a) increases because more products are made, increasing the product to reactant ratio. b) decreases because A is a reactant so the product to reactant ratio decreases. c) does not change because A does not figure into K. d) does not change as long as the temperature is constant. e) depends on whether the reaction is endothermic or exothermic. 5 of 19
20. For a certain equilibrium, the value of K is 0.0010 at 25.0C and the value of K is 0.10 at 50.0C. This means that the forward reaction is a) exothermic. b) endothermic. c) never favorable. d) more information is needed. e) none of above 21‐22. Use the following diagram to answer questions 21 and 22 21. As the cell operates, the cations move towards the a) the Pb electrode and the Pb electrode gains mass. b) the Pb electrode and the Pb electrode loses mass. c) the Zn electrode and the Zn electrode gains mass d) the Zn electrode and the Zn electrode loses mass e) more information is needed 22. As the cell operates, the electrons move towards the a) Zn electrode and the cell voltage decreases over time b) Zn electrode and the cell voltage increases over time c) Pb electrode and the cell voltage decreases over time d) Pb electrode and the cell voltage increases over time e) more information is needed 6 of 19
23. How many electrons are transferred in the following reaction? 2ClO3ˉ + 12H+ + 10Iˉ  5I2 + Cl2 + 6H2O a) 2 b) 5 c) 10 d) 12 e) 30 24. Consider the following spontaneous reactions: 3Cd2+ + 2Np  3Cd + 2Np3+ Cd + Pd2+  Cd2+ + Pd Which is the strongest oxidizing agent? a) Cd2+ b) Cd c) Np d) Np3+ e) Pd2+ 25. Which of the following balanced equations is labeled incorrectly? a) fission: b) fusion: c) alpha decay: d) beta decay e) all the above are labeled correctly + +     + 2 + + + ̅ 26. It is desired to determine the concentration of arsenic in a lake sediment sample by means of neutron activation analysis. The nuclide captures a neutron to form , which in turn undergoes β decay. The daughter nuclide of produces the characteristic  rays used for the analysis. What is the daughter nuclide?
a) b) c) d) e) 7 of 19
27. Which statement is true about the following reaction? The particles masses are given below the particle + 13.992 u a) b) c) d) e)  4.0015 u + 16.9986 u 1.0073 u Energy is absorbed in the reaction. Energy is released in the reaction. No energy change is associated with the reaction. Not enough information is given to determine the energy change. None of these. Section II: Short answer calculations do not need to be shown (19 points total). 28. Circle the dominant intermolecular force in each of the following molecules: (2 points) a) C3H8 b) CH3OCH3 Dispersion Dipole‐Dipole H‐Bond Dispersion Dipole‐Dipole H‐Bond 29. How many electrons can be contained in all of the orbitals with n = 4? (1 point) __32_____ 30. Write the electron configuration for the following and circle if they are diamagnetic or paramagnetic: (4 points) a) a silicon atom (Si) Diamagnetic Paramagnetic Diamagnetic Paramagnetic 1s2 2s2 2p6 3s2 3p2 or [Ne] 3s2 3p2 b) a NO molecule σ1s2 σ1s*2 σ2s2 σ2s*2 σ2p2 π2p4 π2p*1 8 of 19
31. For each of the following molecules and ions draw the best Lewis structure, Name the ideal geometry (from the VSEPR model), give the hybridization of the underline atom and indicate if the molecule is polar or nonpolar. The central atom is underlined. (9 points) Molecule or Ion Best Lewis Structure (show all the electrons and resonance structures)
Hybridization of underlined atom Molecular Geometry Polar or Nonpolar sp² bent polar sp linear polar ∙∙
NOF ∙∙
∶
∙∙
∶ ∶
∶ ↕ ∙∙
∶
SCNˉ ∶
≡
∙∙
↕ ≡
∶ ∙∙
∙∙
∶ 32. Consider the freezing of liquid water at 0°C and 1 atm pressure, H2O()  H2O(s). For this process what are the signs for ΔH, ΔS, and ΔG? Circle the correct answer (3 points) ΔH Negative Zero Positive Need more information ΔS Negative Zero Positive Need more information ΔG Negative Zero Positive Need more information 9 of 19
Section III: Calculations and reasoning must be shown (34 points total) 33. If the binding energy of an electron is 6.45x10‐19 J, what wavelength of photon is required to liberate it from the atom? (2 points) → 6.63 10
3.00 10 6.45 10 3.08 10 308
34. For a particular chemical reaction at constant pressure and temperature ΔH = ‐500 kJ mol‐1 and ΔS = –250 J mol‐1 K‐1 Under what temperature condition is the reaction spontaneous? (2 points) Spontaneous when ΔG < 0 ΔG = ΔH ‐ TΔS < 0 ‐TΔS < ΔH 500,000 Δ
2000 250 Δ
So for T < 2000 K , has a binding energy per nucleon of 8.79 MeV. Determine the difference in mass 35. Iron‐56, between one mole of iron‐56 nuclei and the component nucleons of which it is made. (2 points)
(1 MeV = 1.60x10–13 J) ∆
²
.
.
.
.
5.28 10 /
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0.528 36. Propane gas is used in barbeques. The complete combustion of propane, C3H8, proceeds by the following balanced equation: C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) Given the following information: ΔHf° (kJ/mol) ‐104 –394 –242 Substance C3H8(g) CO2(g) H2O(g) a) What is ΔH° for the reaction C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)? (3 points) ∆
3∆
∆
3
4∆
394 /
4
2046 /
∆
5∆
∆
242 /
104 /
5 0 b) When 0.50 L of propane is burnt at a constant pressure of 0.90 atm and a constant temperature of 20°C, how much heat is transferred with the surroundings? Does heat flow into the surroundings or into the system? Assume that propane behaves as an ideal gas. (3 points) 0.90
0.08206 Δ
0.50
20
0.0187
2046
0.0187
273.15
38.3 So 38.3 kJ of heat flow from the system into the surroundings c) Assume that all the heat released in part b goes into a 200. g hamburger. Assume that the heat capacity of the hamburger is 3.1 J g‐1 °C‐1 and its initial temperature is 20°C. What is the final temperature of the hamburger? (2 points) Δ
→ Δ
Tf = Ti + ΔT = 20°C +62°C = 82°C 11 of 19
200
38.3 10 °
3.1 62° 37. Given: CH3COOH(aq) ⇌ H+(aq) + CH3COOˉ(aq) Ka = 1.8x10‐5 at 25°C What is ΔG° at 25°C for the reaction CH3COOH(aq)  H+(aq) + CH3COOˉ(aq)? (2 points) a) Δ °
Δ °
27.1 /
8.314 25
273
ln 1.8 10
Consider a 25°C solution with [CH3COOH] = 0.10 M, [H+] = 2.0x10‐8 M, [CH3COOˉ] = 0.010 M b) i) What is ΔG for this solution and will the reaction spontaneously proceed to the left or right? (3 points) Δ
Δ °
Δ °
Δ
27.1kJ/mol
8.314 298
0.010
2.0 10
0.10
ln
Δ
27.1 kJ⁄mol 49.6 kJ⁄mol 22.6 kJ⁄mol Since ΔG < 0 the reaction will be spontaneous in going to the right ii) Once equilibrium has been reached, and what are the concentrations of CH3COOH, H+ and CH3COOˉ and what is the pH of the solution? (4 points) CH3COOH(sq) I 0.10 C ‐x E ⇌ H+(aq) + CH3COO‐(aq) 2x10‐8 2x10 + x 2 10
0.10
1.8 10
0.010
0.10
0.010
0.10
0.010
1.8 10
0.010 + x 0.10
2.0 10
1.8 10 x 0.010
2 10
2 10
0.010 ‐8
0.10 ‐ x 1.8 10
x 2 10
0.010
0.10
1.8 10 1.8 10
1.8 10
1.8 10
3.75 12 of 19
0.10 1.8 10 0.010 , ≪ 0.010 38. Refer to the galvanic cell below (the contents of each half‐cell are written beneath each compartment and the anode is not necessarily on the left side): Pt Pt 0.40 M Cr3+ 0.30 M Cr2O72ˉ 0.010 M H+ 0.10 M MnO4ˉ 0.20 M Mn2+ 0.010 M H+ a) What is the balanced reaction occurring in the cell on the left side? (1 point) MnO4ˉ(aq) + 8H+(aq) + 5eˉ  Mn2+(aq) + 4H2O() b) What is the balanced reaction occurring in the cell on the right hand side? (1 point) 2Cr3+(aq) + 7H2O()  Cr2O72ˉ (aq) + 14H+(aq) + 6eˉ c) What is the balanced overall reaction and E°cell? (2 points) 6MnO4ˉ(aq) + 10Cr3+(aq) + 11H2O()  6Mn2+(aq) + 5Cr2O72ˉ + 22H+(aq) E°cell = 1.51 V – 1.23 V = 0.28 V d) What is the cell potential, Ecell at 25°C? (3 points) °
0.28
ln Q
.
°
ln
ln
.
13 of 19
.
.
.
.
0.36V 39. A free particle, V(x) = 0, in one dimension has the wave function: Ψ
where a, b and k are constants. a) What is the energy operator, Hamiltonian operator, for the free particle? Note the potential energy is zero. (1 point) 2
b) Show that the wavefunction is an eigenfunction of the Hamiltonian operator? (2 points) Ψ
Ψ
2
2
2
2
Ψ
c) What is the energy of the particle (a formula in terms of numbers, (a, b, , k and m)? (1 point) The energy is the eigenvalue from part b 2
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Electronegativity Values 15 of 19
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Equations and Formulas PV = nRT 3
2
4
2
2
²
²
2
̅
8
8
3
3
T(K) = T(°C) + 273.15 R = 0.0820575 L atm mol‐1 K‐1 = 8.314 J mol‐1 K‐1 = 8.314 Pa m³ mol‐1 K‐1 k = 1.38x10‐23 J/K c = 2.998x108 m/s me = 9.11x10‐31 kg h = 6.63x10‐34 J s NA = 6.022x1023 1 u = 1.6606x10‐24 g F = 96485 C/mol 1 eV = 1.60x10‐19 J g = 9.81 m s‐2 P = hgd 1
2.178 10
²
Δ
²
²
109,677.58
1
²
1
²
2.178 10
²
²
1
²
1
a0 = 52.9 pm E = hf 
c = f h
mv
Ephoton = EBinding + Eelectron ,
2
whichin1dimensionis
∗
̂
√
,
,
∗
Page 18 of 19
2
∂
∂x
²
Equations and Formulas 1
k ²  ² 2 sinh 2 (a )
1
4k ²  ²
Decay of a particle’s wavefunction into a barrier of height Vo is given by Ψ
√2
,
2
,
sinh
q = ncmT = mcT = CT Heat capacity of water = 4.184 J g‐1 °C‐1 w = ‐PV for PV work E = q + w H = E + PV G = H – TS H =  (Energy of bonds broken) – (Energy of bonds formed) Δ °
Δ
Δ
°
Δ °
° Δ
Δ
Δ
G = H ‐TS G = G° + RT ln(Q) G° = ‐RT ln(K) = ‐nFE° °
ln Q pH = ‐log[H+] pX = ‐log[X] Kw = 1.0x10‐14 at 25°C 14 = pH + pOH at 25°C HA(aq) ⇌ H+(aq) + A‐(aq) B(aq) + H2O() ⇌ OH‐(aq) + HB+(aq) The solution to the quadratic equation ax² + bx +c =0 is ²
2
4
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2