BCIT Winter 2013 Chem 0012 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. Constants, equations, data sheets and tables are provided. Total points = 50 Page 1 of 12
Section I: Multiple choice (25 points total, 1 point each) Choose the BEST answer to the following questions. 1. Which of the following reactions would have the greatest reaction rate at room temperature? a. C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) b. Ca(s) + 2H2O()→ Ca(OH)2(aq) + H2(g) c. AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq) d. Na2CO3(s) + 2HCl(aq) → 2NaCl(aq) + H2O() + CO2(g) 2. An activated complex can be described as a. A particle of maximum kinetic energy and minimum potential energy b. A stable particle found in a reaction mechanism c. An unstable particle that is neither reactant or product d. A particle which is first used and then regenerated in a reaction mechanism 3. Consider the following reaction mechanism and overall reaction: ½O2(g) + NO(g) → NO2(g) NO2(g) → NO(g) + O(g) O(g) + O2(g) → O3(g) 3
/2 O2(g) → O3(g) What is the catalyst in this mechanism? a. O(g) b. NO(g) c. NO2(g) d. O2(g) 4. Two substances are mixed and no reaction occurs. With respect to enthalpy and entropy, which of the following could explain why no reaction occurs? Enthalpy Entropy a. increases increases b. increases decreases c. decreases increases d. decreases decreases Page 2 of 12
5. Which of the following could result in an increase in reaction rate? a. An increase in the activation energy b. An increase in the reaction enthalpy c. An increase in the frequency of collisions d. An increase in the potential energy of the activated complex 6. Consider the following equilibrium: PCl3(g) + Cl2(g) ⇌ PCl5(g) If the volume of the system is decreased, how will the reaction rates in the new equilibrium compare with the rates in the original equilibrium? Forward Rate Reverse Rate a. Increases Increases b. Increases Decreases c. Decreases Decreases d. Decreases Increases 7. For the equilibrium: Cu(s) + 2Ag+(aq)⇌2Ag(s) + Cu2+(aq) We would know the system is at equilibrium because a. [Cu2+] = [Ag+] b. 2[Cu2+] = [Ag+] c. the mass of the Cu(s) remains constant d. the mass of the entire system remains constant 8. Consider the following equilibrium: H = ‐36 kJ H2(g) + Br2(g) ⇌ 2HBr(g) How could the value of Keq be increased? a. add H2 b. add HBr c. increase the pressure d. reduce the temperature Page 3 of 12
9. What is Keq expression for the following equilibrium? 2KClO3(s) ⇌2KCl(s) + 3O2(g) a.
b.
c.
d.
10. Consider the following equilibrium: MgCO3(s) ⇌ Mg²⁺(aq) + CO3²ˉ(aq) Adding which of the following would cause the solid to dissolve? a. HCl b. K2CO3 c. MgCO3 d. Mg(NO3)2 11. Which of the following will not form a precipitate when mixed with an equal volume of 0.2 M AgNO3?: a. 0.2 M NaBr b. 0.2 M NaIO3 c. 0.2 M NaNO3 d. 0.2 M NaBrO3 12. Which of the following is least soluble in water? a. CuI b. BeS c. CsOH d. AgBrO3 13. Which of the following 1.0 M solutions will have the highest electrical conductivity? a. HI b. HF c. HCN d. HNO2 Page 4 of 12
14. Consider the following equilibrium: HOI + Fˉ ⇌ OIˉ + HF Reactants are favoured in this equilibrium. Which of the following describes the relative strengths of the acids and bases? Stronger Acid Stronger Base a. HF Fˉ b. HF OIˉ c. HOI Fˉ d. HOI OIˉ 15. Consider the following equilibrium: H2CO3(aq) + H2O() ⇌ H3O⁺(aq) + HCO3ˉ(aq) What is the equilibrium expression? a.
b.
c.
d.
16. The HC2O4ˉ(aq) ion will act as a. A base since Ka < Kb b. A base since Ka > Kb c. An acid since Ka < Kb d. An acid since Ka > Kb 17. Which of the following describes the net ionic equation for the hydrolysis of a NaNO2 solution? a. NaNO2(s) ⇌ Na⁺(aq) + NO2ˉ(aq) b. NO2ˉ(aq) + H2O()⇌ HNO2(aq) + OHˉ(aq) c. Na⁺(aq) + 2H2O()⇌ H3O⁺(aq) + NaOH(aq) d. NaNO2(s) + H2O() ⇌ NaOH(aq) + HNO2(aq) Page 5 of 12
18. What is the approximate pH and Ka at the transition point for phenol red? a. pH = 6.6, Ka = 3x10‐7 b. pH = 7.3, Ka = 1x10‐14 c. pH = 7.3, Ka = 5x10‐8 d. pH = 8.0, Ka = 1x10‐8 19. What volume of 0.100 M NaOH is required to completely neutralize 15.00 mL of 0.300 M H3PO4? a. 5.00 mL b. 15.0 mL c. 45.0 mL d. 135 mL 20. Consider the following buffer equilibrium: H2CO3(aq) + H2O() ⇌ H3O⁺(aq) + HCO3ˉ(aq) What happens when a small amount NaOH(aq) is added? a. [H3O⁺] increases, then the equilibrium shifts to the left b. [H3O⁺] decreases, then the equilibrium shifts to the left c. [H3O⁺] increases, then the equilibrium shifts to the right d. [H3O⁺] decreases, then the equilibrium shifts to the right 21. What is the oxidation number of iron in magnetite, Fe3O4 a. +4/3 b. +2 c. +8/3 d. +3 22. Which of the following is more difficult to reduce than the H⁺(aq) ion? a. I2 b. Ag⁺ c. Zn²⁺ d. Cu²⁺ Page 6 of 12
Use the following diagram to answer questions 23 to 25 23. In the above galvanic cell, which of the following best describes the movement of the electrons through the wire? a. They move from left to right towards the anode. b. They move from right to left towards the anode c. They move from right to left towards the cathode. d. They move from left to right towards the cathode. 24. In the above cell, which of the following best describes the change in mass of the electrodes? Anode Mass Cathode Mass a. decreases increases b. decreases stays constant c. stays constant increases d. stays constant stays constant 25. What is the standard voltage (E°) for the cell? a. 0.43 V b. 0.77 V c. 1.34 V d. 1.68 V Page 7 of 12
Section II: Written problems (25 points total). 26. The rate law for the decomposition of HI to I2 and H2 is second order with respect to HI. When the HI concentration was 0.0558 M the rate of reaction was found to be 2.5x10‐4M s‐1. What is the value of the rate constant k? (2 points) Rate = k[HI]² ²
2.5 10 0.0558 ²
0.080
27. Use the potential energy diagram to answer the following: (3 points) a. What is the activation energy of the forward reaction? 60 kJ b. What is ΔH for the forward reaction? ‐20 kJ Exothermic c. Is the forward reaction endothermic or exothermic? 28. a) Write the balance formula for the reaction between Na3PO4(aq) and CuCl2(aq). (1 point) 2 Na3PO4(aq) + 3 CuCl2(aq)  Cu3(PO4)2(s) + 6 NaCl(aq) b) Write the net ionic equation for the reaction between Na3PO4(aq) and CuCl2(aq). (1 point) 2 PO43‐(aq) + 3 Cu2+(aq)  Cu3(PO4)2(s) Page 8 of 12
29. Consider the following equilibrium: Fe3+(aq) + SCN‐(aq) ⇌ FeSCN2+(aq) Initially, 50.0 mL of 0.100 M Fe³⁺ is added to 30.0 mL of 0.200 M SCNˉ. At equilibrium, the concentration of FeSCN²⁺ is found to be 0.0500 M. Calculate the value of Keq for the equilibrium. (4 points) . .
. 30.0
50.0
0.0625 . 0.200
30.0
0.0750 + SCNˉ ⇌ FeSCN²⁺ Fe³⁺ I0.0625 0.0750 0 C‐0.0500 ‐0.0500 0.0500 E0.0125 0.0250 0.0500 ²⁺
³⁺
¯
0.0500
0.0125 0.0250
1.60 10² 30. A saturated solution of nickel carbonate, NiCO3, contains 0.090 g in 2.0 L solution. Calculate Ksp for NiCO3. (3 points) NiCO₃(s)⇌ Ni²⁺(aq) + CO₃²ˉ(aq) 0.090 2.0
Ksp = [Ni²⁺][ CO₃²ˉ] = (3.8x10‐4)(3.8x10‐4) = 1.4x10‐7 Page 9 of 12
118.7 3.8 10 31. An acid‐base reaction occurs between HSO3ˉ and IO3ˉ. a. Write the equation for the equilibrium that results (1 point) HSO3ˉ + IO3ˉ ⇌ SO3²ˉ + HIO3 b. Identify one conjugate acid‐base pair in the reaction. (1 point) HSO3ˉ and SO3²ˉ or IO3ˉ and HIO3 c. Are the reactants or products favoured? Explain your reasoning. (2 points) Reactants are favoured Ka (HIO3) > Ka(HSO3ˉ) 32. Balance the following redox reaction in acidic solution: (3 points) Sb + NO3ˉ  Sb2O5 + NO 3(5 H2O + 2 Sb → Sb2O5 + 10 H⁺ + 10 eˉ) 10(4 H⁺+ NO₃ˉ + 3 eˉ → NO + 2 H2O) 6 Sb + 10 NO3ˉ + 10 H⁺ → 3 Sb2O5 + 10 NO +5 H₂O Page 10 of 12
33. About now you may be feeling the need for an aspirin. An extra strength aspirin tablet is dissolved in 250. mL of water. An extra strength aspirin tablet contains 500. mg of ASA (HC9H7O4). ASA has a Ka of 3.3x10‐4. The molar mass of ASA is 180.157 g/mol. What is the pH of the solution? (4 points) 1
250
500
1000
1000
1.11 10
⇌ H⁺(aq) + HC9H7O4 I ‐2
1.11x10 0 0 C ‐x x x E 1.11x10‐2 – x x x C9H7O4ˉ(aq) ²
1.11 10
3.3 10 x² = ‐3.3x10‐4x + (3.3x10‐4)(1.11x10‐2) 0 = x² + 3.3x10‐4x – 3.66x10‐6 3.3 10
3.3 10
2
4
3.66 10
x = 1.76x10‐3 or ‐2.09x10‐3 but x must be positive since x = [H⁺] Therefore x = [H⁺] = 1.76x10‐3 M pH = ‐log(1.76x10‐3) = 2.76 Page 11 of 12
180.157 Chem 0012 Equations and Constants PV = nRT T(K) = T(°C) + 273.15 R = 0.0820575 L atm mol‐1 K‐1 = 8.314 J mol‐1 K‐1 Δ
Kp = Kc(RT) n 1 atm = 760 mm Hg = 760 Torr = 101.325 kPa pH = ‐log[H+] pOH = ‐log[OHˉ] [OHˉ] = 10‐pOH [H+] = 10‐pH pX = ‐log(X) X = 10‐pX Kw = 1.0x10‐14 @ 25°C 14.00 = pH + pOH @ 25°C Kw = Ka Kb  [ A ] 
 pH  pK a  log
 [ HA] 
F = 96485 C/mol °
°
ln
.
log
at 25°C The solution to the quadratic equation ax² + bx +c = 0 is x
 b  b 2  4ac
2a
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