Exam 2 with Answers Key - Principles Chemistry II | CHM 212, Exams of Chemistry

Download Exam 2 with Answers Key - Principles Chemistry II | CHM 212 and more Exams Chemistry in PDF only on Docsity! Chemistry 212 Key 2 Spring 2004 1. The equilibrium expression for the reaction: Ni(CO)4 (l) Ni(s) + 4 CO(g) is: a) Keq = ][Ni(CO) [CO] 4 4 c) Keq = ][Ni(CO) [Ni][CO] 4 4 e) Keq = 4 4 [Ni][CO] ][Ni(CO) b) Keq = ][Ni(CO) 1 4 d) Keq = [CO] 4 Problem 15.7c 2. For the equilibrium H2 (g) + I2 (g) 2 HI(g) which of the following would result in a shift of the equilibrium to the right? a) Increasing pressure c) Adding HI e) Adding NaOH b) Adding a catalyst d) Decreasing pressure 3. At equilibrium, which is/are true? a) All chemical processes have stopped. b) The rate of the forward reaction equals that of the reverse. c) The rate constant for the forward reaction equals that of the reverse. d) The sum of the concentrations of the reactants equals that of the products. e) None of the above. 4. The reaction CO + 2 H2 CH3OH (all gases) has an equilibrium constant equal to what value, if at equilibrium in a 2.00 L vessel, there are 0.302 atm H2, 0.170 atm CO, and 0.0406 atm CH3OH present. a) 0.0955 b) 1.58 c) 2.61 d) 10.5 e) 26.2 5. A Brønsted-Lowery acid is: a) a proton acceptor. c) a proton donor. e) an electron pair donor. b) an electron pair acceptor. d) a substance that ionizes to produce protons. 6. Which of the following is a weak base? a) KOH b) Sr(OH)2 c) NH2OH d) NaNH2 e) Mg(OH)2 7. In which of the following would HCl be the strongest acid? a) pyridine(l) b) HOAc(l) c) H2O(l) d) NH3 (l) e) 1.0 M NaOH(aq) 8. What is the [H+] of a HCl solution that has a pH = 3.21? a) 6.2 x 10-4 M b) 4.0 x 10-2 M c) 3.2 M d) 25 M e) 1600 M Discussion questions 1. Define the following: (20 points) Lewis acid – an electron pair acceptor amphoteric – molecules that can act both as an acid and base homogeneous equilibrium – an equilibrium in which all components are in the same phase reaction order – the sum of the exponents (x + y) in a rate law 2. At 373 K, Keq = 0.416 for the equilibrium 2 NOBr 2 NO + Br2 (all gases). If the pressures of NOBr and NO are equal, what is the equilibrium pressure of Br2? (6 points) Let PNOBr = PNO = x, then PBr2 = ½x 0.416 = ½x (x) (½x)(x) P PP 2 2 2 NOBr Br2 2 NO == x = 0.832 atm ⇒ PBr2 = ½(0.832 atm) = 0.416 atm (Problem 15.37) 3. Consider an equilibrium whose forward reaction is exothermic. Which way will the equilibrium shift if the reaction is heated? Provide a physical explanation for your prediction. (6 points) In exothermic reactions, heat is released. Le Châtelier’s principle tells us that if heat is added, the reaction will shift to consume some of this heat, so the reaction will shift in the reverse direction. The energy diagram below shows equilibrium of an exothermic reaction: reactants products Ea(forward) E n e r g y Reaction progress Ea (reverse) In an exothermic reaction a higher percentage of reactions in the forward direction have sufficient energy to cross the barrier than do reactions moving in the reverse direction. The physical rationale for the prediction is that when heat is added it will affect more reactions going in the reverse direction than the forward reaction because more of them are lacking the necessary energy to go to completion. There will be more reactions in both directions, but there will be a relative increase in the reverse direction. 4. Explain why HNO3 is a stronger acid than HNO2. (6 points) Nitric acid is a stronger acid than nitrous acid because the nitrogen in the former is in the +5 oxidation state as opposed to +3 for nitrous acid. The higher oxidation state on nitrogen in HNO3 polarizes the N-O bond (and hence the O-H bond) more. This makes it easier for an H+ ion to separate from the molecule. (Problem 16.81)