P a g e 1 Chem 123 Practice Questions for EXAM II Spring 2014 Exam II on Wed 3/12/14 This HAS BEEN updated after Monday s lecture (3/10/14) JUST studying these questions is not sufficient preparation. There certainly will be questions that are not shown here. Before you do these practice problems, I strongly recommend you first re-do the examples from the lectures, AND the practice exercises mentioned in the PowerPoints. PLEASE DO NOT ASK ME WHAT THE CORRECT ANSWERS ARE FOR QUESTIONS WHERE YOU CAN CHECK ANSWERS EASILY BY LOOKING IT UP IN THE TEXTBOOK. Multiple Choice: Circle your one BEST answer in each of the following questions. This first question should have been on Exam I. It was left out and so a similar one will be on Exam II. 1. A new compound, vitellium oxide, has been discovered. X-ray crystallographic studies reveal that it has a cubic unit cell with a vitellium ion at each of the corner lattice points, a vitellium ion at the geometric center of the unit cell, and an oxide ion in the center of each of the cube faces in the unit cell. How many vitellium ions are there per cubic cell? How many oxide ions are there per cubic cell? Based on this structure, the correct formula should be A. Vi 3 O B. Vi 3 O 2 C. ViO D. Vi 2 O 3 E. ViO 3 2. The closest-packed structure of crystals fall into classes called A. simple cubic close packed and face centered cubic close packed B. simple cubic close packed and hexagonal cubic close packed C. cubic closest packing and hexagonal closest packing D. cubic close packed and tetrahedral close packed E. hexagonal close packed and tetrahedral close packed 3. Which represents a higher efficiency in packing? 4. Ethanol (C 2 H 5 OH) and water (H 2 O) are both liquids at or near room temperature. When a sample of ethanol is poured into a beaker containing water, the two liquids are A. immiscible because organic and non-organic compounds cannot mix. B. immiscible because each has strong hydrogen bonds that cannot be broken. C. miscible because both are liquids and all liquids are miscible. D. miscible because each can hydrogen bond with the other. 5. Concerning the separation of a solid substance into its component units (molecules or ions), A. the process is exothermic and the potential energy increases. B. the process is exothermic and the potential energy decreases. C. the process is endothermic and the potential energy decreases. D. the process is endothermic and the potential energy increases. E. the process is endothermic and occurs with no change in potential energy. 6. Which is higher in the potential energy diagram? A. the solute and solvent before mixing B. the solution (solute and solvent after mixing) C. not enough information is given 7. What is the algebraic sign for enthalpy of solution? A. positive B. negative C. not enough information is given 8. Which causes an increase in the solubility of a gas in a solvent? (The gas is nonreactive with the solvent.)
P a g e 2 A. Increasing the temperature of the solvent and simultaneously decreasing the pressure of the gas in the space above the solvent. B. Decreasing the temperature of the solvent and simultaneously increasing the pressure of the gas in the space above the solvent. C. Increasing the temperature of the solvent and simultaneously increasing the pressure of the gas in the space above the solvent. D. Decreasing the temperature of the solvent and simultaneously decreasing the pressure of the gas in the space above the solvent. E. Increasing the temperature of the solvent while maintaining the pressure of the gas in the space above the solvent at a set value. 9. During osmosis: A. pure solvent passes through a membrane but solutes do not B. pure solute passes through a membrane but solvent does not C. pure solvent moves in one direction through the membrane while the solution moves through the membrane in the other direction D. pure solvent moves in one direction through the membrane while the solute moves through the membrane in the other direction E. pure solute moves in one direction through the membrane while the solution moves through the membrane in the other direction 10. Which has the higher vapor pressure? A. pure water B. 1 m NaCl C. Neither (They are the same.) 11. Which has the higher boiling point? A. 1 m K 2 SO 4 B. 1 m NaClO 4 C. Neither (They are the same.) 12. Which has the higher freezing point? A. 0.2 M glucose B. 0.2 M NaCl 13. Which direction will the water flow? The two chambers are separated by a semipermeable membrane. 0.2 M NaCl 0.1 M NaCl A. Water will flow to the left. B. Water will flow to the right. 14. Which solution is more likely to produce a van t Hoff factor (i) that is closer to 3? A. 0.1 m K 2 SO 4 B. 0.00001 m K 2 SO 4 15. Which solution has the highest osmotic pressure at 25 C? A. 0.200 M (NH 4 ) 2 SO 4 B. 0.200 M KClO 3 C. 0.200 M C 2 H 6 O, D. 0.200 M NiF 2 E. 0.200 M Fe(NO 3 ) 3 16. In a particular study of the reaction described by the equation, 2 CH 4 O(g) + 3 O 2 (g) 2 CO 2 (g) + 4 H 2 O(g), the rate of consumption of O 2 (g) is 0.400 mol L -1 s -1. What is the rate of formation of H 2 O(g)? A. 0.300 mol L -1 s -1 B. 0.400 mol L -1 s -1 C. 0.533 mol L -1 s -1 d. 0.800 mol L -1 s -1 E. 1.33 mol L -1 s -1 17. The CO 2 gas sealed inside a carbonated beverage bottle has a pressure of 3.750 atm. At this pressure, the solubility of CO 2 in water is 0.65 g CO 2 /100 g H 2 O. If the bottle is opened, as the gas in the space above the liquid escapes, the partial pressure of the CO 2 falls to 0.30 torr. What is the solubility of CO 2 (in g per 100 g H 2 O) in the beverage at this new pressure? SHOW YOUR SETUP CAREFULLY. Ans. 6.8x10 5 gco 2 /100gH 2 O 18. How much water would you need to add to 50.0 g of NaCl in order to make a 12.5% solution by weight? Show your work clearly! 19. A 1.37 M solution of citric acid (C 6 H 8 O 7 ) in water has a density of 1.10 g/ml. a) Calculate the mole fraction of citric acid in the solution. Show your work clearly. Ans. 0.029
P a g e 3 b) Calculate the molality of this solution Ans. 1.6m 20. A solution of NaCl in water has a concentration of 19.5%. Calculate the molality of the solution. Ans. 4.14 m Hvap 1 21. The Clausius equation is lnp C R T A graph is plotted for ln P ve vaporization H vap in units of kj/mol. R is 8.3145 J/(K.mol) Show your setup carefully, including units at every step. 3 K. Calculate the heat of 22. Pressure 760 torr Temperature How will the Phase Diagram change when a nonvolatile solute is added? Draw in the new curves and show how the melting point will change due to the addition of the solute. 23. The K f for benzene is 5.12 C/m and its freezing point is 5.50 C. The freezing point of a solution containing 0.546 g of a human hormone (a nonelectrolyte) dissolved in 15.0 g of benzene is measured. The molar mass of the human hormone is 776 g/mol. a) What is the molality of this solution? b) What is the freezing point of this solution? 24. From Raoult s Law we find that the change in vapor pressure ( P) is equal to the mole fraction of the solute times the vapor pressure of the pure solvent. a) Write the equation to show this relationship. Define your terms. b) Acetone (MM 58.1 g/mol) has a vapor pressure of 162 torr at a given temperature. When a certain number of grams of a nonvolatile stearic acid (MM 284.5 g mol 1 ) is added to 156 g of acetone, the vapor pressure of acetone decreases to 151 torr. i) Calculate P. ii) Calculate the mole fraction of stearic acid (abbreviated SA). Ans. 0.068 iii) How many grams of the stearic acid was added? Ans. 55 g SA 25. Which of the following are units of reaction rate? A. grams B. moles C. g/mol D. moles/s E. none of the above 26. Why is the graph of M of reactant versus time not linear? A. The reaction slows down because the reactant loses kinetic energy over time. B. The reaction slows down because the temperature of the mixture decreases over time. C. All of the above. D. None of the above. 27. Above is a graph of M versus time. What is being measured? A. concentration of the reactant
P a g e 4 B. concentration of the product 28. In the above graph, draw in the line that shows initial rate. In the above graph, draw in the line that shows instantaneous rate half through the reaction. Assume the reaction is stopped at the end of the curve. 29. If the line drawn for an instantaneous rate contains the point (35 s, 0.0053M) and (58 s, 0.0032M), calculate the rate. Be sure to show units in your setup as well as your answer. Watch our significant figures. 30. List the five factors that affect reaction rate. 31. Why would stirring the reaction mixture affect the rate. Which of the five factors above is involved? 32. In a particular study of the reaction described by the equation, 2 CH 4 O(g) + 3 O 2 (g) 2 CO 2 (g) + 4 H 2 O(g), the rate of consumption of O 2 (g) is 0.400 mol L -1 s -1. What is the rate of formation of H 2 O(g)? A. 0.300 mol L -1 s -1 B. 0.400 mol L -1 s -1 C. 0.533 mol L -1 s -1 d. 0.800 mol L -1 s -1 E. 1.33 mol L -1 s -1 33. A chemical reaction has been the subject of intense study, and a rate law, rate = k[a][b] 2, was developed which summarized the mechanistic findings. Which change to the system will not cause an increase in the rate constant under any circumstances? A. raising the temperature by 25 degrees B. adding a catalyst C. doubling the concentration of B D. lowering the temperature by 0.5 degrees 34. For the reaction, 2 M + 2 N 2 P + Q, studies on how the initial rate of the reaction varied with concentration were carried out. The data is given below. Experiment [M] [N] Rate (mol L -1 s -1 ) 1 0.100 0.100 0.000230 2 0.100 0.200 0.000920 3 0.200 0.200 0.000920 a) Give the rate law for this reaction. To get any credit you MUST show your work to support your rate law. b) Calculate the rate constant, including units. c) What is the overall reaction order? 35. Sodium-24 is a diagnostic radioisotope used to measure blood circulation time. How much of a 20.0-mg sample remains after 1 day and 6 hours if Na-24 has a half-life of 15 hours? Show your work. 36. The rate constant for a first order decomposition reaction is 0.0111 min-1. What is the half-life of the reaction? 37. The reaction of substance A with substance C was carefully studied under conditions where the [C] remained essentially constant. The graphs of [A] vs. time and that of ln[a] vs. time both gave curves, but the graph of 1/[A] vs. time gave a straight line. What is the order of reaction with respect to A? 38. The decomposition of an aldehyde solution in carbon tetrachloride is a first order reaction with a rate constant of 1.20 x 10-3 min-1. If we start with [aldehyde] = 0.0500 M, what will the concentration be 150 minutes later? 39. What are the 3 factors that affect the number of collisions that are effective? 40. The letter A in the Arrhenius Equation is composed of two factors. Which are the two factors and explain how each factor affects the rate of a reaction? Why are there only two factors as opposed to three factors in Question 37 above? 41. Make a sketch of a kinetic energy diagram and show on the diagram the following: A) how an increase of temperature affects the curve B) how it may or may not affect the activation energy C) how temperature would affect the rate of reaction BASED ON THE DIAGRAM.
P a g e 5 42. (a) Convert Arrhenius equation k = Ae Ea/(RT) into a linear equation. Show your work. (b) Given a set of rate constants at various temperatures, what would you plot on the x-axis and the y-axis in order to determine the activation energy? Answer in full sentences. (c) If the slope is 8.67x10 3 K, what is the activation energy in kj/mole? Show your work. Ans.72.1 kj/mol 43. (6 pts) The activation energy of the reaction cyclobutane 2 ethylene is 262 kj/mol. If the rate constant is 0.032 s 1 at 525 C, what is the rate constant at 635 C? Show your work. Include units in your setup and in your answer. Ans. 4 s 1 44. Examine the integrated rate laws given. If you suspect a reaction to be first order, what would you plot on the x axis and y axis in order to get a linear graph and be able to determine the rate constant from the graph? Explain how you would get the rate constant from the graph. zero: [A] = [A] - kt first: ln[a] = ln[a] kt 1 1 second: = kt + [A] [A] o o o 45. Consider the equation given for the first order integrated equation provided. A graph is plotted for [A] vs. t and the slope is 3.2x10 5, (a) What would the units of the slope be? (b) What is the rate constant k? Reaction Mechanism 46. Answer these question based on the potential energy diagrams shown below. Potential E Fig. A Potential E Fig. B Reaction Coordinate Reaction Coordinate A) Mark onto each figure how you would measure the activation energy B) On each figure show how you would measure the enthalpy of reaction. C) Which reaction is faster? A or B D) Which reaction is endothermic? A or B Exam II will cover only up through Question 46. Questions 47 and 48 will be on the next exam.
P a g e 6 47. PE Reaction Coordinate Indicate in the graph above where the following is: (a) Activation energy of the rate determining step. (b) intermediate(s) if any (c) transition state(s) if any (d) H of the overall reaction (e) Is this an endothermic or exothermic reaction? (f) In the reverse reaction, which step is rate determining? 48. The destruction of ozone occurs via a 2-step mechanism, where X can be any of several species: (1) X + O 3 XO + O 2 (slow) (2) XO + O X + O 2 (fast) a) Write the overall reaction. (b) Write the rate law for each step. (c) Are there any intermediates in this reaction? (d) Is a catalyst involved in this reaction? (e) What would you expect the experimental rate law to look like?
P a g e 7 46. (2 pts) Can you tell from a potential energy diagram below whether the reaction is reversible or not? Which of the following diagrams show a reversible reaction? Circle as many as applies: A B C D PE PE PE PE Rxn Coordinate Rxn Coordinate Rxn Coordinate Rxn Coordinate A B C D 47. In the PE Diagram below, which is the rate determining step? PE A. Step 1 B. Step 2 Rxn Coordinate This will be updated after Monday s lecture on 3/10/14