Chem. 1B Midterm 2 Version A

Transcription

1 Chem. 1B Midterm 2 Version A First letter of last name Name Student Number All work must be shown on the exam for partial credit. Points will be taken off for incorrect or no units. Calculators are allowed. Cell phones may not be used for calculators. On fundamental and short answer problems you must show your work in order to receive credit for the problem. If your cell phone goes off during the exam you will have your exam removed from you. Fundamentals (of 36 possible) Problem 1 (of 16 possible) Problem 2 (of 18 possible) Multiple Choice (of 30 possible) Midterm Total (of 100 possible) 1

2 Fundamental Questions Each of these fundamental chemistry questions is worth 6 points. You must show work to get credit. Little to no partial credit will be rewarded. Make sure to include the correct units on your answers. 1) 6 pts Consider this reaction: 2CH3OH(g) +3O2(g) 2CO2(g) + 4H2O(l) At a certain temperature it obeys this rate law. Rate= 3.30 L mol s [CH3OH]2 Suppose a vessel contains CH3OH at a concentration of 1.20 M. Calculate the concentration of CH3OH in the vessel seconds later. You may assume no other reaction is important. 2) 6 pts The rate constant of the reaction O(g) + N2(g) NO(g) + N(g) is L kj at 800. C. The activation energy of the reaction is 315 mol s mol Determine the rate constant at 700. C. 3) 6 pts A voltaic cell, at 25 C, represented by the following cell diagram has Ecell =1.45 V. What must be [Ag + ] in the cell? Zn(s) Zn 2+ (1.00M) Ag + (x M) Ag(s) 2

3 4) 6 pts If the rate law was determined to be rate k[ A] [ B] And the following data was collected [A]o (M) [B]o (M) Initial Rate( mol Ls What is k? Remember to include units. ) 5) 6 pts Draw the following galvanic cell: Pb(s) + Cl2(g) Pb 2+ (aq) + 2Cl - (aq) Calculate E, show the direction of electron flow and the direction of ion migration through the salt bridge, and identify the cathode and anode. 6) 6 pts How much time is required to electroplate 1.50 g of silver from a silver nitrate solution by using a current of 13.6 ma? 3

4 Short Answer Questions Each of the following short answer questions are worth the noted points. Partial credit will be given. You must show your work to get credit. Make sure include proper units on your answer. 1) 16 pts Consider the cell described below: Al Al 3+ (1.00 M) Pb 2+ (1.00 M) Pb Calculate the cell potential after the reaction has operated long enough for the [Al 3+ ] to have change by 0.60 M. (Assume T=25 C) 4

5 2a) 10 pts In the gas phase, production of phosgene from chlorine and carbon monoxide is assumed to proceed by the following mechanism: k 1 Cl 2 2Cl k 1 k 2 Cl + CO COCl k 2 COCl + Cl 2 k 3 COCl 2+ Cl Slow 2Cl k 4 Cl 2 Fast Write the rate law for this reaction. Fast equilibrium Fast equilibrium 2b) 8 pts Is this rate law consistent with the following experimental data. Expt. [CO] (M) [Cl 2] (M) Rate ( mol L s You must show what the rate law is for this data in order to get credit. 5

6 Multiple Choice Questions Each of the following multiple choice questions are worth 5 points. Your answers need to be filled in on the Scantron provided. Note: Your Scantrons will not be returned to you, therefore, for your records, you may want to mark your answers on this sheet. On the Scantron you need to fill in your perm number, test version, and name. Failure to do any of these things will result in the loss of 1 point. Your perm number is placed and bubbled in under the ID number. Do not skip boxes or put in a hyphen. In addition, leave bubbles blank under any unused boxes. The version number (A) is bubbled in under the test form. Use the following to answer question 1: You make a galvanic cell with a copper electrode in a solution of copper nitrate and a silver electrode in a solution of silver nitrate. Cu e Cu E = 0.34 V Ag + + e Ag E = 0.80 V 1. If you could increase the concentration of Ag +, which of the following would be true about the cell potential? A) It would remain constant. B) This cannot be determined. C) It would decrease. D) It would increase. 2. A galvanic cell is constructed with copper electrodes and Cu 2+ in each compartment. In one compartment, [Cu 2+ ] = M, and in the other compartment, [Cu 2+ ] = 1.4 M. Calculate the potential for this cell at 25 C. The standard reduction potential for Cu 2+ is V. A) V B) 0.42 V C) V D) 0.42 V 3. In which of the following cases can E be equal to zero? I. In any cell at equilibrium II. In a concentration cell III. E can never be equal to zero. A) III B) I only C) I and II D) II only 6

7 4. When the equation for the following reaction in basic solution is balanced, what is the sum of the coefficients? MnO4 (aq) + CN (aq) MnO2(s) + CNO (aq) A) 20 B) 13 C) 8 D) 10 Use the following to answer questions 5-6: For the reaction A products, select the reaction order(s) that best fit(s) the observations. 5. A plot of [A] 2 vs. t gives a straight line. A) second order in A B) zero order in A C) first order in A D) all of these 6. The half-life decreases over time. A) second order in A B) zero order in A C) first order in A D) all of these 7