Placement Exam for Exemption from Chemistry 120 Fall 2006

Transcription

1 Placement Exam for Exemption from Chemistry 120 Fall 2006 Name: By submitting this exam you affirm that you have not consulted any person, book, website or any other source for assistance. You may use a calculator. Please attempt every question whether or not you think that you can answer it completely. The purpose of the exam is placement, and we need to see how you approach each question. Please be brief. One or two sentences is usually adequate for any question that requires an explanation. Please your completed exam, as a Word or pdf file, with the subject Chemistry 120 Exemption, to chemplacement@wellesley.edu by 5:00 p.m. on Wednesday August 31, You may wish to scan handwritten work and then incorporate that into your Word or pdf file. The time limit on the exam is 3 hours.

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3 Potentially Useful Information Avogadro's number = h = J s c = m/s 1amu = kg 1MeV = J 1Å=10 10 m c E = λν = hν 2 E = mc h h λ = = p m v h ( x)( p ) 4 π 1 2 m v = h ν φ J E n = 2 n E = q + w w = P V H = E + PV q = H q = mc ( T ) q = C ( T ) K S o c RT = K P p c o surr H = T G = H TS sys n G = H T S G = G + RT ln P G = G + RT ln Q G = RT ln K H S ln K = + RT R K = C R = J mol 1 K 1 = liter atm mol 1 K 1 = cal mol 1 K 1 1 atm = 760 Torr PV = nrt aa +bb cc +dd d A d C rate = 1 [ ] 1 [ ] = a dt c dt x Rate = k [ A] [ B ] 1 1 = kt [ A ] [ A ] ln[ A] = kt + ln[ A ] [ A] = kt + [ A ] t 1 / 2 t 1 / 2 t 1 / = k [ A ] [ A ] = 2 k = k k = A e Ea RT 0 y 0

4 E = IR pk = log K Q I = t I =96, C mole G = RT ln K E = E E o o o cell cathode anode welec welec w = QE = nfe rev, elec = G G = nfe cell o RT Ecell = ln K nf o RT Ecell = Ecell ln Q nf 1 + ph = log[h ] poh = log[oh ] pk w = ph + poh = 14 K h = K K w a ph = p K u rms = E P a [anion] +log [acid] 0 3 RT M = 3 2 RT = X P A A T 0 o V E cell = E cell log Q n

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6 Question 1 (8 points) (a) (6 points) The gas in interstellar space consists primarily of hydrogen atoms (either neutral or ionized) at a concentration on the order of one atom per cubic centimeter. The hydrogen in the interstellar space near a star is largely ionized by the high energy photons from the star. Calculate the longest wavelength of light that will ionize a ground state hydrogen atom. (b) (2 points) What do the two cones represent in the diagram below? (c) (2 points) What do the + and signs signify?

7 Question 2 (8 points) Use Lewis structures and VSEPR to predict the shapes and bond angles of the following two chemical species. In addition, specify whether the molecule or ion will possess a dipole moment and indicate the formal charges, if any. The central atom is underlined in each case. (1) POCl 3 (2) BrF 3

8 Question 3 (8 points) When a photon of light hits the cis isomer of retinal, it is converted to the trans retinal. This process initiates an electrical impulse that is perceived by the brain as seeing light. The incomplete Lewis structure for cis and trans retinal are shown below. The carbon atoms labeled 2, 3, and 4 are attached to two hydrogen atoms each (not shown). The H atom attached to carbon 14 is also not shown in the cis retinal. The conversion of cis to trans retinal involves rotation about the bond between C 11 and C 12. None of the atoms in the structures shown below contain a formal charge. (a) Complete the Lewis structure of either the cis or the trans retinal (6 points) (b) Identify all the atoms that have sp 3 hybridization by circling them for one of the above structures. (4 points)

9 Question 4 (8 points) (a) In the simplest approach to MO theory, linear combinations of atomic orbitals are used to form molecular orbitals. Draw a diagram to demonstrate this concept for the specific case of the π antibonding molecular orbital in a homonuclear diatomic molecule. Make sure you completely label the atomic and molecular orbitals in your diagram and indicate the signs of the wavefunctions (5 points). (b) Calcium carbide (CaC 2 ) is an intermediate in the manufacture of acetylene (C 2 H 2 ). It is the calcium salt of the carbide ion ( C ). Sketch a complete MO diagram for the carbide ion, 2 2 showing the relative energy levels of the atoms and the bonding and antibonding orbitals of the molecular ion. Represent with arrows the electron occupation of the molecular orbitals (5 points)

10 Question 5 (9 points) At a certain temperature, the partial pressures of an equilibrium mixture of N 2 O 4 (g) and NO 2 (g) are 0.34 atm and 1.20 atm, respectively. N 2 O 4 (g) 2NO 2 (g) The volume of the container is instantaneously doubled (Hint: what happens to the individual pressures?) without changing the temperature. (i) (2 point) Does the equilibrium constant change, and if so how? (ii) (2 point) What happens to the equilibrium position? (iii) (6 points) Set up an equation involving the equilibrium constant to find the partial pressures of the two gases when a new equilibrium is reached. Do not solve the equation. There should be only one unknown in your final answer.

11 Question 6 (8 points) (a) (2 points) Discuss the validity of the following statement: A bimolecular reaction is often a second order reaction while a second order reaction is always bimolecular. (b) (6 points) The following reaction has a second order rate law: Rate = k [H 2 ][ICl] (c) H 2 (g) +2ICl(g) I 2 (g) + 2HCl (g) HI has been found to be an intermediate in this reaction. Devise a possible reaction mechanism.

12 Question 7 (8 points) (a) (5 points) What is the mass of the diethylamine (C 2 H 5 ) 2 NH, in ml of an aqueous solution if it requires ml of M HCl to titrate it to the equivalence point? (b) (5 points) What is the ph at the equivalence point if K b = ?

13 Question 8 (8 points) a. (3 points) Describe an experiment that would enable you to determine which electrode is the cathode and which electrode is the anode in an electrochemical cell using copper and zinc electrodes. b. (5 points) In a neuron (a nerve cell), the concentration of K + ions inside the cell is 30 times as great as that outside. Calculate the potential difference between the inside and outside of the cell if the difference is due only to imbalance of potassium ions.

14 Question 9 (9 points) grams of NaCH 3 COO are added to L of a 0.1 M solution of CH 3 COOH at 25 C. (a) (3 points) Calculate the ph of this solution using the Henderson Hasselbalch equation. (b) (3 points) What is the ph of the solution after the addition of mole of HCl to the buffer. (c) (3 points) In what way is the Henderson Hasselbalch equation merely an approximation to a more accurate treatment?

15 QUESTION 10 (8 points) (a) (7 points) The diffusion of gases is often illustrated by the following lecture demonstration: two cotton plugs, one soaked in ammonia (NH 3 ) and the other soaked in hydrochloric acid (HCl) are simultaneously placed at the ends of a long tube. A white ring of ammonium chloride (NH 4 Cl) forms where the NH 3 and HCl molecules meet several minutes later. The distances traveled by the NH 3 and HCl molecules can be measured, and their ratios obtained. Using the kinetic molecular model for gases, what value do you predict for this ratio? (10 points) (b) (3 points) The diffusion of gases through the tube is surprisingly slow in light of the fact that the velocities of HCl and NH 3 molecules at 25 C are about 450 and 660 meters per second, respectively. Why does it take several minutes for the NH 3 and HCl molecules to meet? (5 points)

16 QUESTION 11 (8 points) In hemoglobin the iron ion is bound to four nitrogen atoms from the poryphin ring, the nitrogen in a histadine residue, and a sixth ligand that is oxygen, a strong ligand, in the case of oxyhemoglobin, and water, a weaker ligand, in deoxyhemoglobin. Oxyhemoglobin is diamagnetic, while deoxyhemoglobin is diamagnetic to the extent of four unpaired electrons. Using appropriate diagrams, explain these results. What is the oxidation state of iron in the two forms of hemoglobin? Does the addition of oxygen result in oxidation of the iron?