First initial of last name Chem. C Midterm Version B April 6, 07 Name: Print Neatly. You will lose point if I cannot read your name or perm number. Perm Number: All work must be shown on the exam for partial credit. Points will be taken off for incorrect or missing units. Calculators are allowed. Cell phones may not be used as calculators. On fundamental and challenge 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 (of 7 possible) Problem (of 7 possible) Multiple Choice (of 30 possible) Midterm Total (of 00 possible)
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 awarded. Make sure to include the correct units on your answers. ) 6 pts Circle the following molecule/atom that fits the description Most Polarizable Cl I Greatest Viscosity CH 3CH OH CH 3OCH 3 Least Surface Tention CCl 4 CHCl 3 Highest Bioling Point C 5H C 0H Lowest Freezing Point Highest Vapor Pressure (at a given temperature) KI HF ) 6 pts Complete the table below, which lists information about some diatomic molecules or molecular ions. In particular: Decide whether each molecule is stable or not Decide whether each molecule would be diamagnetic or paramagnetic. Calculate each molecule s bond order. molecule stable? diamagnetic or paramagnetic bond order + O yes diamagnetic (8 no paramagnetic ) = 3 Be yes diamagnetic ( ) = 0 no paramagnetic B yes diamagnetic (4 no paramagnetic ) = + O (0 V.E.) Be (4 V.E.) B (6 V.E.) σ * p π * p πp σp σ * s σs σ * p π * p 3) 6 pts List three ways that Lewis/VSPER and the Local Electron Model break down (ways in which they incorrectly predict bonding or bonding properties). σp σ * s σs πp σ * p π * p σp πp σ * s σs ) Incorrect magnetic properties of O ) Not all molecules can be explained (B H 6) 3) Does not account for resonance
4) 6 pts Carbon tetrachloride (CCl 4) has a vapor pressure of 3 torr at 40.0 C and 836 torr at 80 C. What is the normal boiling point of CCl 4? ln ( P vap ) = H vap P vap R ( ) T T 3 torr ln ( 836 torr ) = H vap J 8.345 H vap = 3,400 J mol Find the normal boiling point mol K J mol J 8.345 mol K 3 torr ln ( ) = 3,400 760 torr ( 353K 33K ) ( T 33K ) T = 350. K = 77 5a) 3 pts 3- What is the molecular shape and angle(s) of the PO 3 ion? You must draw the Lewis structure to get credit. Shape: Trigonal pyramidal Angle: <09.5 5b) 3 pts What is the molecular shape and angle(s) of TeCl 4? Use the structure that minimizes formal charge. You must draw the Lewis structure to get credit. Shape: Seesaw Angle: <90, <0 6) 6 pts What type of orbital(s) (example: s orbital) overlap to make what type of bond(s) - in CO 3? If there are any lone pair(s) of electrons, what type of orbitals are they - in? In other words give the local electron description of the bonding in CO 3. The carbon atom forms one σ bond to each of the single bonded oxygen atoms. This bond is formed from the overlap of a sp hybridized orbital on the carbon atom and a sp 3 hybridized orbital on the single bonded oxygen atoms. A third σ bond is formed from the overlap of an sp hybridized orbital on the carbon atom and a sp hybridized orbital on the double bonded oxygen atom. The π bond between the double bonded oxygen atom and the carbon atom is formed from the overlap of the unhybridized p orbitals on both the carbon and oxygen atoms. The loan pair electrons on the double bonded carbon sit in sp hybridized orbitals and the loan pair electron on the single bonded oxygen atoms sit in sp 3 hybridized orbitals. 3
Challenge Problems 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 to include proper units on your answer. ) 7 pts Many important compounds in the chemical industry are derivatives of ethylene (C H 4). Two of them are acrylonitrile and methyl methacrylate. Acrylonitrile Methyl methacrylate Complete the Lewis structures for these molecules, showing all lone pairs. Give approximate values for bond angles a through f, and give the hybridization of all carbon atoms. In acrylonitrile and methyl methacrylate indicate which atoms in each molecule must lie in the same plane. How σ bonds and how many π bonds are there in acrylonitrile and methyl methacrylate? 3(H) N 3(C) Total Valence e - 3() 5 3(4) 0 H H Wanted e - 3() 8 3(8) 38 C C wanted valence 38 0 H # bonds = = = 9 C # e = valence 3(#bonds) = 0 (9) = N Carbon hybridization (listed in the order that they appear in the structure) = sp, sp, sp Bond angles: a= 0., b=0., c=80. σ bonds = 6 π bonds = 3 All atoms are in the same plane. 8(H) O 5(C) Total Valence e - 8() (6) 5(4) 40 Wanted e - 8() (8) 5(8) 7 wanted valence 7 40 # bonds = = = 6 # e = valence 3(#bonds) = 40 (6) = 8 Carbon hybridization (listed in the order that they appear in the structure) = sp, sp, sp 3, sp, sp 3 Bond angles: d = 0., e = 0., f = <09.5 σ bonds = 4 π bonds = Atoms with circles around them have to be in the same plane. Atoms with squares around them have to be in the same plane. Depending on the rotation, the atoms with the circles and the atoms with the squares can be in the same plane. 4
a) 0 pts The density of cesium chloride is 3.988 g cm 3. The unit cell of cesium chloride is shown to the right; the dark spheres are the chloride ions and the light spheres are the cesium ions. Calculate the percentage of empty space in the cesium chloride lattice with the ions treated as hard spheres. The ionic radii of Cs + is 70 pm and of Cl - is 8 pm. Determine the volume of the unit cell. The unit cell has Cs atom and Cl atom Determine the mass of both the atoms mol Cs g Cs atom( 6.0 0 3 )(3.9 ) =. atoms mol Cs 0 g mol Cl 35.45 g Cl atom ( ) ( ) = 5.89 6.0 0 3 atoms mol Cl 0 3 g m tot =. 0 g + 5.89 0 3 g =.80 0 g V cell =.80 0 g ( cm3 ) = 7.0 3.988 g 0 3 cm 3 Determine the volume that the atoms take up Determine the volume of Cs atoms and Cl atom r Cs + = 70 pm ( m cm ) 0 (00 ) =.70 pm m 0 8 cm V Cs + = 4 3 πr3 = 4 π(.70 3 0 8 cm) 3 =.06 0 3 cm 3 m cm ) 0 (00 pm r Cl = 8 pm ( ) =.8 m 0 8 cm V Cl = 4 3 πr3 = 4 π(.8 3 0 8 ) 3 =.49 0 3 cm 3 V filled =.06 0 3 cm 3 +.49 0 3 cm 3 = 4.55 0 3 cm 3 Determine the percentage empty space V empty = V cell V filled = 7.0 0 3 cm 3 4.55 0 3 cm 3 V empty =.47 0 3 cm 3 (.47 0 3 cm 3 7.0 0 3 cm3) 00% = 35.% b) 4 pts Calculate the metallic/covalent radii of cesium. Cesium forms a body centered cubic structure with unit cell side of 64. pm. For body centered cubic structures there are four touching radii if you cut through the upper corner of the unit cell to the far back corner of the unit cell (see picture). Therefore (4r) = a + ( a) = 3a r = 3 4 3 a = (64.5 pm) = 66 pm 4 c) 3 pts Compare the size of the ionic (given a) and the metallic/covalent (calculated in b) radii of cesium. Justify the size difference for the two radii. Ionic Radii = 70pm, metallic/covalent radii = 66pm The ionic radii is smaller than the metallic/covalent radii because to form an ion the cesium lost one electron which results in a smaller electron cloud/radii. 5
Multiple Choice Questions On the ParScore form you need to fill in your answers, perm number, test version, and name. Failure to do any of these things will result in the loss of point. Your perm number is placed and bubbled in under the ID number. Do not skip boxes or put in a hyphen; unused boxes should be left blank. Bubble in your test version (B) under the test form. Note: Your ParScore form will not be returned to you, therefore, for your records, you may want to mark your answers on this sheet. Each multiple choice question is worth 5 points.. Of the following, which molecule has the smallest bond angle? A) NF3 B) SCl C) SO D) CCl4 E) NO -. Which of the following statements is correct? A) σ bonds result from the head-to-head overlap of atomic orbitals. B) π bonds have electron density on the internuclear axis. C) A triple bond is composed of two σ bonds and one π bond. D) Free rotation may occur about a double bond. E) More than one of these statements are correct. 3. Which of the following statements is false? A) Atoms or molecules with an odd number of electrons are paramagnetic. B) N molecules are diamagnetic. C) Atoms or molecules with an even number of electrons are diamagnetic. D) Paramagnetic molecules are attracted toward a magnetic field. E) Paramagnetism cannot be deduced from the Lewis structure of a molecule alone. 4. Choose the correct statement about the diagram below. A) The diagram is qualitatively correct for water. B) The diagram shows that the melting point of the solid increases with increasing pressure. C) The diagram shows the triple point above atm pressure. D) The diagram could represent the phase diagram of CO. E) None of these statements is correct. 6
5. All of the following are polar except? Make sure to minimize the formal charges. A) SF4 B) XeF4 C) IF4 + D) ClO - E) All of the above are nonpolar 6. Calculate the interplanar distance that has a second order reflection of 43. for x-rays of wavelength of 0.4 nm. A) 0.93 nm B) 0.4 nm C) 0.03 nm D) 0.69 nm E) None of the above B, A, C, B, B, E 7