Spring 2006 CHEM 3500 Exam: Atomic & Molecular Orbitals - MCQs & T/F, Exams of Physical Chemistry

Download Spring 2006 CHEM 3500 Exam: Atomic & Molecular Orbitals - MCQs & T/F and more Exams Physical Chemistry in PDF only on Docsity! CHEM 3500 Exam 1 Spring 2006 February 15, 2006 Name: _________________________________ A. TRUE/FALSE Questions (30 points) (11 questions, +3 points for the correct answer, –1 point for the incorrect answer, 0 points for no answer, lowest scored question in this section dropped) __T __ __ __ __ __ __ __ __ __ ____ 1. For the radiation coming from a blackbody, the wavelength of maximum intensity (λmax) decreases as the temperature increases. F __ 2. The angular momentum of the electron in hydrogen atom is completely determined by the quantum number ml. __ 3. If radiation of wavelength 230 nm strikes a silver surface with a workfunction of 4.58 eV, electrons are ejected from the surface. T __ 4. The probability of finding a particle at a certain position is given by the square F root of its wavefunction at that position. __ 5. LUMO for F molecule is an antibonding π orbital. F 2 __ 6. The first ionization energy of N is larger than the first ionization energy of C. T __ 7. The ionization energy of He+ is twice as big as the ionization energy of H. F F __ 8. A π molecular orbital is cylindrical symmetric around the internuclear axis. T __ 9. The 2s orbital in a hydrogen atom has one radial node. +F __ 10. The bond order of Cl2 species is equal to 2. __ 11. The d2sp3 hybridization is associated with an octahedral electronic structure. T 1 CHEM 3500 Exam 1 Spring 2006 B. MULTIPLE CHOICE Questions (40 points) (9 questions, +5 points for the correct answer, 0 point for an incorrect answer, +1 point for no answer, lowest scored question in this section dropped) ____ 12. A number of phenomena and their interpretation lead to the development of quantum mechanics. Which of the following was not one of them? e a. Heat capacity of materials b. Photoelectric effect c. Blackbody radiation d. Wave-particle dualism e. Born-Oppenheimer approximation ____ 13. If the uncertainty in the location of an electron is 20 pm, then what is the uncertainty in its speed? d a. 1.6 × 103 m⋅s–1 b. 1.4 × 106 m⋅s–1 c. 1.2 × 107 m⋅s–1 d. 2.9 × 106 m⋅s–1 e. 3.6 × 107 m⋅s–1 __c __b__ 14. Which of the following best describes the representation of the molecular orbital? a. A non-bonding orbital b. A πu antibonding orbital c. A πg antibonding orbital d. A πu bonding orbital e. A πg bonding orbital __ 15. A ground-state hydrogen atom absorbs a photon of light that has a wavelength of 93.78 nm. It then gives off a photon that has a wavelength of 1182 nm. What is the principal quantum number of this final state? a. 2 b. 3 c. 4 d. 5 e. 6 2 CHEM 3500 Exam 1 Spring 2006 22. (a) Explain the relationships between a wavefunction, an orbitals, and a boundary surface. u I t A wavefunction ψ is a solution of the Schrödinger equation for a particular system. If the system is an electron moving around a proton (i.e., the hydrogen atom) the wavefunction will depend on three quantum numbers (n, l, and ml) and will be called an orbital. Because it does not depend on the fourth quantum number (ms) which can have two different values, the wavefunction can be the same for up to two electrons (i.e., two electrons can occupy an orbital). Chemists prefer to deal with probability of finding the electron, which is given by the square of the wavefunction, ψ2. This probability of finding the electron is not constant in space. Chemists are sing a geometric representation of the orbital as a surface that engulfs the smallest space where the electron has a probability of being 90% of the time. This is called a boundary surface of an orbital. (b) Explain the similarities and the difference between a bonding and an antibonding orbital. Both bonding and antibonding orbitals are molecular orbitals. Within molecular orbital theory, both bonding and antibonding orbitals are obtained as a linear combination of atomic orbitals. f the linear combination of atomic orbitals increases the electron density between he two atoms (more precisely, between the two nuclei) through constructive interference, the molecular orbital is called bonding. If the linear combination of atomic orbitals decreases the electron density between the two atoms (more precisely, between the two nuclei) through destructive interference, the molecular orbital is called antibonding. This linear combination makes the wavefunction to become zero in regions between the two nuclei, which is also equivalent to a region of zero electron density or a nodal plane. 5 CHEM 3500 Exam 1 Spring 2006 23. Employing Valence Bond Theory terminology, describe the bonding and molecular geometry in ethanal (or acetaldehyde) with as much detail as you can. (Include sketches if necessary.) .... C OC H H H H sp3 sp2 sp2 There is a σ bond between an sp3 hybrid orbital of C in methyl group and an sp2 hybrid orbital of the aldehydic C. The bonding between C and O involves a σ bond between an sp2 hybrid orbital of C and an sp2 hybrid orbital of O as well as a π bond between a pure p orbital of C with a pure p orbital of O. Oxygen atom has also two lone pairs in sp2 hybris orbitals. 6 CHEM 3500 Exam 1 Spring 2006 24. Draw a molecular orbital energy level diagram for homonuclear diatomic molecules. Label and sketch all molecular orbitals in the diagram. Describe if the orbitals are bonding or antibonding. Fill the diagram with the appropriate number of electrons for N2 – (not N2) and determine the bond order. 1πg* 1πu 2σg 1σu* 1σg 2p 2s 2p 2s 2σu* Electronic configuration of N2 – : (1σg)2 (1σu)2 (2σg)2 (1πu)4 (1πg)1 Bond order = (8 – 3)/2 = 2.5 7