Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110

General Chemistry

Summer 2006

© R. Paselk 2002
 
 

Exam II Study Guide

 

Summer 2006 - Final Version

Electrochemistry

Batteries and Fuel Cells: What is a battery? What are some of the most important commercial batteries? What is so great about a lead acid battery? Why would we like to get rid of it? Why are standard "dry cells" not rechargeable? Compare and contrast a battery to a fuel cell. What are their comparative advantages and disadvantages? Which do you think will be most important in the future?

Electrolysis: What is electrolysis? How is an electrolytic cell related to a galvanic cell? What are electrolytic cells useful for? Give some examples. Be able to diagram an electrolytic cell, identifying the anode and cathode (are they the same as in the galvanic cell with the same components?). What is the decomposition potential (external voltage needed to begin electrolysis). Will the calculated potential of an electrolytic cell always produce electrolysis? Why or why not? In principle galvanic cells are reversible to give electrolytic cells. In fact it is often not the case. Why not?

Bonding Review

Chemical Bonding: What is an ionic bond? How is it formed? What forces maintain it? Anion. Cation. Ion pair. What is the octet rule? Be able to apply the octet rule to guess the ionic forms of the various "A" group elements. Why are outer electron shell octets so stable?

What is a Lewis Structure? What is it intended to show? For which elements are Lewis structures most useful? (representative elements = "A" grps) Be able to draw Lewis structures for all the representative elements in their atomic and predicted ionic forms. Kernel vs. inert gas core.

What is a covalent bond? How do ionic and covalent bonds differ? Which electrons are involved in covalent bonding? What is electronegativity? Know electronegativity values for H and Li-F. How is it used? Be able to use electronegativity (both quantitatively and qualitatively [hi,lo rules]) to predict the ionic/covalent nature of a bond. Be able to draw a correct Lewis structure for any covalent molecule made up of atoms from the representative elements. (Remember-you use Electronegativity to determine covalent or ionic first!) When do you need to use multiple bonds? Resonance? Are multiple bonds real? What does resonance represent in the real molecule? Be able to make proper resonance based sets of Lewis structures. For which representative elements does the octet rule not hold? (H - B) When is valence expansion required? Be able to draw correct expanded valence shell Lewis structures. Which orbitals are involved in expanded valence shells? What is the outermost shell of an atom? Does the outer-most shell of an atom need to have any electrons in it? Polar vs. non-polar bonding.

VSEPR Theory: What is VSEPR theory? Be able to use the VSEPR method to determine the geometry of a molecule, including demonstrating the process involved-what assumptions are made with this model. Know the various orientations of electron pairs around a central atom and the various molecular geometry's they predict. What relations are there between electronic configuration and molecular shape? Does a given electron configuration always give the same molecular shape? Explain. Be able to determine whether a given molecule is polar or not. Be able to defend your decision (think charge separation and geometry).

A Quantum View of Bonding: When atomic orbital sets are filled, or half-filled they become completely symmetrical. Orbitals are orbitals: 1) Only two electrons can be accommodated in any orbital. 2) No two electrons can have the same "address" (the same set of quantum numbers). 3) For a molecules the "address" becomes the molecule over which the electrons are shared rather than the atom. 4) We have conservation of orbitals - a molecule will have the same number of orbitals as the atoms which make up the molecule. 5) For our purposes we can also assume a conservation of orbital energy.

Hybrid Atomic Orbitals: What are hybrid atomic orbitals? What do we mean when we say this is a "localized electron" model? What does it tell us about the electron distribution in molecules? Why are hybrid orbitals popular? (relatively easy to calculate/visualize). What are their limitations? (NOT looking at molecules, so can only be a limited approximation.) How are bonds visualized with hybrid orbitals? Know the five common hybrid orbital sets (sp, sp2, sp2, dsp3, d2sp3) and their geometries (linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral). Be able to describe the molecular geometries we discussed with VSEPR Theory in terms of hybrid orbitals. Does the hybrid orbital set correspond to the electronic or to the molecular geometry?

What are the two basic bond types in molecular orbitals? (Sigma (s) bonds - cylindrically symmetrical around the axis connecting the bonded atoms, and Pi (p) bonds - made up of two lobes with planar symmetry round a plane though the nuclei of the two bonded atoms.) Note that single bonds are always sigma bonds, and in a multiply bonded system the "central" bond is a sigma bond. The "second" and "third" bond of multiply bonded atoms are pi bonds. For systems with two pi bonds the bond panes are perpendicular to each other and the two bonds form a symmetrical cylinder around the sigma bond. What is resonance and why is it needed in hybrid orbital theory? How is this a "failure" of the theory? Why does hybrid orbital theory not give information about electronic energy levels in molecules?

Be able to describe the hybridization of atoms in molecules given their structures, and be able to specify the bond types (sigma or pi) between atoms in a structure. (Sample question: Give the hybridization for each atom and the bond types for: CH3CCCH2COH.)

Molecular Orbital Model of Bonding: How does MO theory differ from hybrid orbital theory? What advantages does it have over hybrid orbital theory? Disadvantages? Be able to draw energy diagrams. Know and be able to explain the molecular energy diagrams we discussed in lecture. How is bonding described in MO theory? Are bond shapes the same (sigma & pi)? Be able to determine when molecules are diamagnetic or paramagnetic and explain why. Be able to give the bond order of a diatomic molecule. Define bond order. (Sample questions: 1) Draw a Molecular Orbital Energy-level diagram for NO. Include all of the valence electrons in your diagram, and determine the bond order and whether the molecule should be diamagnetic or paramagnetic based on your diagram. 2) Draw a molecular orbital energy-level diagram for NO showing the original atomic energy levels and the MO energy-levels. )

The Chemistry of the Representative Elements: Know periodic trends and patterns within Groups and Periods. Be able to illustrate trends with properties of Groups of elements. You should be able to give and/or interpret examples of trends and properties!

Be able to write net ionic equations for the reactions discussed in the lab book for the Representative Elements! (e.g. Na, K; Mg, Ca, Ba; Al; Sn, Pb; Sb, Bi; S; N; Cl, Br, I.)

 

You may bring a single sheet of 8.5 x 11 paper with formulae etc. on one side only

as a "cheat sheet" for the exam.

Make sure you have all relevant formulae and constants on your sheet!

You should be careful and thorough in designing and creating this sheet.

However, remember it is for reference and insurance, the less you rely on it the better off you will be!

You will be provided with a Periodic Chart.


Syllabus/Schedule
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© R A Paselk

Last modified 23 June 2006