Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109	General Chemistry	Summer 2002
Lecture Notes:: 9 July	© R. Paselk 2002
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Polarity in Covalent Molecules

Polarity: So now we can predict bonding and shape in representative group molecules (and thus most biomolecules), how about electron density and thus charge distribution? Need two bits of information:

• Shape (based on VSEPR Theory)
• Electron distribution within a bond (based on electronegativity)

Examples:

Molecule Geometry Structure Electronegativities Bond Dipoles Molecular Dipole Carbon monoxide  linear    ENC= 2.5, ENO= 3.5     Carbon dioxide  linear   ENC= 2.5, ENO= 3.5    None: two dipoles are of equal magnitude, but opposite in direction and cancel.  Water  bent      ENH= 2.1, ENO= 3.5      Ammonia  trigonal pyramidal     ENH= 2.1, ENN= 3.0     Ammonium ion tetrahedral      ENH= 2.1, ENN= 3.0  None: four dipoles are symmetrically arranged to cancel each other out and give a spherically charged but non-polar ion.

Formal Charge

This is another mode of "electron bookkeeping." Like oxidation numbers it uses a very simple set of rules to enable us to make realistic guesses about how atoms behave in molecules without having to have a pocket supercomputer to do that "quick" quantum mechanics calculation.

Formal charge is a simple model for determining how charges are distributed on atoms in a molecule or molecular ion. It is not always terribly accurate, but is very useful for approximating how molecules will behave in some situations. It is particularly useful in choosing among resonance structures in organic chemistry to determine which are likely to make the greatest contribution to the "real" structure.

Formal Charge (FC) = the charge an atom would have if all bonding pairs were shared equally (polar bonds don't exist in this model).

To assign Formal Charges:

1. Draw a correct Lewis Structure.
2. Assign both electrons of a lone pair to its associated atom.
3. Divide all bonding pairs, giving one electron of each pair to each atom in the bond.
4. Calculate FC = # electrons on the unbonded (elemental) atom - # electrons assigned to the bonded atom.

Examples:

• Phosphoric acid (H₃PO₄)
  • LS:
    • FC_P: 5 - 4 = +1
    • FC_H: 1 - 1 = 0
    • FC_O: 6 - 7 = -1
    • FC_{3 O's}: 6 - 6 = 0
  • S FC = 1 + 3 (0) + (-1) = 0. Notice that the Formal charges add up to give zero, the charge on the molecule.
• Perchlorate ion (ClO₄^-)
  • LS:
    • FC_Cl: 7 - 4 = +3
    • FC_O: 6 - 7 = -1
  • S FC = +3 + 4 (-1) = -1. Notice that the Formal charges add up to give -1, the charge on the molecular ion.
• Note can also determine FC's for compounds containing transition metals. Just don't worry about transition metal, use octet rule for representative elements, and find transition metal FC by difference. For example, permanganate ion MnO₄^-
  • Oxygen will have an octet in normal oxygen compounds, and will share one bonding pair, thus
    • FC_O: 6 - 7 = -1
  • S FC = FC_Mn + 4 (FC_O) = -1, the charge on the ion, thus
    • FC_Mn = -1 - 4 (FC_O) = -1 - 4 (-1) = +3

Energy of Formation for Ionic Compounds

It turns out that the transfer of an electron from a metal to a non-metal will not generally provide enough energy to favor the process. So how is it that these are in fact favorable reactions?

Let's look at the energy of the process by breaking it into steps and looking at the enthapies of formation starting with free atoms (the reality will be somewhat more complex since we would start with solid metal and molecules, each of which must first react to give free atomic state, but the results are similar). Of course we can get away with this because we are looking at state functions, which as we saw before are pathway independent!

Bond Energies and Enthalpies of Reaction

Bond energies, as tabulated in Table 8.4 of your text (p 373) can be used much like heats of formation to calculate the heat (energy) involved in a reaction. Note that in the table all of the bond energies are positive values, so we have to think and assign the appropriate sign depending on what's occuring. Thus, it takes energy to break a bond (in a sense a bond is a situation where the energy is lower, or it wouldn't be a bond) - the bond energy is positive, but energy will be released when a bond is made - the bond energy is negative.

Let's try an example: How much energy is released in the complete combustion of methane?

Writing a balanced equation:

CH₄ + 2 O₂ Æ CO₂ + 2 H₂O

From the table the bond energies are:

• C-H 413 kJ/mol
• O=O 495 kJ/mol
• C=O 799 kJ/mol
• O-H 467 kJ/mol

Combining the bond energies (reactants - products):

4 (413 kJ/mol) + 2 (495 kJ/mol) - 2 (799 kJ/mol) - 4 (467 kJ/mol)

2642 kJ/mol - 3466 kJ/mol = -824 kJ/mol

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