Laboratory Follow-Up

Ionic bonding - understand the relationship between: (1) melting point and ion size (2) melting point and charge difference [see previous post for a link]

Solubility:
-if it dissolves in water, it is polar, thus possesses LD & DD (maybe even HB)
-if it does not dissolve, it is nonpolar, thus possesses only LD (no DD/HB of any significance)

Consider the following links for some alcohol solubility information
http://www.solubilityofthings.com/water/alcohols/
This next one discusses solubility much further down the "page", though the initial information fits in nicely with the next organic functional group...
http://www.chemguide.co.uk/organicprops/alcohols/background.html

Question: Why do white bears dissolve in water? Answer: Because they are polar.

Paper

Can paper melt?

Solid State Bonding

Time to consider four solid state bonding forces. Solid state bonding forces are indicated by melting points.

(1) Molecular (CO2)

-weak between molecule forces





(2) Metallic (Fe(s), Pb(s))

-electron sea

http://www.chemguide.co.uk/atoms/bonding/metallic.html#top



(3) Ionic (NaCl(s))

-attraction of oppositely charged ions

-greater charge difference - greater attraction

-smaller ions, greater attraction

-greater attraction = higher melting point

http://www.saskschools.ca/curr_content/chem20/ionicpds/ionicex.html



(4)Covalent Network (SiO2, C(diamond))

-very strong force = highest melting point

-there are not really individual molecules

-each 'unit' is bonded to each surrounding 'unit'

http://www.chm.davidson.edu/vce/Crystals/NetworkSolids.html

http://www.chem.queensu.ca/people/faculty/mombourquette/FirstYrChem/Molecular/Solids/index.htm



The following link summarizes both liquid & solid state bonding forces. [Although it seems to exclude molecular solids - the weakest solid force.]

Liquid State Bonding

Three intermolecular forces to consider

• London Disperson Forces
• Dipole-dipole
• Hydrogen bonding

The strength of the bonding force is indicated by the boiling point of a substance.

Remember that boiling separates one molecule from another. Boiling DOES NOT break molecules apart - that is, DOES NOT separate atoms from each other.

A few useful links:

(1) London & dipole-Dipole

http://www.chemguide.co.uk/atoms/bonding/vdw.html#top

(2) H-bonding

http://www.chemguide.co.uk/atoms/bonding/hbond.html#top

(3) All 3 intermolecular forces

http://www.science.uwaterloo.ca/~cchieh/cact/c123/intermol.html

In order of increasing strength:

London (LDF) - weakest

• dependent upon the number of electrons
• greater number of electrons, greater London forces
• shape plays a role too, straight chain hydrocarbons tend to have higher bpt than branched chains of similar atom count

Dipole-Dipole (DDF)

• force of polar molecules
• polar molecules rely on shape of molecule
• polar molecules also have LD forces

H-bonding - strongest

• molecule must have H-N, H-O and/or H-F bond
• molecules will also have DDF and LDF

A typical liquid state bonding question would ask you to explain the difference in boiling points of various substances. Given a chemical formula, you need to be able to identify the type of liquid state bonding force involved with each molecule.

[The end of a much delayed post. Since water has all three bonding forces present, the chosen font colour for this little section is blue. Ready for a joke? Question: What did the sink say to the water faucet? Answer: You're a real drip. :<]

Hybridization II

For any molecule given, you should be able to provide:

• a Lewis structure (knowing the Lewis, you can predict the expected shape and then the type of hybridization involved)
• a 3-D structural representation (including -name, bond angle, molecular dipole)
• an energy level diagram using valence electrons to show hybrid orbital formation and final bonding
• an orbital sketch (include sigma and pi bonds)

From today, you should have done all of the above for:

BH3 PCl3 N2H4 HCN C2Br2 CH2O C2Br4 CH3CH=CH2

[A happy thought to end this puppy today. Why was the polar bear not white anymore. Asnwer: Because it was tickled pink.)

Hybridization

"If you know the shape, then you know the type of hybridization."

Hybridization maximizes orbital overlap. Hybridization provides the electron densities with the maximum separation. Without hybridization, measured bond angles (ex. 109.5 of tetrahedral) would conflict with predicted 90 of p-orbitals of valence bond theory without hybridization.

The link below reviews the common shapes, briefly overviews hybridization then provides a 'flow chart' to help connect central attributes (atoms attached & lone pairs) to the type of hybridization. At the bottom of the page are some sample questions. If you construct the Lewis structure for each, you should be able to identify the hybridization type and then the bond angle.
http://misterguch.brinkster.net/VSEPR.html

Once the last hybridization lesson is given, for any molecule you should be able to:

• draw its Lewis structure
• draw a 3-D representation (include bond angle, shape name, polarity of bonds & overall dipole of molecule)
• use energy level diagrams to show required hybridization of central atom
• provide a labeled* orbital sketch of the bonding [*sp3/sp2/sp & pi/sigma bonds]

From today you were asked to complete energy level diagrams and orbital sketches for both CHCl3 and NF3 , plus the questions on page 235 (11-14)

[I am a little pressed for time this afternoon, so I am going to fall back on the old polar jokes. Ready? What's the difference between a polar bear and a panda? {Answer: About 2500 kilometers. another? Where do polar bears keep their money? In snow banks.]

[Still looking for the link to explain bond angle measuring. :<]

Valence Bond Theory

• as the name valence bond theory implies, it is the theory of bonding on a valence (electron) level
• a bond: the overlap of an orbital with a single electron with another orbital with a single electron


• the resulting bond orbital contains 2 electrons of opposite spin

From today: p232 (1 to 5; with #4 provide both energy level & orbital sktech for each molecule) and complete the hybridization preparation handout

At this point you should be able to represent bonding in terms of (1) Lewis structure (2) 3-D shape (3) energy level diagram and (4) orbital sketch

There are a few issues with VBT as it has been described so far...

Considered the conflict between bond angles as indicated by the VBT depiction of H2O compared to the VSEPR bond angles. The link below will address the bond angle conflict as well as look ahead at hybridization.

http://www.sparknotes.com/testprep/books/sat2/chemistry/chapter4section9.rhtml

The above link is an SAT test preparation site. Have a look around the site for other interesting gems.

I am running out of time for today. I shall leave you with another polar [bear] joke: Why do polar bears win so many races? Answer: Because they are always in the "pole" position. Another question : Where will Woody be tomorrow?