notes_14C_noncoval01

Notes_14C_noncoval01 - Non-Covalent Molecular Forces How does this reaction occur H2O(liquid H2O(gas Add energy H2O gas molecules are very far

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2/27/06 – 3/1/06 Non-Covalent Molecular Forces How does this reaction occur: H 2 O (liquid) H 2 O (gas) ? Add energy H 2 O gas: molecules are very far apart H 2 O liquid: bonding between molecules Use heat to add energy. i.e. boil water to turn H 2 O to gas Boiling point o How much energy do you need to change a substance from liquid to gas state? o Water’s boiling point is high for a molecule weighing only 18amu. Why? Data Boiling Point NaCl 1413 ºC BrCl 5 ºC H 2 O 100 ºC Ar - 186 ºC Ionic: NaCl Is it Na—Cl or Na + Cl - ? Covalent? Ionic? EN (change in electronegativity) ~ 1.6 + EN = 3.0 (Chlorine) – 0.9 (Sodium) = 2.1 ionic Ionic Strong bond o The negative ion of chlorine is electro-statically attracted to the positive charge on sodium o 188 kcal/mol to break the bond. Huge number, but typical for ionic compounds. o Therefore, NaCl’s boiling point is high (1413 ºC) because it takes a lot of energy (in the form of heat, in this case) to break the bonds and turn the molecule into its gas state Dipole-Dipole: BrCl Is it Br—Cl or Br + Cl - ? EN = 3.0 (Chlorine) – 2.8 (Bromine) = 0.2 covalent? Sharing of electrons not completely even and covalent, so BrCl is best described as polar covalent.
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Electrostatic Attraction = opposite charges that attract bonds and have permanent dipoles, dipole-dipole bonding. (Strength of attraction follows Coulomb’s Law: force of electrostatic attraction is proportional to magnitude of the difference between charges) Polar Covalent One BrCl molecule has a covalent bond where chlorine is more electronegative (electron-greedy) than bromine. This causes a shift in electron density towards chlorine, resulting in a slight negative dipole. Thus bromine obtains a slight positive dipole because it is lacking electron density. o The negative dipole from the chlorine of one BrCl molecule then becomes attracted to the positive dipole from a bromine atom of a different BrCl molecule (opposite charges attract!). Dipole-dipole attraction accounts for BrCl’s 5 ºC boiling point In ionic case, force of attraction is stronger and bonds are harder to break compared to dipole- dipole (_ + , _ - ) attraction. Bond dipole depends on:
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This note was uploaded on 07/27/2009 for the course CHEM 14C-14D taught by Professor Hardinger during the Fall '08 term at UCLA.

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Notes_14C_noncoval01 - Non-Covalent Molecular Forces How does this reaction occur H2O(liquid H2O(gas Add energy H2O gas molecules are very far

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