CHEMICAL BONDING Set 6 Cocaine SAVE PAPER AND INK!!! When you print out the notes on PowerPoint, print "Handouts" instead of "Slides" in the print setup. Also, turn off the backgrounds (Tools>Options>Print>UNcheck "Background Printing")!

Credits Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website m m Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

Bond Polarity HCl is POLAR because it has a positive end and a negative end. (difference in electronegativity) Cl has a greater share in bonding electrons than does H. Cl has slight negative charge (-  ) and H has slight positive charge (+  )

Molecular Polarity REMEMBER: Polarity refers to a separation of charges. Entire molecules can be polar or nonpolar, too! Polar Nonpolar

Determining Molecular Polarity 1)Draw the Lewis dot structure of the molecule. each 2)Find the polarity of each bond within the molecule. ALL 3)If ALL the bonds are nonpolar, the molecule is nonpolar. 4)If one or more of the bonds is polar, determine the symmetry of the molecule.

Molecular Symmetry Molecular symmetry is NOT the same as geometrical symmetry! and/or identical Molecular symmetry occurs when all items (bonds and/or lone pairs) on the central atom are identical. whathow NOTE: It matters what is attached, not how it is attached (single, double, triple bonds count the same!)

Symmetry symmetrical NONPOLAR If a molecule is symmetrical, the molecule is NONPOLAR regardless of what types of bonds it contains. not symmetrical POLAR If a molecule is not symmetrical AND it has at least one polar bond, the molecule is POLAR. Lone pairs ≠ H atoms (not symmetrical) All atoms are F (symmetrical)

Intermolecular Forces Intermolecular forces inter Intermolecular forces: forces of attraction between molecules (like international = between countries) Intramolecular forces Intramolecular forces: forces of attraction within a molecule, i.e. a bond

Types of Intermolecular Forces (IMFS) Dipole-dipole forces Dipole-dipole forces: occurs between polar molecules – partial positive end of molecule A attracts partial negative end of molecule B

Types of IMFs Hydrogen “bonding”: Hydrogen “bonding”: a special subset of dipole-dipole interactions – occurs between H atom of molecule A and F,O,N or S atom of Molecule B

Types of IMFs Dipole-induced dipole forces Dipole-induced dipole forces: attraction between a polar molecule and nonpolar molecule – the polar molecule creates a temporary dipole in the nonpolar molecule.

Types of IMFs London dispersion forces dipsersion forces van der Waal’s forces induced dipole- induced dipole forces London dispersion forces (or dipsersion forces or van der Waal’s forces or induced dipole- induced dipole forces): attractions between two nonpolar molecules

Effects of IMFs “Like Dissolves Like” “Like Dissolves Like” – Polar dissolves Polar – Nonpolar dissolves Nonpolar Other properties affected: melting points, boiling points Other properties affected: melting points, boiling points

Properties of Three Molecular Compounds CompoundMolecular FormulaMolar Mass (g)Boiling Point (°C) WaterH2OH2O MethaneCH AmmoniaNH Hydrogen bonds explain why water is a liquid at room temperature, while compounds of comparable mass are gases. The difference between methane and water is easy to explain – because methane is nonpolar, the only forces holding the molecules together are relatively weak dispersion forces. Ammonia and water is not as obvious – molecules of both can form hydrogen bonds, but the ammonia is a gas which indicates its IMFs are not as strong. This is because the EN difference between N-H is a lot less than the EN difference between O-H.

This is why oil and water will not mix! Oil is nonpolar, and water is polar. This is why oil and water will not mix! Oil is nonpolar, and water is polar. The two will repel each other, and so you can not dissolve one in the other The two will repel each other, and so you can not dissolve one in the other IMFs and Dissolving

HOMEWORK 1)Explain what determines a substance’s state at a given temperature. 2)Compare and contrast intermolecular forces and describe intramolecular forces. 3)Evaluate which of the molecules listed below can form hydrogen bonds. For which of the molecules would dispersion forces be the only intermolecular force? Give reasons for you answers. a)H 2 b) H 2 Sc) HCld) HF

MORE HOMEWORK 4) In a methane molecule (CH 4 ), there are four single covalent bonds. In an octane molecule (C 8 H 18 ), there are 25 single covalent bonds. How does the number of bonds affect the dispersion forces in samples of methane and octane? Which compound is a gas at room temperature? Which is a liquid?