CHEMICAL CHEMICAL BONDINGBONDING

Set 6Set 6

CHEMICAL CHEMICAL BONDINGBONDING

Set 6Set 6

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Credits

• Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website www.chemistrygeek.com

• Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

Bond PolarityBond PolarityBond PolarityBond Polarity

HCl is HCl is POLARPOLAR because it because it has a positive end and a has a positive end and a negative end. (difference negative end. (difference in electronegativity)in electronegativity)

Cl has a greater share in Cl has a greater share in bonding electrons than bonding electrons than does H.does H.

Cl has a greater share in Cl has a greater share in bonding electrons than bonding electrons than does H.does H.

Cl has slight negative charge Cl has slight negative charge (-(-)) and H has slight and H has slight positive charge positive charge (+ (+ ))

H Cl••

••

+ -••H Cl

••

••

+ -••

Molecular PolarityMolecular Polarity

• REMEMBER: Polarity refers to a separation of charges.

• Entire molecules can be polar or nonpolar, too!

PolarNonpolar

Determining Molecular PolarityDetermining Molecular Polarity

1) Draw the Lewis dot structure of the molecule.

2) Find the polarity of eacheach bond within the molecule.

3) If ALLALL 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 SymmetryMolecular Symmetry

• Molecular symmetry is NOT the same as geometrical symmetry!

• Molecular symmetry occurs when all items (bonds and/orand/or lone pairs) on the central atom are identicalidentical.

• NOTE: It matters whatwhat is attached, not howhow it is attached (single, double, triple bonds count the same!)

SymmetrySymmetry

• If a molecule is symmetricalsymmetrical, the molecule is NONPOLARNONPOLAR regardless of what types of bonds it contains.

• If a molecule is not symmetrical not symmetrical AND it has at least one polar bond, the molecule is POLARPOLAR.

Lone pairs ≠ H atoms (not symmetrical)

All atoms are F (symmetrical)

Intermolecular ForcesIntermolecular Forces

• Intermolecular forcesIntermolecular forces: forces of attraction between molecules (like interinternational = between countries)

• Intramolecular forcesIntramolecular forces: forces of attraction within a molecule, i.e. a bond

Types of Intermolecular Forces (IMFS)Types of Intermolecular Forces (IMFS)

• Dipole-dipole forcesDipole-dipole forces: occurs between polar molecules – partial positive end of molecule A attracts partial negative end of molecule B

Types of IMFsTypes 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 IMFsTypes of IMFs

• Dipole-induced dipole forcesDipole-induced dipole forces: attraction between a polar molecule and nonpolar molecule – the polar molecule creates a temporary dipole in the nonpolar molecule.

Types of IMFsTypes of IMFs

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

Effects of IMFsEffects of IMFsEffects of IMFsEffects of IMFs

• ““Like Dissolves Like”Like Dissolves Like”– Polar dissolves PolarPolar dissolves Polar– Nonpolar dissolves Nonpolar dissolves

NonpolarNonpolar

• Other properties Other properties affected: melting affected: melting points, boiling pointspoints, boiling points

Properties of Three Molecular CompoundsCompound Molecular Formula Molar Mass (g) Boiling Point (°C)

Water H2O 18.0 100

Methane CH4 16.0 -33.4

Ammonia NH3 17.0 -164

•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 This is why oil and water will not mix! Oil is nonpolar, and water is polar.nonpolar, and water is polar.

• The two will repel each other, and so you The two will repel each other, and so you can not dissolve one in the othercan not dissolve one in the other

IMFs and DissolvingIMFs and DissolvingIMFs and DissolvingIMFs and Dissolving

HOMEWORK1) 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) H2 b) H2S c) HCl d) HF

MORE HOMEWORK

4) In a methane molecule (CH4), there are four single covalent bonds. In an octane molecule (C8H18), 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?