Properties of Covalent (Molecular) Substances

Properties of Covalent Properties of Covalent (Molecular) Substances(Molecular) Substances

Properties of Covalent (Molecular) Properties of Covalent (Molecular) SubstancesSubstances

• Poor conductors of heat & electricity in any phase. No charged particles!

• Low melting & boiling points: easy to pull molecules apart from each other.

• Majority of solids are soft• Low Hf and Hv compared to ionic & metallic

substances• High vapor pressure compared to ionic &

metallic substances

Properties

• Depend on strength of forces between “particles” or separate units.

• In covalent substances, the units are molecules.

Intermolecular Forces

• Forces between molecules = intermolecular forces

• 3 kinds: What are they?

Intermolecular ForcesIntermolecular Forces

• Dispersion forcesDispersion forces occur between nonpolar molecules. (Van der Waals)

• Dipole-dipole forcesDipole-dipole forces occur between polar molecules.

• Hydrogen bondingHydrogen bonding occurs between molecules with an H-F, H-O, or H-N bond.

Intermolecular forces determine phase!Intermolecular forces determine phase!

Weakest intermolecular force = dispersion Weakest intermolecular force = dispersion force. Occurs between force. Occurs between nonpolarnonpolar molecules. molecules.

1) Monatomic molecules: He, Ne, Ar, Kr, etc.1) Monatomic molecules: He, Ne, Ar, Kr, etc.

2)2) Diatomics with 2 atoms of the same element. Diatomics with 2 atoms of the same element.

3) Very symmetric molecules.3) Very symmetric molecules.

Dispersion forces are stronger for Dispersion forces are stronger for larger molecules.larger molecules.

Dipole-dipole forces Dipole-dipole forces occur between occur between molecules with a molecules with a permanent charge permanent charge separation, i.e., separation, i.e., polar molecules.polar molecules.

HCl and HBr are HCl and HBr are examples of polar examples of polar diatomic molecules.diatomic molecules.

Hydrogen bonding influences properties!Hydrogen bonding influences properties!

Molecular Polarity

• For diatomics, the molecular polarity is the same as the bond polarity.

• For molecules with 3 or more atoms, the polarity depends on the types of bonds and the shapeand the shape of the molecule!

MolecularMolecular Polarity Polarity

• Depends on 2 factors.–Type of bonds in the molecule

–Arrangement of bonds or shape of molecule

For larger molecules, look at the kind For larger molecules, look at the kind & arrangement of bonds to determine & arrangement of bonds to determine overall polarity of molecule.overall polarity of molecule.

Hydrogen bonding occurs between Hydrogen bonding occurs between molecules containing an H-F, H-O, or molecules containing an H-F, H-O, or H-N bond. It is the strongest H-N bond. It is the strongest intermolecular force.intermolecular force.

Properties of Covalent (Molecular) Properties of Covalent (Molecular) SubstancesSubstances

• Poor conductors of heat & electricity in any phase. No charged particles!

• Low melting & boiling points: easy to pull molecules apart from each other.

• Majority of solids are soft• Low Hf and Hv compared to ionic & metallic

substances• High vapor pressure compared to ionic &

metallic substances

Melting pt, Boiling pt, Melting pt, Boiling pt, HHff and H and Hvv and vapor and vapor

pressure depend on pressure depend on how hard it is to pull how hard it is to pull the particles apart.the particles apart.

Weak intermolecular Weak intermolecular forces – it’s easy to forces – it’s easy to pull them apart.pull them apart.

Strong intermolecular Strong intermolecular forces – it’s hard. forces – it’s hard.

Which substance has the strongest Which substance has the strongest intermolecular forces?intermolecular forces?The weakest?The weakest?

WaterWaterEtherEther

Polarity of MoleculesPolarity of Molecules

• A molecule may contain polar bonds, but not be polar! Depends on the geometry of the molecule.

• If molecule is symmetric, the “pull” of one polar bond is offset by the “pull” of another polar bond.– It’s a tug-of-war that no one can win!

Symmetric MoleculesSymmetric Molecules

• Contain at least one mirror plane.

Symmetric vs. Asymmetric

• CO2 is nonpolar.

• Can’t tell the ends apart.

• H2O is bent.• Electron cloud is lopsided.• H2O is polar.

Polarity of CO2?

Draw an arrow along each bond pointing Draw an arrow along each bond pointing to the more electronegative atom.to the more electronegative atom.If the arrows cancel out, the molecule is If the arrows cancel out, the molecule is NONPOLAR.NONPOLAR.

Polarity of H2O?

The green The green arrows do not arrows do not cancel out, so cancel out, so water is polar!water is polar!

Symmetry of Larger MoleculesSymmetry of Larger Molecules

CFCF44 is fairly is fairly

symmetric. symmetric. Overall, it is Overall, it is nonpolar.nonpolar.

The arrows The arrows cancel out.cancel out.

Use the structural formula to predict Molecular Polarity!

HH

H H C C H H HH

Ethane = CEthane = C22HH66

Ethene = CEthene = C22HH44

Ethyne = CEthyne = C22HH22

These molecules are symmetric and These molecules are symmetric and the electron cloud is the same on both the electron cloud is the same on both ends. Overall, they are nonpolar.ends. Overall, they are nonpolar.

Molecular Polarity

• If you know the shape, you can use the arrow technique to determine the polarity.

• So how do you get the shape?