Giant Covalent Molecules

Covalent BondsCovalent Bonds

Simple molecular structureex: CH4

Giant covalent structureex: diamonds

covalent bond between atoms or molecules-strong forces between

atoms (within molecule)-weakweak forces between forces between

moleculesmoleculesdepend on the polarity of the

molecules

– sometimes, a solid lattice is held together with covalent bonds

• giant covalent structures: very hard, very high melting/boiling points– Ex.: diamonds and graphite (both C),

SiO2 (crystals)

Intermolecular Forces

IMF

• IMF: Forces (not bonds) that hold covalent molecules together– For solids and liquids

• IMF control the physical properties of covalent compounds.– Melting and boiling point

– Viscosity (how thick/sticky a liquid is)

– Solubility (what will dissolve in a liquid)

• IMF explains why WATER is the liquid required for life!

Three types of IMF

• Van der Waal’s

• Dipole-dipole

• Hydrogen bondsIncreasing strength

1. van der Waal’s Forces

• Between non-polar molecules• Electrons move randomly, so by

chance, many e- can end up at one end of a molecule– “temporary dipole”– even in non-polar molecules!

• Ex: CCl4

• This repels (pushes away) the electrons of nearby molecules

• Temporary dipoles do not last because the e- are constantly moving.

• vdW Force depends on:– surface area of molecules

– molar mass of molecules

2. Dipole-dipole Forces

• Between polar molecules

• Because of the electrostatic attraction between molecules– opposite charges attract one another

• stronger than vdW Forces

• Ex: HCCl3

• Affect the boiling point of a substance

3. Hydrogen Bonding• Between polar molecules that have H and

one of N, O or F• H-bonds are the attraction between a non-

bonding e- pair on N, O or F and the H atom.– N, O or F has a slight negative charge, H has

a slight positive charge.

• H-bond is the strongest IMF

• H-bonds affect the physical properties of substances (ex: boiling point)

• Ex: H2O

• H-bonds cause:a. higher boiling point, lower volatility

• volatility: how easily something evaporates

b. greater solubility in water

c. higher viscosity

• viscosity: resistance to flow, “thickness”

• All IMF affect the properties of a substance.

• The stronger the IMF, the…– Higher the boiling point– Higher the melting point– Greater the viscosity (resistance to flow)

• IMF also affect the solubility of a substance

• http://www.youtube.com/watch?v=cgiNk94XyaI

• Practice Quiz on Thursday

• I will give you the formula for a molecule. You need to provide the:– Lewis structure

– Molecule shape/VSEPR Theory

– Polarity

– Intermolecular Force present

– Also, know something about giant covalent structures

Example Quiz Question:• For the molecule HCCl3:

a) Draw the Lewis Structure

b) Draw the shape including the bond angle, and label it with the name of the shape.

c) Identify polarity using delta + and delta –

d) Label the molecule as Polar or Non-polar.

e) What kind of intermolecular forces would exist between molecules of this compound?