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Giant Covalent Molecules. Covalent Bonds. Simple molecular structure ex: CH 4. Giant covalent structure ex: diamonds. - strong forces between atoms (within molecule) - weak forces between molecules. covalent bond between atoms or molecules. depend on the polarity of the molecules. - PowerPoint PPT Presentation
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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?