liquids, solids, and intermolecular forces · intermolecular forces: the attraction forces that...
TRANSCRIPT
Chapter 12
Liquids, Solids, and
Intermolecular Forces
Intermolecular Forces: The Attraction Forces that Hold
Matter Together in a Condensed State (Liquids and Solids)
• The strength of the attractions between the particles of a
substance determines its state (i.e., gas, liquid, solid).
• At room temperature, moderate to strong attractive forces
result in materials being solids or liquids.
• The stronger the attractive forces are, the higher will be the
boiling point of the liquid and the melting point of
the solid.
– Other factors also influence the melting point.
Trends in the Strength of
Intermolecular Attraction
• The stronger the attractions between the atoms or molecules,
the more energy it will take to separate them.
• Boiling a liquid requires adding enough energy to overcome all
the attractions between the particles.
– However, it does not require breaking the
covalent bonds.
• The higher the normal boiling point of the liquid, the stronger
the intermolecular attractive forces.
Ionic Compounds CRYSTALS
NaCl
Ionic Bonding and the Crystal Lattice
• The extra energy that is released comes from the formation of a
structure in which every cation is surrounded by anions, and
vice versa.
• This structure is called a crystal lattice.
• The crystal lattice is held together by the electrostatic
attraction of the cations for all the surrounding anions.
• The crystal lattice maximizes the attractions between cations
and anions, leading to the most stable arrangement.
What is Polarity?
• Polarity in covalently bonded molecules refers to a
separation of charge and can describe a bond or an
entire molecule.
• Experimentally, bond polarity is measured by its dipole
moment.
• Bonds connecting atoms of different electronegativity
are polar with a higher density of bonding electrons
around the more electronegative atom giving it a partial
negative charge (designated as δ-).
• The less electronegative atom has some of its electron
density taken away giving it a partial positive charge
(δ+).
Examples
Carbon DioxideWater
µ = 1.8 D
Shapes of Molecules - Review
• The shapes of molecules are determined by
both bonds as well as the non-bonding
valence shell electrons.
• That is why Carbon Dioxide is linear and
non-polar
• Water is bent and polar due to remaining 2
sets of non-bonding lone pair electrons.
Hydrogen bonds (H-bonding)
• An especially strong dipole–dipole
attraction results when H is covalently
attached to a Oxygen, Nitrogen, or
Fluorine atom.
• These are called hydrogen bonds (H-
bonding).
• Strong intermolecular attraction. Very
Important in Biology.
DNA – The Molecule of Life