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Chapter 12

CHEM 110 Chapter 12 Smart Notes.pdf

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CHEM 110
Bryan Sanctuary

CHEM 110 Chapter 12 Smart Notes October, 2011 CHEM 110 Chapter 12 Smart Notes  12.1: Intermolecular Forces o Van Der Waals Forces  Definition  The term Van der Waals forces is used to describe, collectively intermolecular forces of the London type and interactions between permanent dipoles.  Polarizability  Definition o Polarizability is the term to describe the relative tendency for a charge distribution to distort from its normal shape in an atom or a molecule.  Polarizability increases as electrons increase  Polarizability increases as molar mass increases  Polarizability increases as dispersion forces increase  Elongated less compact molecules have greater Polarization due to more intermolecular interactions. CHEM 110 Chapter 12 Smart Notes October, 2011  Types  Dipole-Dipole Interactions o In a polar substance, the molecules have permanent dipole moments, so the molecules tend to line up with the positive end of one dipole directed toward the negative ends of neighbouring dipoles.  Dipole-Induced Dipole o A polar with a non-polar. The polar molecule induces a temporary dipole in the non-polar molecule.  Induced Dipole-Induced Dipole or London Dispersion Forces o Dispersion forces exist between all molecules. o Dispersion (London) Forces are intermolecular forces associated with instantaneous and induced dipoles.  An instantaneous dipole is when a non-polar species become momentarily polar when the electrons are concentrated in one region of an atom or a molecule. CHEM 110 Chapter 12 Smart Notes October, 2011  An induced dipole occurs when a molecule that has an instantaneous dipole moment induces a neighbouring atom or molecule to produce a dipole.  Hydrogen Bonding o Definition  Hydrogen Bonding occurs when Hydrogen covalently bonds with Fluorine, Oxygen, or Nitrogen (intramolecular hydrogen bonding), which are highly electronegative atoms. o Hydrogen Bonding in Water (Intermolecular Hydrogen Bonding)  O-H Bonds  Ice has a low density. When ice melts, some of the hydrogen bonds are broken, this allows molecules to be more compactly arranged, accounting for the increase in density when ice melts. CHEM 110 Chapter 12 Smart Notes October, 2011  Water’s density decreases as temperature increases.  Without hydrogen bonding, all lakes would freeze from the bottom up. This is because the denser water sinks to the bottom of the lake and the colder surface water freezes. Furthermore, the ice over the top of the lake then tends to insulate the water below from further heat loss. CHEM 110 Chapter 12 Smart Notes October, 2011 CHEM 110 Chapter 12 Smart Notes October, 2011  12.2: Some Properties of Liquids o Surface Tension  Definition  Surface tension is the energy, or work, required to increase the surface area of a liquid. o Representation  o Units  Energy per Unit Area (typically in Joules per Square Meter)  As temperature decreases, surface tension increases.  Cohesive Forces  Definition o The forces exerted between molecules holding them together in the drop are cohesive forces  Adhesive Forces  Definition o The forces between liquid molecules and the surface are adhesive forces.  When cohesive > adhesive = water drop forms  When adhesive > cohesive = water drop collapses CHEM 110 Chapter 12 Smart Notes October, 2011 CHEM 110 Chapter 12 Smart Notes October, 2011  The water is drawn slightly up the walls of the tube by adhesive forces between water and glass. The interface between the water and the air above it, called a meniscus, is concave, or curved in. With liquid mercury, the meniscus is convex, or curved out. o Viscosity  Definition  A liquid’s resistance to flow o Stronger intermolecular forces = Greater Viscosity. o Viscosity generally decreases with increased
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