Intermolecular Forces, Liquids, and Solids

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Intermolecular Forces, Liquids, and Solids

• Chapter 11 Brown-LeMay

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I. Kinetic Molecular Description

• Intermolecular forces (I.F.) attractive
  electrostatic interactions that occur between
  molecules, atoms, or ions of a substance.
• Liquids I.F. are strong enough to hold sub
  together but weak enough to allow movement
• Solids I.F. are strong so no movement of
  molecules occurs (they vibrate)

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Types of solids

• Crystalline solids molecules and ions arranged
  in repeating patterns
• Amorphous solids molecules and ions arranges in
  random fashion

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II. Intermolecular Forces tend to be weak Type
of Interaction Aprox. E kj/mol
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A. Importance of I.F.

• The stronger the I.F. the greater the boiling and
  melting point and the lower the vapor pressure
  the weaker the I.F. the lower the melting and
  boiling point and the higher the vapor pressure

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Woops Debbie Deeeeeeeees

• B. Van der Walls forces are all
  electrostatic-2-types
• 1. Dipole-Dipole (D.D.) Debbie De result from
  the tendency of polar molecules that position
  themselves so that the pos. and neg. ends of
  different molecules are near to each other

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Debbieeeeeeeee D

• For molecules with the same approximate mass and
  size I.F. increases with increasing polarity
• 2. London forces (dispersion forces-L.D.) in
  non-polar molecules no D.D. forces exist small
  dipoles exist because of electron movement
  position of electrons in clouds- all molecules
  and compounds have these forces.

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London Dispersion Forces
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London Dispersion Forces

• L.D. forces ten to increase with increasing
  molecular weight
• Polarizable- larger atoms have electron clouds
  that are easily distorted

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3. Hydrogen Bonding (x oxygen very negative
atom y- hydrogen)

• Exists between a hydrogen atom covalently bonded
  to a very electronegative atom X, and a lone pair
  of electrons on another small, electronegative
  atom Y.

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Hydrogen Bonding

• The hydrogen bond in water may be explained in
  part on the basis of the dipole moment of the
  OH
• when the electrons in the hydrogen atom are
  furthest away from the oxygen atom
  
• X Y atoms are usually F, N, or O

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Strength of Hydrogen Bonding

• H---N lt H---O lt H---F
• 4. Ion-dipole forces attraction between an ion
  and an opposing charge pole of a molecule

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III. Properties of Liquids

• A. Viscosity resistance of a liquid to flow
• Ex. Motor oil SAE (society of automotive
  engineers) viscosity of motor oil increases as
  temperature decreases SAE ratings are for 0oF
  or -18oC increased ratings indicate greater
  viscosity for a given tempt.
• The greater the rating the thicker the oil.
• SAE 10w/40 behaves like a 10 in the winter and
  a 40 in the summer. The thicker grade is needed
  in the summer because the oil is heated and
  becomes thinner.

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B. Surface Tension

• The energy required to increase the surface area
  of a liquid by a unit amount (1m2)
• Why do liquids have this property?
• molecules at the surface experience a net force
  towards the middle

• Molecules at the center experience no net force

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Surface Tension Forces

• Cohesive - bind like molecules together
• Adhesive bind a substance to a surface
• Meniscus-curved surface of a liquid caused by the
  combination of adhesive forces between the liquid
  and container

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Surface Tension Forces

• Convex Meniscus ex mercury cohesive forces are
  greater

• Concave Meniscus ex water adhesive forces greater

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IV. Changes of State

• A. Phase changes or transitions substance
  structure is altered
• solid ??liquid??gas
• More ordered to less ordered state energy is
  supplied to overcome IF (endothermic)
• Less ordered to more ordered state energy is
  released (exothermic)

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B. Energy Changes and Changes of State

• liquid-gas- vaporization
• solid- liquid- melting
• solid- gas- sublimation
• endothermic processes
• gas- Liquid condensation
• liquid-solid- freezing
• gas solid deposition
• exothermic processes

• Gas

Gas
liquid
Solid
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• Transitions between solid, liquid, and gaseous
  phases typically involve large amounts of energy
  compared to the specific heat. If heat were added
  at a constant rate to a mass of ice to take it
  through its phase changes to liquid water and
  then to steam, the energies required to
  accomplish the phase changes (called the latent
  heat of fusion and latent heat of vaporization )
  would lead to plateaus in the temperature vs.
  time graph. The graph below presumes that the
  pressure is one standard atmosphere.

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• B-C melting, D-E boiling, heat is added but no
  change in temperature occurs because all the
  energy is used to break IF. Less energy is
  required to freeze water (delta H fusion) than to
  vaporize it (delta H vaporization)
• The energy required by the 5 processes warming
  the solid(1), melting(2), warming the liquid(3),
  boiling(4), and warming the gas(5) is determined
  by the identity of the substance ant the amount
  of the sample present.

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• Warming processes qs x m x delt t
• s specific heat- different for each substance
• Phase transitions q (number of moles) x delta H

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Heating curve for water