Phases of Matter and Intermolecular Forces

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Phases of Matter and Intermolecular Forces

• Adapted from
• Wilbraham, Anthony. Chemistry, Addison-Wesley.
• Upper Saddle River, NJ Prentice Hall, Inc.,2002.

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Objectives

1. Review the organization of particles in the three
   phases of matter gases, liquids and solids.
2. Name and describe the weak attractive forces that
   hold groups of molecules together.
3. Determine which type of intermolecular force is
   important to overcome in converting a substance
   from a liquid to a gas.

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States of Matter - Gases

• Gases can be compressed because the particles are
  spaced far apart.
• Gases fill their container uniformly and
  completely, and they flow because there are only
  weak forces attracting molecules to one another.
  
• The particles move randomly in a gas.

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States of Matter - Liquids

• Liquids are condensed states, their particles are
  in close contact with each other.
• Liquids take the shape of the container and they
  can flow because the particles are not rigidly
  packed and the attraction between particles is
  weak.
• The particles of a liquid move randomly.

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States of Matter Solids

• Solids are condensed because the particles are
  packed tightly together.
• Solids have a definite shape and volume.

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The three states of matter

• Solids
• Liquids
• Gases

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Intermolecular Forces

• Intermolecular forces occur between molecules
• Intramolecular forces occur inside the molecules

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Three types of Intermolecular Forces (IMF) in
order of increasing strength

• 1. Dispersion forces
• 2. Dipole Interactions
• 3. Hydrogen bonding
• Dispersion forces and dipole interactions are the
  weakest and are sometimes referred to a van der
  Waals forces.

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Dispersion Forces

• Dispersion forces are the weakest of all
  molecular interactions.
• Important in nonpolar substances, such as F2, N2,
  and CO2
• They are also called London forces and induced
  dipole forces.

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About dispersion forces

• Dispersion forces are caused by the motion of
  electrons. When two nonpolar molecules encounter
  one another, attractions and repulsions lead to
  distortions in their electron clouds, inducing
  momentary dipoles.
• The strength of dispersion forces increases as
  the number of electrons in a molecule increases.

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Question

• Why are F2 and Cl2 gases at room temperature ,
  while Br2 is a liquid and I2 is a solid?
• Answer F2 has 18 electrons and Cl2 has 34
  electrons, while Br2 has 70 electrons and I2 has
  106 electrons. More electrons causes a greater
  force of attraction.

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Dipole Interactions

• Dipole interactions occur when polar molecules
  are attracted to one another. They are stronger
  than dispersion forces.
• The oppositely charged ends of polar molecules
  create electrostatic attractions, and these hold
  polar molecules in the liquid or solid state.
  They are much weaker than ionic bonds.

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Hydrogen Bonding

• Hydrogen bonds are attractive forces in which a
  hydrogen atom that is covalently bonded to a very
  electronegative atom is attracted to an unshared
  pair of another very electronegative atom.
• Hydrogen bonding is an extreme case of dipole
  interaction.
• This occurs only when H is directly bonded to
  either N, O, or F.

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Question

• Which has the higher boiling point? (consider
  which has the stronger IMF)
• 1. O2 or N2?
• 2. SO2 or CO2?
• 3. HF or HI?
• 4. SiH4 or GeH4?

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Answers

• 1. O2 both N2 and O2 are nonpolar, both have
  dispersion forces, but O2 has more electrons
• 2. SO2 CO2 is nonpolar (has dispersion forces),
  while SO2 is polar (has stronger dipole
  interactions)
• 3. HF HI has dipole interactions, HF has
  stronger hydrogen bonding
• 4. GeH4 both SiH4 and GeH4 are nonpolar, but
  GeH4 has more electrons

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Challenge

• Place these in order of increasing strength of
  intermolecular forces
• SCl2, NH3, CH4, Cl2

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Answer

• CH4 (nonpolar, dispersion, few electrons),
• Cl2 (nonpolar, dispersion, more electrons),
• SCl2 (polar, dipole interactions),
• NH3 (H bonded to N, O, or F, hydrogen bonding)

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Question

• Which has the higher boiling point?
• dimethyl ether (H3C-O-CH3) or
• ethyl alcohol (H3C-CH2-OH)?
• (both have the general formula C2H6O.)

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Answer

• Ethyl alcohol.
• (Dimethyl ether is slightly polar and has dipole
  interactions, while ethyl alcohol has hydrogen
  bonding.)