Intermolecular Forces

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Chapter 6 Intermolecular Forces

• Outline
• 1. Intermolecular Forces and States of Matter
• 2. Intermolecular Forces and Solubility

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Intramolecular and Intermolecular Forces
Intramolecular forces determine such molecular
properties as molecular geometries and dipole
moments
Intermolecular forces determine the macroscopic
physical properties of liquids and solids, e.g.
melting/boiling point, solubility.
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States of matter Solid, Liquid, Gas
1. Intermolecular Forces and States of Matter
(a) Particles in solid (b) Particles in
liquid (c) Particles in gas
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Some Properties of Solids, Liquids, and Gases
Property Solid Liquid
Gas
Shape Has definite shape Takes the shape
of Takes the shape the container
of its container Volume Has a definite
volume Has a definite volume Fills the volume
of the container Arrangement
of Fixed, very close Random, close
Random, far apart Particles Interactions between
Very strong Strong Essentially
none particles
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What are the Forces holding matter in the
condensed phases (liquid and solid)?
Ionic compounds Ionic bonds are the forces that
hold matter in the condensed states. e.g. NaCl
Molecular compounds Intramolecular forces, e.g.
ice
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Attractive forces and state
Stronger attractive forces between particle ?
close contact
Separation ?Higher Thermal energy (temperature)
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Changes in Physical state

• 3 phases of matter solid, liquid, and gas

The melting point and boiling point give a
indication of strength of attractive forces among
particles.
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Intermolecular Forces

• Melting and boiling points measure the relative
  strength of attractive forces between particles.
• Ionic bonding are stronger than intramolecular
  forces. e.g.
• NaCl melts at 800 C
• Water (H2O) melts at 0 C

What are the intramolecular forces?
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Intramolecular forces Dipole Forces
Dipole Forces Polar molecules exist as dipoles.
These oppositely charged ends will attract each
other.

• Not as strong as ionic bonding, e. g.
• HCl melts at 112 oC, NaCl melts at 800 oC.

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Intramolecular forces Hydrogen Bonds

• Hydrogen Bonds When a hydrogen atom is
  covalently bonded to a highly electronegative
  atom like nitrogen, oxygen, or fluorine (N,O,F),
  it can exhibit an additional polar attraction to
  another electronegative atom called a hydrogen
  bond.

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Hydrogen Bonding in Ice
Solid water is less dense than liquid water due
to _______________
Hydrogen bonding is also the reason for the
unusually high boiling point of water
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Intramolecular forces Dispersion Forces

• Present in all molecules
• Important in nonpolar compounds
• Due to attraction of momentary induced dipole.
• The strength of this force increases with (i)
  molecular weight and (ii) surface area of
  molecules.
• It is also known as dispersion forces or London
  dispersion forces.

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Exercise 1. Which of the following has a higher
boiling point?
(a)
(b)
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Exercise 2. Explain the following observation.
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Relative strength of intermolecular Forces
ExerciseRank the melting points of (a) H2O, (b)
CH3CH3 and (c) LiF
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Solutions

• Solutions are homogeneous mixtures of two or more
  substances
• Homogeneous thoroughly mixed even composition
  throughout
• Solute substance being dissolved
• Solvent substance doing the dissolving

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Solubility
Intermolecular Forces and solubility

• When one substance (solute) dissolves in another
  (solvent) it is said to be soluble.
• Salt is soluble in water.
• Bromine is soluble in methylene chloride.
• When one substance does not dissolve in another
  it is said to be insoluble.
• Oil is insoluble in water.
• The solubility of one substance in another
  depends on two factors natures tendency towards
  mixing and the types of intermolecular attractive
  forces.

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Will It Dissolve?

• Chemists rule of thumb
• Like dissolves like
• A chemical will dissolve in a solvent if it has a
  similar structure to the solvent.
• When the solvent and solute structures are
  similar, the solvent molecules will attract the
  solute particles at least as well as the solute
  particles to each other.

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Solubility and Solution

• Like dissolves like
• Nonpolar compounds dissolve best in nonpolar
  solvents (lipophilic)
• Oil and gasoline mix
• Polar compounds dissolve best in polar solvents
  (hydrophilic)
• Water and HCl mix

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Classifying Solvents
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Will It Dissolve in Water?

• Ionic compounds
• Many ionic compounds dissolve in water. (ions are
  attracted to polar solvents)
• Polar molecules
• Many polar molecules (e.g. table sugar, ethyl
  alcohol, and glucose) dissolve well in water.
• Nonpolar molecules
• Mostly are not water soluble.
• Many molecules have both polar and nonpolar
  structureswhether they will dissolve in water
  depends on the kind, number, and location of
  polar and nonpolar structural features in the
  molecule.

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Ionic Compounds in Solution
Ionic substances dissolve in water through
ion-dipole interactions.

• e.g. A salt dissolves in water
• Charges on ions are attracted to partial charges
  on water

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Solvated Ions
When materials dissolve, the solvent molecules
surround the solute particles due to the
solvents attractions for the solute. This
process is called solvation. Solvated ions are
effectively isolated from each other.
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Exercise. Decide if each of the following will be
significantly soluble in water.

• potassium iodide, KI
• octane, C8H18
• methanol, CH3OH
• copper, Cu
• cetyl alcohol, CH3(CH2)14CH2OH
• iron(III) sulfide, Fe2S3

• potassium iodide, KI
• octane, C8H18
• methanol, CH3OH
• copper, Cu
• cetyl alcohol, CH3(CH2)14CH2OH
• iron(III) sulfide, Fe2S3