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Intermolecular Forces. Chemistry Jokes of the Day. Why did the white bear dissolve in water?   Because it was polar.  What do dipoles say in passing?  "Have you got a moment?“ Why are chemists great for solving problems?   They have all the solutions. . States of Matter .

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chemistry jokes of the day
Chemistry Jokes of the Day
  • Why did the white bear dissolve in water?   Because it was polar.
  •  What do dipoles say in passing?  "Have you got a moment?“
  • Why are chemists great for solving problems?   They have all the solutions.
states of matter
States of Matter
  • Solid, Liquid, and Gas
  • Which of the 3 states do you think would have similar properties?
  • Solid & Liquid
they are similar
They are similar
  • incompressible; constant density
  • These similarities are due
    • to the molecules being close together in solids and liquids
    • but far apart in gases
  • What holds them close together?
intermolecular forces1
Intermolecular Forces
  • Intermolecular Forces: forces exist between molecules.
  • Intramolecular Forces: forces formed within the molecule. Bonding between atoms.

Why do we need to know about intermolecular forces?

- Because these intermolecular forces are responsible for the existence of the liquid state and explained some unusual properties of liquids: i.e. Water - high boiling point

types of intermolecular forces
Types of Intermolecular Forces
  • There are 3 types of Intermolecular Forces
  • Dipole-Dipole (DDF)
  • London Dispersion (LDF)
  • Hydrogen Bonding (H-bond)
  • These intermolecular forces (DDF & LDF) are VERY WEAK, 1% as strong as a covalent bond
  • Then what are the strong forces of attraction?
  • Covalent Bonding & Ionic Bonding
in terms of the strength of attractions
In terms of the Strength of Attractions

Strong: Covalent Bonding & Ionic Bonding

Medium Strength: Hydrogen Bond

Weak: Dipole-Dipole & London Dispersion

dipole dipole forces
Dipole-Dipole Forces
  • Exist between Polar Molecules
  • The partial negative ends is attracted to the partial positive ends of the molecules.
  • Weak Forces
  • Example:
  • H-Cl
what about non polar molecules
What about non-polar molecules?
  • Does that mean no forces of attraction exist?
  • Then, how does solid or liquid state exist?
london dispersion forces
London Dispersion Forces
  • Exist in ALL molecules (regardless of polarity)
  • In any molecule, even non-polar, there will be an instantaneous dipole moment due to random electrons movement.
  • This instantaneous dipole moment induces near by molecules to have a partial negative and partial positive.
  • These dipole-induced molecules orient themselves to attract one another. (very weak attraction)
  • Attractive Forces are said to be induced.
  • Also known as: induced-dipole-dipole interactions
london dispersion forces1
London Dispersion Forces

For Example: N2, H2, CH4

london dispersion forces2
London Dispersion Forces
  • Very Weak Forces
  • Strength of LDF increase as the # of electrons in a molecule increases.
  • Similarly, strength of LDF increase with the size of the molecules.
  • The larger the molecule the more polarizable it is. (more easily distorted to give instantaneous dipoles because electrons are farther from the nuclei)
  • Also, the longer the chain, the easier it is to polarize the molecule.
  • Therefore, higher Melting Point & Boiling Point.
london dispersion forces3
London Dispersion Forces
  • Try to predict the boiling point for SiH4 , GeH4, and SnH4
  • Higher or Lower?






Boiling Points




what about isoelectric molecules of same molecular weight
What about isoelectric molecules of same molecular weight?
  • Like pentane, isopentane, and neopentane?
  • All have the same molecular formula, C5H12
  • Which one has

a higher Boiling


  • Pentane
  • Longer Chain
empirical data revealed to researchers more than what they were looking for
Empirical Data Revealed to Researchers More Than What They Were Looking For!
  • Let’s take a look at some of their findings
  • Results are consistent with Intermolecular Forces suggested: Dipole-Dipole Forces, and London Dispersion Forces.
  • But something else interesting was revealed to them
  • They found that H2O, HF, and NH3 repeatedly yield higher boiling point than predicted.
  • How do we explain such high boiling points for these 3 molecules?
  • We are onto something here!!! Let’s take a look














Boiling Points




hydrogen bonding
Hydrogen Bonding
  • Very strong dipole-dipole forces exist when H is attached to F, O, or N
  • Strictly between a molecule with Hydrogen and another molecule with F, O, or N in it.
  • Use dotted line to indicate hydrogen bonding.
  • These three because-
    • They have high electronegativity.
    • They are small enough to get close.
  • Affect certain physical properties like boiling point.
  • Explained the empirical data obtained.




properties of liquids
Properties of Liquids
  • Aside from Melting & Boiling Points,
  • Surface Tension
  • Viscosity
  • Capillary Action
  • Beading

All of these properties are greatly depended on the strength of the intermolecular forces: DDF, LDF, H-bond.

surface tension
Surface Tension
  • Intermolecular Forces are responsible for the ability (tendency) of a liquid to reduce its surface area.
  • This is why raindrops are nearly spherical (sphere has the smallest surface area for a given volume of any geometrical shape)
surface tension1
Surface Tension
  • Molecules at the top are only pulled inside.
  • Molecules in the middle are attracted in all directions.
  • Minimizes surface area.
surface tension2
Surface Tension
  • Surface Tension: the energy required to increase the surface area of a liquid by a unit amount.
  • Because of surface tension, a liquid behaves as though it had as “skin”.
mini investigation
  • The effect that soap has on the surface tension of water.
  • What did you observe?
  • Explain, in terms of surface tension & surface area, what you have seen?
  • Viscosity: the resistance to flow.
  • Greater Intermolecular Forces = More Viscous
  • Larger Molecule = Greater LDF = More Viscous
  • Cyclohexane vs. Hexane : which one is more Viscous ?
capillary action
Capillary Action
  • Liquid slowly rise in a narrow tube.
  • Intermolecular Forces responsible: H-Bond, DDF, and LDF
  • Cohesion – between water molecules
  • Adhesion – between water + something else

Like glass, or cellulose of cell wall.

  • Glass is polar, therefore, attracts water molecules up the column.
  • If a polar substance is placed on a non-polar surface.
    • There are cohesive,
    • But no adhesive forces.
  • Same with non-polar substances on a polar surface
  • P.260 # 1-4
  • P. 264 # 9
  • P. 266 # 1-5
  • P. 282 # 18, 28