Intermolecular forces
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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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Intermolecular Forces

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Intermolecular forces

Intermolecular Forces

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?

Intermolecular forces






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

Intermolecular forces














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.

Capillary action1

Capillary Action



  • 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

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