Forces between molecules

Intermolecular Forces (IMFs)
 Different molecules have different forces that act between them.
 These forces attracting the separate molecules together control many physical properties
 Boiling Point
 Melting Point
 Solubility
 Viscosity
 Surface Tension

Three Main IMFs
 Dipole-dipole forces
 Hydrogen bonding
 London dispersion forces (LDFs)

Dipole-Dipole Forces
 Electronegativity – the ability of an atom in a compound to attract electrons to itself.
 Fluorine has the highest electronegativity
 The electrons in a compound spend more time around the most electronegative atoms than the other atoms.

Electronegativity
 If the electrons are spending more time around the one particular atom, how will that atom be different from the other atoms?
 It will have a more negative charge than the other atoms in the compound.
 Does not have a full negative charge but a partial negative charge ( δ , lower case delta)

Hydrofluoric Acid (HF)
The red end (fluorine) has a partial negative charge.

Polar Molecules
 One end of the molecule is positive while the other end of the molecule is negative.
 This difference in charge is called a
“dipole”

What effect does this have?
 How does this change the way two molecules interact?
 The positive end of the molecule is attracted to the negative end of a different molecule.

How to spot dipole-dipole forces?
 Look for molecular shapes that have uneven placements of atoms.
 Bent
 Trigonal pyramidal
 Anything that has more than one type of atom around the outside

Is Carbon Dioxide Polar?

Is carbon tetrafluoride polar?

Is Water Polar?

Why is Polarity Important
 Things that are polar or have charges dissolve in things that are polar.
 Things that are nonpolar dissolve in things that are nonpolar.

“Like dissolves Like”
 Opposing types do not dissolve in each other.

A Bio Reminder
 Hydrophilic – “water loving” – polar
 Hydrophobic – “water fearing” - nonpolar

Dissolution Process
 How are strong ionic bonds broken in water?
 Dissolving_NaCl-Electrolyte.exe
 The polar nature of water creates attractions between the water and ionic compound.

Hydrogen Bonding
 Special case of dipole-dipole forces.
 The difference in electronegativity between some atoms and hydrogen is so strong that it creates a very strong dipole

What elements can do this?
 Which elements have the strongest electronegativity?
 Anything with an N-H bond, O-H bond, or
F-H bond will have hydrogen bonding.

Hydrogen Bonding is Very Important
It is the reason why ice floats.

Hydrogen Bonding is Very
Important

DNA Base Pairs

London Dispersion Forces (LDFs)

Often called “induced dipoles”
 A momentary change in where the electrons are in one molecule, “induces” a dipole in another molecule.

LDFs
 The more electrons you have in a molecule, the more likely you are to have momentary imbalances in charges.
 The more electrons in an atom, the stronger the London Dispersion Forces.
 Any molecule can have London Dispersion
Forces.

LDFs
 This explains why the boiling point goes up as you move down a column.
 Hydrogen telluride has more electrons than hydrogen sulfide
 Hydrogen telluride has stronger LDFs
 Hydrogen telluride has a higher boiling point

Crude Oil
 Crude oil is refined based on differences in LDFs.
 Longer carbon chains have higher boiling points
 Have larger number of electrons

IMF Comparison
 LDFs are the weakest
 Dipole-dipole are in the middle
 Hydrogen bonding is the strongest.

Practice Problems
 List all of the intermolecular forces acting on two phosphorus trichloride molecules
 Figure out the formula
 Draw a Lewis structure
 Figure out the molecular geometry
 Check to see what IMFs it has.

Practice Problem
 Explain why ammonia (-33.4
º C)has a higher boiling point than phosphine, PH
3
(-87.8
º C).
Justify your answer.

Figure out ammonia’s formula
 Draw Lewis structures for both compounds
 Figure out the molecular geometry for both compounds
 Figure out what IMFs each compound has.
 Compare the two compounds.

Practice Problems
 Explain why ammonia (-33.4
º C)has a higher boiling point than phosphine, PH
3
(-87.8
º C).
Justify your answer.

Practice Problem
 Hexane (C
6
H
14
) is a liquid at room temperature. Its Lewis structure has each carbon connected to another in a long chain. Will sodium chloride dissolve in hexane? Justify your answer.