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IntermolecularIntermolecular ForcesForces
How are molecules held How are molecules held together?together?There are two types of attraction in
molecules: ◦ Intramolecular forces ◦ Intermolecular forces
Intermolecular Forces – the forces that bond molecules to each other
Intramolecular Forces – the forces that bond the atoms to each other within a molecule
Until now, we have been discussing intramolecular forces (ionic, polar, non-polar bonds)
How are molecules held How are molecules held together?together?
Intermolecular forces are weak relative to covalent bonds
Intramolecular forces are strong covalent bonds
We can conclude that it does not take very much energy to break the bonds that hold molecules to each other (intermolecular forces)
These forces are also referred to as van der Waals forces after the physicist who studied them
We will be discussing 4 types of intermolecular forces◦Dipole-Dipole Forces◦Ion-Dipole Forces◦Dispersion (London) Forces◦Hydrogen Bonding
Dipole-Dipole ForcesDipole-Dipole ForcesDipole is a polar
moleculeThese molecules will
orient themselves so that oppositely charged ends of molecules are near to one another
The electrostatic attractions between these ends are called dipole-dipole forces
H Cl
+ –
+–
+ –
+
–
+ –
Lets see it!
Result of Dipole-Dipole Result of Dipole-Dipole ForcesForcesPolar molecules will tend to attract
one another more than similarly sized non-polar molecules
Therefore the energy required to separate polar molecules from one another is greater than that needed to separate non-polar molecules
Ion-Dipole ForcesIon-Dipole ForcesThe force of attraction between an
ion and a polar molecule (a dipole)
Example: NaCl breaks up because the ion dipole with water is stronger than the attraction of Na+ to Cl-
Let’s See it!
Dispersion (London) ForcesDispersion (London) ForcesWeakest intermolecular force that
act between non-polar molecules
Shared pairs of e- can temporarily occupy positions that create momentary, uneven distributions of charge
This causes non-polar molecules to become slightly polar for just an instant
At this instant it is capable of inducing a dipole in a nearby molecule resulting in an intermolecular force of attraction
This is called a dispersion force
Hydrogen BondingHydrogen BondingAn attraction between the
hydrogen atom, bonded to a highly electronegative atom, and the negative end of a dipole nearby
Like Dissolves LikeLike Dissolves LikeIonic solutes dissolve in polar
solvents ◦(ex: NaCl and H2O)
Non polar solutes dissolve in non polar solvents◦(ex: solid I2 and liquid Br2)
PracticeSheet on polar molecules is for
homework!
On this sheet please add this question:
Based on what we know about intermolecular forces predict the solubility of this molecule in a)
water and b) acetone