4.4 Intermolecular forces
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What are intermolecular forces responsible for?
Physical properties like melting point, boiling point and solubility
What are Van der Waal’s forces made of?
Dipole-dipole and London dispersion forces
Are London dispersion forces strong?
No. They are the weakest of the 3 bonds
Where are London dispersion forces found?
They exist between all atoms and molecules
What are the two types of London dispersion forces?
• Temporary/instantaneous dipoles - Induced dipoles
What are temporary dipoles caused by?
Changes in electron density within an atom or molecule
Why are there changes in electron density in atoms or molecules?
Shared electrons revolve around both nuclei but in an instant, it is more on one side so there is a temporary charge. They become more concentrated on one side
What is the possible effect of a temporary dipole on another molecule?
It can cause a neighboring molecule to have an induced dipole
What does strength of temporary dipole depend on?
It depends on surface area and polarizability (ease with which electrons form the dipole)
Relation between mass and polarizability and hence formation of dipole
As mass increases, so does polarizability because electron number increases = larger dipole = stronger forces.
Where are dipole-dipole forces found?
Only exist between polar molecules with permanent dipoles
Why does HCl have dipole-dipole forces?
Because of its electronegativity, there is a permanent dipole
What is the order of strength of bond for: H bonds, dipole-dipole forces and London dispersion forces
London dispersion forces
Where is hydrogen bonds found?
In molecules that have an electronegative N, O or F directly bonded to hydrogen
How does strength of H bonds relate to water?
As they are strong, it results in higher melting and boiling point for water
Why do H2O have higher boiling points than molecules with similar molecular masses i.e. ethane?
Because they have hydrogen bonds which are stronger than the Van der Waal forces of ethane and hence require more energy to overcome
Put in order of increasing boiling point and explain why: H2S, H2O, H2Te
H2S
Why does ice have lower density than water?
Hydrogen bonds hold water molecules in a rigid structure when frozen. This means there’s actually less water molecules in the area as compared to a liquid
Why are certain covalent substances soluble in water?
Because they can form H2 bonds with water. Mostly polar
Can non-polar substances dissolve in water?
No but they can form solutions with other non-polar substances
Why do longer chain alcohols become less soluble?
As they lengthen, they become progressively less soluble as OH is the only polar part. Therefore it becomes more and more hydrophobic