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Chapter 6 - Shapes of molecules and intermolecular forces II - Coggle…
Chapter 6 - Shapes of molecules and intermolecular forces II
Forces between molecules
Intermolecular froces
Weak interactions between dipoles of different molecules
Three types
Induced dipole-dipole interactions (London forces)
Permanent dipole-dipole interactions
Hydrogen bonding
Responsible for physical properties
Induced dipole-dipole interactions
London forces - weakest intermolecular forces existing between all molecules
Process
Movement of electrons produces a changing dipole in a molecule
An instantaneous dipole exists, but positions constantly change
Instataneous dipole induces a dipole on a neighbouring molecule
Induced dipole induces further dipoles on neighbouring molecules, which attract eachother
Temporary, in an instant, induced dipoles may disappear and repeat elsewhere
Strength
More electrons in each molecule:
Larger instantaneous and induced dipoles
The greater the induced dipole-dipole interactions
Stronger attractive forces between molecules
Larger number of electrons means larger induced dipoles, so more energy is needed to overcome the intermolecular forces, so a higher boiling point
Permanent dipole-dipole interactions
There are permanent dipole-dipole intereactions between polar molecules
Extra energy is needed to break the additional intermolecular forces
Boiling point of Polar molecules is higher than non-polar
Simple molecular substances
Made up of simple molecules - small units containing a definite number of atoms
Simple molecular lattice
Molecules held in place by weak intermolecular forces
Atoms in each molecule bonded together strongly by covalent bonds
Properties
Boiling points
Weak intermolecular forces can be broken very easily using small amounts of energy
Simple molecular substances have low melting and boiling points
During melting
Only weak intermolecular forces break
Covalent bonds don't break
Solubility
Non polar substances
Non polar solvent
Intermolecular forces break in the simple lattice and compound dissolves
Non-polar substances tend to be soluble in non-polar solvents
When non-polar substances are added to a non-polar solvent, intermolecular forces form between the molecules
Polar solvents
When non-polar substances are added to a polar solvent, there is little intereaction between the molecules
Intermolecular forces within the polar solvent are too strong to be broken
Non polar substances tend to be insoluble in polar solvents
Polar substances
Dissolve in polar solvents as polar solute molecules and the polar solvent molecules attract eachother
Solublity depnds on the strength of the dipole
Conductivity
No mobile charged particles in simple molecular structures
They cannot conduct electricity
Hydrogen bonds
Special type of permanent dipole-dipole interaction
Found in...
Molecules containing an electronegtive atom with a lone pair of electrons (oxygen, nitrgoen or florine)
A hydrogen atom attatched to an electronegative atom
Hydrogen bond acts between lone pair of electrons on an electronegative atom and a hydrogen atom in a different molecule
Strongest intermolecular force
Shown by dashed line
Anomalous properties of water
Solid water is less dense than liquid water
Hydrogen bonds hold water molecules apart in an open lattice structure, and the water moleces in ice are further apart than in water
Hydrogen bonds extend outwards, holding water moleces slightly apart and forming an open tetrahedral lattic full of holes
Holes decrease the density of the water, but when the ice melts, the lattice collapses and the molecules move closer together
Melting/ boiling point
Due to the hydrogen bonds, more energy is required to break the intermolecular forces, so water has a much higher melting/ boiling point than if it had just london forces
When ice lattic breaks, rigid arrangement of hydrogen bonds in ice is broken
When water boils, the hydrogen bonds break completely
Other properties
Relatively high surface tension and viscosity