6.3 Intermolecular forces
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What are intermolecular forces
Weak interactions between dipoles of DIFFERENT molecules
What are the three main categories of intermolecular forces
Induced dipole-dipole interactions (London forces) Permanent dipole-dipole interactions Hydrogen bonding
What are intermolecular forces responsible for
Physical properties such as melting and boiling points
What are covalent forces responsible for
The identity and chemical reactions of molecules
Bond enthalpy/kJmol^-1 of London forces
1 to 10
Bond enthalpy/kJmol^-1 of permanent dipole-dipole interactions
3 to 25
Bond enthalpy/kJmol^-1 of hydrogen bonds
10-40
Bond enthalpy/kJmol^-1 of single covalent bonds
150 to 500
What are London forces
Weak intermolecular forces that exist between ALL molecules whether polar or non-polar Act between induced dipoles in different molecules
Explanation of the origin of induced dipoles
Movement of electrons produces a changing dipole in a molecule At any instant, an instantaneous dipole will exist but its position is constantly shifting The instantaneous dipole induces a dipole on a neighbouring molecule The induced dipole induces further dipoles on neighbouring molecules, which then attract one another
Induced dipoles are only…
… temporary
The more electrons in each molecule:
The larger the instantaneous and induced dipoles The greater the induced dipole-dipole interactions The stronger the attractive forces between molecules Larger numbers of electrons mean larger induced dipoles More energy is then needed to overcome the intermolecular forces, increasing the boiling point
What was the term van der Walls, forces, used to describe
Sometimes used to describe induced dipole-dipole interactions The IUPAC recommends it also be used for permanent dipole-dipole interactions so the term is ambiguous
Where do permanent dipole-dipole interactions act
Between the permanent dipoles in different polar molecules
Are fluorine molecules polar
No
Do fluorine molecules have London forces
Yes, they ONLY have London forces
Are hydrogen chloride molecules polar
Yes
Do hydrogen chloride molecules have London forces
Yes, and ALSO permanent dipole-dipole interactions between molecules Extra energy is needed to break the additional permanent interactions so the boiling point is higher than fluorine
What is a simple molecular substance
A substance made up of simple molecules
What is a simple molecule
Small units containing a definite number of atoms with a definite molecular formula
What do simple molecules form in the solid state
A simple molecular lattice
In a simple molecular lattice:
The molecules are held in place by weak intermolecular forces The atoms within each molecule are bonded together strongly by covalent bonds
All simple molecular substances can be…
…solidified into simple molecular lattices by reducing the temperature
Are the forces easily broken in a simple molecular lattice
Yes, they can be broken even by energy present at low temperatures