Physical - IMF Flashcards
Instantaneous –
existing at a particular point in time
Induced –
when something is caused by something else
IMF
attractive forces between molecules.
This means only covalently bonded compounds
They are not bonds they are attractions
These are much weaker than any covalent bonds
IMF decreasing in strength order
Hydrogen bonding
Perm dip-dip
VdWs
Dipole
means two different charges.
𝛅-
𝛅+
Van der Waals’ forces
The distribution of electrons around an atom is changing at every instant because there is electron movement
If most/all of the electrons in the molecule are located at one end of the molecule it
creates an instantaneous dipole.
This causes an induced dipole in the other molecule
There is electrostatic attraction between the δ+ and δ- areas on the adjacent
molecules. This attraction is called a Van der Waals force of attraction between
molecules
Instantaneous dipole
Most of the electron density
is on the left hand side.
There is a higher density of
negative charge here. We
show this with a 𝛅-
The more electrons there are
the larger the instantaneous
dipole will be, and the stronger the forces between the
atoms/molecules will be, therefore the higher the melting
point
Which will have a higher boiling point: Butane or Hexane?
✓ Hexane has more electrons than butane (1)
✓ So has stronger/greater/more Van der
Waals’ forces between the molecules. (1)
✓ This means more energy is needed to
overcome the forces (1)
✓ So hexane has a higher boiling point. (1
Van der Waals’ forces exist between all molecules.
Explain how these forces arise. (3)
Electron movement in first molecule
Induces a dipole in another molecule
(Induced-temporary) attraction or δ+ attracts δ- in different/adjacent molecules
Dipole – Dipole
interactions occur
between
all polar molecules
* Attraction between atoms
of opposite partial charge
on different molecules due
to electronegativity
difference
To have permanent dipole dipole forces
A molecule must have both polar bonds and the molecule must be polar.
Electronegativity of c and h
Not much difference
A molecule is symmetrical
if it has multiple planes of symmetry –
this will mean that the
dipoles will be equal and opposite to each other and cancel out
Symmetrical shapes
Linear
Square planar
Trigonal planar
Octahedral
Tetrahedral
Trigonal bipyramidal
The hydrogen bonding
occurs
because of the large electronegativity difference between H and
N/O/F.
Drawing Hydrogen bonds
All lone pairs shown (1)
All partial charges shown (1)
Bond shown correctly (from lone pair on O to H) with 180O bond angle.
Hydrogen bonding in HF
Large electronegativity difference between H and F
Creates delta + on H and delta – on F
H atom attracted to/forms hydrogen bond to lone pair of electrons
On an F atom in a different HF molecule
Hydrogen bonding gives water
its unique properties
Surface tension
*Excellent solvent
*Liquid at room temperature
*Highly polar molecule
Why does ice float on water
In ice, each water molecule is hydrogen bonded to 4 other water
molecules. Liquid can only make 2
- Ice has an “open structure” with lots of spaces betweenmolecules
- As a result the molecules are held further apart than in liquid water
- When the ice melts, the structure is lost and the molecules
collapse closer together.
- This means ice is less dense than water
