Polarisation and Intermolecular Forces Flashcards
Electronegativity Definition
The ability to withdraw e- density from a covalent bond
Most electronegative elements?
top right to bottom left of periodic table, starting with Fluorine
Covalent Bonds….
May be polarised by differences in electronegativity
For Example…
H–H = non-polar - same electronegativities
C–H = essentially non-polar - similar electronegativites
H–Cl = polar
C=O = polar
Dipole definition
The difference in charge between two atoms caused by a shift in e- density in the bond.
Permanent Dipole
Caused by a difference in electronegativity between the two atoms in a polar bond. Greater difference = more polar bond = stronger permanent dipole
Polar Molecules
ONLY occur when there is an uneven distribution of charge in a molecule containing polar bonds.
If polar bonds are arranged SYMMETRICALLY
i.e. O-=C+=O-
Then charges CANCEL OUT and there is NO permanent dipole
Permanent Dipole-Dipole forces
In a substance made up of molecules that have permanent dipoles, there will be weak electrostatic forces of attraction between opposite charges on neighbouring molecules. Stronger permanent dipole = stronger permanent dipole-dipole forces.
Intermolecular forces strength
Are much weaker than covalent than covalent, ionic or metallic bonds
=> Hydrogen Bonding is the strongest
=> Permanent dipole-dipole forces
=> Induced dipole-dipole or Van Der Waals are the weakest
Describe Van Der Waals intermolecular forces
Found between all atoms and molecules
Cause all atoms and molecules to be attracted to each other
1) Caused by ASYMMETRICAL e- DISTRIBUTION in charge cloud, forming a temporary dipole.
2) This dipole causes ANOTHER OPPOSITE temporary dipole - two dipoles are attracted to each other
3) Second dipole causes ANOTHER dipole - CHAIN REACTION
4) e- are constantly moving therefore dipoles CREATED AND DESTROYED all the time
5) overall effect = atoms attracted to each other
Structure w/ Van Der Waals intermolecular forces
Can hold MOLECULES in a lattice
Effect of Mr on VDW
Larger molecules = larger electron clouds = stronger VDW
Hydrogen Bonding
ONLY occurs when H is covalently bonded to F/N/O
F/N/O very electronegative so draw bonding e- away from H
Combination of highly polarised molecule with H’s high charge density allows H to form weak bonds with lone 2e- on F/N/O of other molecules.
Effect of H bonding
Extra energy needed to break H bonds => Substances with H bonds have higher melting and boiling points
H bonding and ice
liquid water cools to form ice => molecules make more H bonds => form regular lattice structure => molecules further apart in solid form than liquid form => ice is less dense than water
Effect of shape on VDW
Longer and/or straighter molecules = more surface contact between molecules/molecules in closer proximity to each other = stronger VDW
