Intermolecular forces
- Created by: Jasmin
- Created on: 23-01-14 09:52
View mindmap
- Intermolecular Forces
- Forces between molecules
- Much weaker than covalent bonds
- London Forces
- Van der Waals forces or dispersion forces
- Exist between all molecular substances an also in monotomic rare gases like helium
- Caused by instantaneous unequal electron distribution in an atom, which gives rise to a temporary or instantaneous dipole
- Origin: instantaneous sipole-induced dipole interactions
- Electrons in a covalent molecule osciallte within the covalent bonds
- Causes an induced dipole in the opposite direction on a neighbouring atom, so the two dipoles attract
- Induced dipole can cause further induced dipoles on the neighbouring atoms. Net result is a weak attractive force
- More electrons in a molecule, the larger the induced dipoles and so the larger the London forces
- Overall effect is for the atoms to be attracted to each other
- Molecules with greater SAs have stronger London forces
- Stronger London forces means HMP/HBP
- Long chain alkanes have higher BP than short-chain alkanes
- Straight chain alkanes have higher BP than branched alkanes
- Induced dipole can cause further induced dipoles on the neighbouring atoms. Net result is a weak attractive force
- Causes an induced dipole in the opposite direction on a neighbouring atom, so the two dipoles attract
- Permanent dipole-dipole forces
- Occur between permanent dipoles in neighbouring polar molecules
- Polar molecules have small opposite charges S+ and S-
- The S+ on one molecule attracts the S- on another polar molecule
- The S+ and S- charges on polar molecules cause weak electrostatic forces of attraction between molecules
- Occur between permanent dipoles in neighbouring polar molecules
- Hydrogen bonds
- H must be bonded directly to a highly electronegative atom - F, O, N
- For H bonds to form there must be a lone pair of electrons on the highly electronegative atom
- H-F, H-O and H-N bonds are strongly polar
- S+ H atom is within the orbital of the non-bonding pair of electrons on the O atom. As if the H has formed a second weak covalent bond - H bond is at 180 degrees to the normal covalent bond between O and H
- Strongest force
- Higher BP and MP because of extra energy needed to break the H bonds
- H20 molecules in ice are further apart on average, making ice less dense than liquid water
- H-F-H bond angle 109.5 degrees F-H-F 180 degrees
Comments
No comments have yet been made