Intermolecular forces Flashcards
force of attraction in chemical bonds is
The force of attraction in all chemical bonds (intramolecular and intermolecular) is electrical
temporary dipole or temporary polarity. AKA London Dispersion forces.
electrons are mobile, and at any one instant, they might find themselves towards one end of the molecule, making that end -. The other end will be temporarily short of electrons and so becomes + PARTIALLY. Another instant, the electrons may move to the opposite side of molecule reversing its polarity. This causes rapid dipole fluctuation even in most symmetrical molecule. This happens even to noble gases in monoatomic form.
how do temporary dipoles give rise to intermolecular attractions?
molecule with temporary dipole approaches a non-polar molecule (or one who is less polar making the other molecule more dominant) and the non-polar one would be slightly attracted to the positive end of the temporary dipole molecule. This sets up an induced dipole in the other molecule so that the +ve end is attracted to the -ve end. A second later, the electrons may have moved to the other end and the 2 molecules may repel. The polarity of both molecules reverses but you have + attracted -ve. As long as molecules stay close to each other, the polarities will continue to fluctuate in synchronization so attraction is always there. This affects massive numbers of molecules.
Strength of London dispersion forces
Dispersion forces between molecules are much weaker than the covalent bonds within molecules. They are the weakest intermolecular forces but every molecule has it.
How does London Dispersion affect boiling points?
The bigger the molecule, the more electrons, and the larger the electron cloud and the more distance over which they can move, the bigger the possible temporary dipoles and therefore the bigger the dispersion forces. Bigger molecules have more electrons and more distance over which temporary dipoles can develop - and so the bigger molecules are “stickier”.
Molecular shape and London Dispersion Forces
long chains of molecules are stronger since they fit together more easily that molecules that have odd limbs and stuff. More linear chain, more attraction, more higher boiling point.
Dipole-Dipole interactions
- when molecules with dipoles attract other molecules
- only POLAR molecules have this
- Dipole-dipole interactions are not an alternative to dispersion forces - they occur in addition to them. Molecules which have permanent dipoles will therefore have boiling points rather higher than molecules which only have temporary fluctuating dipoles.
- sometimes however, dispersion forces can be stronger because the other molecule may have significantly MORE electrons
Symmetrical molecules?
- symmetrical means that there is no dipole on molecule
When do dipoles cancel out?
- dipoles only cancel out if the electronegativity difference in each bond is the same (and if each bond has same ion at each end)
Hydrogen Bonding
strong dipole – dipole interaction that occurs between molecules in which hydrogen is bonded to a highly electronegative atom (F, O, N- they are small and electronegative). only POLAR molecules have this.
- Only works when hydrogen is positive in molecule
- higher hydrogen bonding potential, higher the boiling point
- THIS IS THE STRONGEST FORM OF INTERMOLECULAR FORCES since the electronegativity difference is as high as it can be without becoming ionic
- not a true bond since it is just interactions between other molecules
Dipole-Induced Dipole Interaction
This form of interaction exists between a polar molecule and a non-polar molecule. When a non-polar molecule approaches a polar molecule, its electrons will tend to be attracted by the slightly positive end of the left hand one. This sets up an induced dipole in the approaching molecule, which is orientated in such a way that the + end of one is attracted to the - end of the other.
ion- dipole interaction
this form of interaction occurs between polar molecules and ions. For example, as an ionic solid such as NaCl dissolves in a polar solvent such as water, water molecules surround Na+ and Cl- ions forming ion dipole-interactions. The partially negative end of the water molecule (the oxygen atom) attracts the cation, Na+ and the partially positive end of the water molecule (hydrogen atoms) attract the anion, Cl-.
