2.3 Intermolecular Forces

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2.3 Intermolecular forces

- Permanent Dipole Interactions- Polar molecules have permanent separation of charge (dipole). This means that polar molecules are attracted to other polar molecules as one end of the molecule will be positive and the other end will be negative.

- Instantaneous Dipole Interactions- 2 non-polar molecules. Electron distribution around a molecule moves all the time so at one point one end of the molecule might be positive (due to fewer electrons) whilst the other end might be negative (due to more electrons). This creates a temporary (or instantaneous) dipole (separation of charge).

- Induced Dipole- If a molecule next to another has an instantaneous dipole, then that molecule will have a delta negative end and a delta positive end and this will be attracted to the delta negative end or delta positive end of the molecule.

-London forces is an instantaneous dipole and an induced dipole interaction.

-Intermolecular forces- attractions between one molecule and another.

-Intramolecular forces- forces of attractions within a molecule.

-Hydrogen bonding occurs between a hydrogen molecule and and a molecule that has a lone pair. (H-F, H-N, H-O)

-Melting and boiling point of alkanes increase with chain length as the number of intermolecular forces increases.

- The more branched a molecule is, the lower the boiling point. This is because straight chain molecules can lie side by side creating more intermolecular forces which require more energy to overcome.

-Alcohols are more volatile (how easily molecules escape from a liquid) because the OH molecule attracts oxygen molecules atoms from another molecule.

-Alcohols have a higher boiling point because they form hydrogen bonds which requires more energy than just London forces alone.

-H-F is high because it contains hydrogen bonding. Cl- Br- I then increases size and mass which leads to increased London forces.

-Solubility of ionic solid in water- The positive ions in the solid are attracted to the delta negative oxygen molecules found in water whilst the negative ions are attracted to delta positive hydrogen atom in water. This breaks up lattice and the individual ions are spread throughout the water. The water molecules arrange themselves around the ions, with positive ions the oxygen molecule is more attracted to it, with negative ions the hydrogen molecules are more attached to it.

-Solubility of alcohols and carboxylic acids in water- Alcohols are soluble because the OH group can hydrogen bond to the OH groups in water. Solubility decreases as chain length increases because less of the molecule is polar.

-Non-polar substances are insoluble in water- very weak forces between the non-polar substance cannot overcome the hydrogen bonds in water so the molecules can't disrupt the water structure. This also happens in halogenoalkanes because only a small proportion of the molecule (the carbon-halogen bond) is polar whilst most of the molecule is non-polar. There is not enough polar character to overcome the water structure.

-Two organic (non polar) molecules- are soluble because of the weak London forces throughout the two substances.