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London Forces or van der Waals Forces    Dipole-Dipole Attraction    Hydrogen Bonding


Hydrogen Bonding


  Hydrogen bonds occur between molecules that have a permanent net dipole resulting from hydrogen being covalently bonded to either fluorine, oxygen or nitrogen. For example, hydrogen bonds operate between water (H2O) molecules, ammonia (NH3) molecules, hydrogen fluoride (HF) molecules, hydrogen peroxide (H2O2) molecules, alkanols (alcohols) such as methanol (CH3OH) molecules, and between alkanoic (caboxylic) acids such as ethanoic (acetic) acid (CH3COOH) and between organic amines such as methanamine (methyl amine, CH3NH2). 

Are a stronger intermolecular force than either Dispersion forces or dipole-dipole interactions since the hydrogen nucleus is extremely small and positively charged and fluorine, oxygen and nitrogen being very electronegative so that the electron on the hydrogen atom is strongly attracted to the fluorine, oxygen or nitrogen atom, leaving a highly localised positive charge on the hydrogen atom and highly negative localised charge on the fluorine, oxygen or nitrogen atom. This means the electrostatic attraction between these molecules will be greater than for the polar molecules that do not have hydrogen covalently bonded to either fluorine, oxygen or nitrogen.



If only dispersion forces are present, then the more electrons the molecule has (and consequently the more mass it has) the stronger the dispersion forces will be, so the higher the melting and boiling points will be. Consider the hydrides of Group IV, all of which are non-polar molecules, so only dispersion forces act between the molecules. CH4 (molecular mass ~ 16), SiH4 (molecular mass ~ 32), GeH4 (molecular mass ~ 77) and SnH4 (molecular mass ~ 123) can all be considered non-polar covalent molecules. As the mass of the molecules increases, so does the strength of the dispersion force acting between the molecules, so more energy is required to weaken the attraction between the molecules resulting in higher boiling points.

If a covalent molecule has a permanent net dipole then the force of attraction between these molecules will be stronger than if only dispersion forces were present between the molecules. As a consequence, this substance will have a higher melting or boiling point than similar molecules that are non-polar in nature. Consider the boiling points of the hydrides of Group VII elements. All of the molecules HF (molecular mass ~ 20), HCl (molecular mass ~ 37), HBr (molecular mass ~ 81) and HI (molecular mass ~ 128) are polar, the hydrogen atom having a partial positive charge (H) and the halogen atom having a partial negative charge (F, Cl, Br, I). As a consequence, the stronger dipole-interactions acting between the hydride molecules of Group VII elements results in higher boiling points than for the hydrides of Group IV elements as seen above. With the exception of HF, as the molecular mass increases, the boiling point of the hydrides increase. HF is an exception because of the stronger force of attraction between HF molecules resulting from hydrogen bonds acting between the HF molecules. Weaker dipole-dipole interactions act between the molecules of HCl, HBr and HI. So HF has a higher boiling point than the other molecules in this series.

London Forces or van der Waals Forces    Dipole-Dipole Attraction    Hydrogen Bonding

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