What is the difference between a Hydrogen bond and a London dispersion force?
Answer:
A hydrogen bond is an intermolecular attraction between a hydrogen atom and a small electronegative atom such as flourine or oxygen.
London Dispersion forces are also intermolecular forces and are explained as follows: In a molecule, there are free floating valence electrons from the outer valence electron shell of each atom. Electrons have no set position-their position is described by a probability diagram. Hence, at any given instant, there may be a greater concentration of electrons in one area then in another. This causes a temporary dipole moment.
Weak London dispersion forces or van der Waal's force These forces alway operate in any substance. The force arisen from induced dipole and the interaction is weaker than the dipole-dipole interaction. In general, the heavier the molecule, the stronger the van der Waal's force of interaction. For example, the boiling points of inert gases increase as their atomic masses increases due to stronger Landon dispersion interactions.
Hydrogen bond
Certain substances such as H2O, HF, NH3 form hydrogen bonds, and the formation of which affects properties (mp, bp, solubility) of substance. Other compounds containing OH and NH2 groups also form hydrogen bonds. Molecules of many organic compounds such as alcohols, acids, amines, and aminoacids contain these groups, and thus hydrogen bonding plays a important role in biological science.
london dispersion forces are due to temporary dipoles of a molecule. over time, these molecules are seen as neutral, but at any one moment in time, the charges within the molecule may not be equally dispersed. this intantaneous dipole (one end positive, the other negative) induces a temporary dipole on its neighbor (induced dipole), causing the molecules to be temporarily attracted. this can occur between all molecules.
hydrogen bonding can only occur between a molecule that has a highly electronegative atom (N, O, F) bound to a hydrogen atom (O-H) and a molecule with a highly electronegative atom with lone electron pairs (O:). a common example is water (H-O-H). the oxygen has unbound electron pairs that give the oxygen a partial negative charge. this in turn gives the hydrogen a partial positve charge. since like charges repel and opposite charges attract, a group of water molecules will align themselves accordingly: the H of one molecule is attracted to the O of another. this is a hydrogen bond. these are stronger than dispersion forces, but still relatively weak.
in summary, london dispersion forces are caused by temporary dipoles. hydrogen bonding is caused by permanent dipoles.
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