4 Intermolecular bonding

Intermolecular bonding Induced dipole–dipole interactions Induced dipole–dipole interactions are also called London forces. They occur between all simple covalent molecules and the separate atoms in noble gases. In any molecule the electrons are moving constantly and randomly. As this happens the electron density can fluctuate and parts of the molecule become more or less negative i.e. small temporary or transient dipoles form. These temporary dipoles can cause dipoles to form in neighbouring molecules. These are called induced dipoles. The induced dipole is always the opposite sign to the original one. Main factor affecting size of Induced dipole–dipole interactions The more electrons there are in the molecule the higher the chance that temporary dipoles will form. This makes the Induced dipole–dipole interactions stronger between the molecules and so boiling points will be greater. Permanent dipole-dipole forces •Permanent dipole-dipole forces occurs between polar molecules •It is stronger than van der waals and so the compounds have higher boiling points •Polar molecules have a permanent dipole. (commonly compounds with C-Cl, C-F, C-Br H-Cl, C=O bonds) •Polar molecules are asymmetrical and have a bond where there is a significant difference in electronegativity between the atoms. The increasing boiling points of the alkane homologous series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the induced dipole–dipole interactions between molecules. Permanent dipole forces occur in addition to induced dipole–dipole interactions The increasing boiling points of the halogens down the group 7 series can be explained by the increasing number of electrons in the bigger molecules causing an increase in the size of the induced dipole–dipole interactions between the molecules. This is why I2 is a solid whereas Cl2 is a gas. The shape of the molecule can also have an effect on the size of the induced dipole–dipole interactions forces. Long chain alkanes have a larger surface area of contact between molecules for induced dipole–dipole interactions to form than compared to spherical shaped branched alkanes and so have induced dipole–dipole interactions. Induced dipole–dipole interactions occur between all molecular substances and noble gases. They do not occur in ionic substances. Permanent dipole–dipole and induced dipole–dipole interactions can both be referred to as van der Waals’ forces. The molecules are held further apart than in liquid water and this explains the lower density of ice O H H O H H O H H O H H O H H H2O H2S H2Se HF H2Te HCl HBr HI NH3 PH3 AsH3 SbH3 SiH4 CH4 GeH4 SnH4 100 200 300 400 Molecular mass 25 50 75 100 125 Boiling point K 7 N Goalby chemrevise.org It occurs in compounds that have a hydrogen atom attached to one of the three most electronegative atoms of nitrogen, oxygen and fluorine, which must have an available lone pair of electrons. e.g. a –O-H -N-H F- H bond. There is a large electronegativity difference between the H and the O,N,F Hydrogen bonding Always show the lone pair of electrons on the O,F,N and the dipoles and all the δ – δ + charges Hydrogen bonding occurs in addition to van der waals forces Water can form two hydrogen bonds per molecule, because oxygen is very electronegative, and it has two lone pairs of electrons. Molecular: Iodine There are covalent bonds between the Iodine atoms in the I2 molecule The crystals contain a regular arrangement of I2 molecules held together by weak induced dipole– dipole interactions intermolecular forces. Hydrogen bonding is stronger than the other two types of intermolecular bonding. The anomalously high boiling points of H2O, NH3 and HF are caused by the hydrogen bonding between the molecules The general increase in boiling point from H2S to H2Te is caused by increasing induced dipole– dipole interactions between molecules due to an increasing number of electrons. Alcohols, carboxylic acids, proteins, amides all can form hydrogen bonds