7 Intermolecular Forces PDF

7: Intermolecular Forces London Forces London forces occur between all molecular substances and noble gases. They do not occur in ionic substances. London Forces are also called instantaneous, induced dipole- dipole interactions. They occur between all...

7: Intermolecular Forces London Forces London forces occur between all molecular substances and noble gases. They do not occur in ionic substances. London Forces are also called instantaneous, induced dipole- dipole interactions. 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 London Forces The more electrons there are in the molecule the higher the chance that temporary dipoles will form. This makes the London forces stronger between the molecules and more energy is needed to break them so boiling points will be greater. 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 London forces between the molecules. This is why I2 is a solid whereas Cl2 is a gas. 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 London forces between molecules. The shape of the molecule can also have an effect on the size of the London forces. Long straight chain alkanes have a larger surface area of contact between molecules for London forces to form than compared to spherical shaped branched alkanes and so have stronger London forces. Permanent dipole-dipole forces Permanent dipole-dipole forces occurs between polar molecules It is stronger than London forces 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. Permanent dipole forces occur in addition to London forces N Goalby chemrevise.org 1  Hydrogen bonding 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 + δ- δ- Hδ δ+ 180o N O 180o H δ+ δ - δ - + δ + + H F H F Hδ H Hδ δ+ δ+ δ- H δ+ N H Always show the lone pair of electrons on the δ- O,F,N and the dipoles and all the δ- δ+ charges δ+ H O Hδ + H δ+ δ+ Hydrogen bonding occurs in addition to London forces δ+ H H The hydrogen bond should have an bond angle of 180o with one O of the bonds in one of the molecules δ- The bond angle is 180O around the H atom because there are two Water can form two hydrogen bonds pairs of electrons around the H atom involved in the hydrogen per molecule, because the bond. These pairs of electrons repel to a position of minimum electronegative oxygen atom has two repulsion, as far apart as possible. lone pairs of electrons on it. It can therefore form stronger Alcohols, carboxylic acids, proteins, amides all can form hydrogen bonds hydrogen bonding and needs more energy to break the bonds, leading to a higher boiling point. Alcohols form hydrogen bonds. This means alcohols have higher boiling points and relatively low volatility compared to Ice H H alkanes with a similar number of electrons O H H O O H H H HO O H H In ice the molecules are held further apart by the hydrogen bonds than in liquid water and this explains the lower density of ice Hydrogen bonding is stronger than the other two 400 types of intermolecular bonding. H2O Boiling point K The anomalously high boiling points of H2O, NH3 and 300 HF H2Te HF are caused by the hydrogen bonding between H2Se SbH3 these molecules in addition to their London forces. NH3 H2S HI AsH3 The additional forces require more energy to break 200 HBr PH3 HCl SnH4 and so have higher boiling points GeH4 SiH4 The general increase in boiling point from H2S to H2Te or 100 CH4 from HCl to HI is caused by increasing London forces between molecules due to an increasing number of electrons. 25 50 75 100 125 N Goalby chemrevise.org Molecular mass 2  Solvents and Solubility Solubility of a solute in a solvent is a complicated balance of energy required to break bonds in the solute and solvent against energy given out making new bonds between the solute and solvent. Ionic substances dissolving in water hydration of the ions When an ionic lattice dissolves in water it involves breaking up the bonds in the lattice and forming new bonds between the metal ions and water molecules. The negative ions are attracted to the δ+ hydrogens on the polar water molecules and The higher the charge density the greater the hydration the positive ions are attracted to the δ- oxygen enthalpy (e.g. smaller ions or ions with larger charges) on the polar water molecules. as the ions attract the water molecules more strongly. Solubility of simple alcohols δ- δ+ δ- + O H O H δ H The smaller alcohols are soluble in water because they can form hydrogen bonds C H H δ+ with water. The longer the hydrocarbon H chain the less soluble the alcohol. C H H Insolubility of compounds in water Compounds that cannot form hydrogen bonds with water molecules, e.g. polar molecules such as halogenoalkanes or non polar substances like hexane will be insoluble in water. Solubility in non-aqueous solvents Compounds which have similar intermolecular forces to those in the solvent will generally dissolve Non-polar solutes will dissolve in non-polar solvents. e.g. Iodine which has only London forces between its molecules will dissolve in a non polar solvent such as hexane which also only has London forces. Propanone is a useful solvent because it has both polar and CH3 non polar characteristics. It can form London forces with some non polar substances such as octane with its CH3 groups. Its δ- δ+ δ- δ+ polar C=O bond can also hydrogen bond with water. O H O C δ + H CH3 N Goalby chemrevise.org 3