Chem: Topic 7: INTERMOLECULAR FORCES

• Induced dipole-dipole interaction

  - aka London forces/dispersion forces/van der Waals forces

  - weakest type of intermolecular force

  - non-polar molecules (noble gases and hydrocarbons) lack a permanent dipole, however, they can experience temporary dipoles due to the movement of electrons

  - these temporary dipoles induce the neighbouring molecules/dipoles, resulting in attractive forces --> temporary imbalances in charge

  
• Permanent dipole-dipole interactions

  - a type of intermolecular force that acts between molecules with a polar bond

  - this is due to a difference in electronegativity

  - the δ+ and δ− regions of neighbouring polar molecules attract each other and hold together in a lattice-like structure

  - stronger than London forces

  
• hydrogen bonding

  - strongest type of intermolecular force

  - only acts between hydrogen and the 3 most electronegative atoms (nitrogen, oxygen, fluorine)

  - the lone pairs on these atoms form a bond with a δ+ hydrogen atom (shown w a dotted line)

  - examples: H2O, NH3, and HF all have hydrogen bonds

  
• properties of hydrogen bonding

  - molecules held together with hydrogen bonds have much higher melting and boiling points compared to similar-sized molecules without hydrogen bonding

  - H2O: simple molecular structure but unusually high boiling point due to the presence of hydrogen bonds that require a lot of energy to overcome

  - hydrogen bonds also result in ice having a much lower density than liquid water --> molecule is in a rigid structure with lots of air gaps

  - hydrogen bonding is also the reason alcohols have a higher boiling point than alkanes with similar Mr --> the lone electron pair (on the oxygen atom) is able to form hydrogen bonds with another hydrogen
• boiling point trends in alkanes

  - Van der Waals forces act between organic alkane chains

  - straight-chain alkanes: as the chain length increases, so does the Mr --> number of electrons per molecule increases therefore a larger electron cloud --> greater fluctuation in electron density --> larger instantaneous and induced dipoles --> stronger London forces --> higher boiling point

  - branched alkanes: less able to pack closely together, fewer electrons per molecule, smaller electron cloud --> distance over which the intermolecular forces act is increased, weakening the forces --> lower boiling point
• boiling point trends in hydrogen halides

  - hydrogen flouride has the highest boiling point

  - boiling point increases down a group (past HF) as the halide increases in size --> number of electrons increase --> stronger London forces--> more energy required to overcome them

  
• boiling point trends in hydrogen halides: ANOMALIES

  - the boiling points of H2O, HF, and NH3 are all higher than the other hydrides in their groups

  - this is because H-bonding is present between the molecules in each of these compounds --> stronger intermolecular forces
• solvents

  - H2O: its hydrogen bonding capabilities allow it to dissolve some ionic compounds by solvating the individual ions, and to dissolve some alcohols by forming hydrogen bonds with their hydroxyl group.

  - HOWEVER: both water and alcohols are poor solvents for the dissolving of some polar molecules such as halogenoalkanes that cannot form hydrogen bonds

  - non-aqueous solvents are used for compounds which have the same type of intermolecular force.