INTERMOLECULAR FORCES OF ATTRACTION - A- A- A- Attraction existing between molecules in a given substance - R- R- R- Responsible for the phase of the substance at a given temperature; for example, strong IMFA – solid at room temperature; weak IMFA – gas at room temperature

COMPARISON OF MOLECULAR MASS, BOILING POINT SUBSTANCE MOLECULAR WEIGHT (amu) BOILING POINT (K) PROPANE CH 3 CH 2 CH DIMETHYL ETHER CH 3 OCH METHYL CHLORIDE CH 3 Cl ACETALDEHYDE CH 3 CHO ACETONITRILE CH 3 CN 41355

TYPES OF IMFA (weakest to strongest) 1. VAN DER WAAL’S FORCES a. London dispersion force – caused by the movement of the electrons around the nucleus - effect is very temporary - present in all substances and only force present in non-polar molecules

LONDON DISPERSION FORCE

Induced dipole – temporary charges due to presence of electrons from neighboring atoms

BOILING POINTS OF HALOGENS substance Boiling point ( o C) F2F2F2F2-188 Cl Br 2 60 I2I2I2I2185 The higher the number of electrons, the stronger is the London dispersion force.

SHAPE OF THE MOLECULE AFFECTS BOILING POINT

b. dipole-dipole interaction – present only in polar covalent molecules - caused by the dipole moment (differences in the electronegativy of the elements present)

Ethanol is Polar

F O N c. Hydrogen bonding - present in polar covalent compounds where hydrogen is directly attached to Fluorine, Oxygen and Nitrogen ( the three most electronegative elements)

Hydrogen bonding only occurs if H is attached to F, O and N.

Hydrogen bonding in water

H bonding in ice

2. IONIC BONDING – -p-p-p-present only in ionic compounds. -E-E-E-Electrostatic force of attraction.

An ionic compound dissolved in water.

FORMULA TO DETERMINE THE NUMBER OF RINGS AND/OR DOUBLE AND TRIPLE BONDS 2 + 2C + N – H – X 2 ρ = where C = number of carbon atoms N = number of nitrogen atoms H = number of hydrogen atoms X = number of halogen (Group 17) atoms