Intermolecular Forces Unit 8 Intermolecular Forces

Intermolecular forces are attractive forces between molecules. (Example: water molecule to water molecule) Intramolecular forces hold atoms together within in a molecule. (Example: H to O bond within a water molecule). Intermolecular vs Intramolecular 41 kJ to vaporize 1 mole of water (inter) 930 kJ to break all O-H bonds in 1 mole of water (intra) “Measure” of intermolecular force boiling point melting point DHvap DHfus DHsub Generally, intermolecular forces are much weaker than intramolecular forces. 11.2

Intermolecular Forces 1. London Forces (a.k.a. Dispersion Forces) Weakest Dipole-Dipole Interactions Ion-Dipole Interactions -(Salt dissolving in solution; Na+ and Cl- ) 4. Hydrogen Bonding (STRONGEST)

London Dispersion Forces: Weakest Occur between every compound and arise from the net attractive forces amount molecules which is produced from induced charge imbalances Figure 10-8 Olmsted Williams The larger the molecule the greater it’s Dispersion Forces are. Animation: http://chemmovies.unl.edu/ChemAnime/LONDOND/LONDOND.html Figure 10-9 Olmsted Williams

The boiling point of long molecules increase with the length of the carbon chain. Olmsted Williams Fig 10-10 Pg 437

How molecular shape affects the strength of the dispersion forces The shapes of the molecules also matter. Long thin molecules can develop bigger temporary dipoles due to electron movement than short fat ones containing the same numbers of electrons.                                                                                                                                                              Butane has a higher boiling point because the dispersion forces are greater. http://www.chemguide.co.uk/atoms/bonding/vdw.html

Polarizability the ease with which the electron distribution in the atom or molecule can be distorted. Polarizability increases with: greater number of electrons more diffuse electron cloud Dispersion forces usually increase with molar mass. 11.2

Is the Molecule Polar? The more Electronegative atom will pull the electron density of the bond Closer to itself giving it a partial negative charge leaving the other Atom with a partially positive charge. This is a dipole moment.

Molecules with 3 Atoms CO2 HCN SO2 Even though the C-O bond is polar, the bonds cancel each other out because the molecule is linear the dipole moments are equal and in opposite directions.Therefore CO2 is non-polar. CO2 The dipole moment between H-C points in the direction of C. The dipole moment points between C-N points in the direction of the N. Therefore the dipole vectors are additive and HCN is polar HCN SO2 SO2 is a polar molecule because the S-O dipole Moments don’t cancel each other out due to the angle

Molecules with 4 Atoms CCl4 is non-polar CHCl3 is polar

How to Determine if a Molecule Is Polar Draw Lewis Structure If all of the regions of electron density are bound to the same thing (CCl4; CO2 ) than the molecule is non-polar If the regions of electron density are not bound to the same thing (atom) than the molecule is polar (HCN; SO2)

Which of the following molecules are polar (have a dipole moment)?H2O, CO2, SO2, and CH4 O H S O dipole moment polar molecule dipole moment polar molecule C H C O no dipole moment nonpolar molecule no dipole moment nonpolar molecule 10.2

Orientation of Polar Molecules in a Solid Dipole-Dipole Forces Attractive forces between polar molecules Orientation of Polar Molecules in a Solid Animation: http://chemmovies.unl.edu/ChemAnime/DIPOLED/DIPOLED.html 11.2

Dipole Forces occur between molecules containing a dipole moment. The positive end of the dipole moment on one mole is attracted to the Negative end of the dipole moment on a nearby molecule. 2-methyl propane (left) and acetone (right) Both compounds are about Equal in size and shape Figure 10-11 Olmsted Williams

Ion-Dipole Interaction Ion-Dipole Forces Attractive forces between an ion and a polar molecule Ion-Dipole Interaction The larger the charge the stronger the force 11.2

A molecular picture showing the ion-dipole Olmsted Williams Fig 10-34 A molecular picture showing the ion-dipole Interaction that helps a solid ionic crystal dissolve in water. The arrows indicate ion-dipole interactions.

What type(s) of intermolecular forces exist between each of the following molecules? HBr HBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules. CH4 CH4 is nonpolar: dispersion forces. S O SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules. 11.2

The Hydrogen Bond STRONGEST INTERMOLECULAR FORCE http://chemmovies.unl.edu/ChemAnime/HYBOND/HYBONDD.html Animation

water has the highest heat of fusion water has the highest melting point water has the highest boiling point water has the highest heat of vaporization water has the lowest molar mass The melting point, boiling point, heat of fusion and heat of vaporization of water are extremely high and do not fit the trend of properties relative to molar mass within Group VIA.

Water exhibits these unusual properties because of hydrogen bonding between water molecules.

A hydrogen bond is an intermolecular bond. A hydrogen bond is formed between polar molecules that contain hydrogen covalently bonded to a small, highly electronegative atom: F, O, N. F—H O—H N—H

it will be attracted to another F, O, or N, on another molecule. A dipole-dipole bond bond will be formed between the two molecules which is called a hydrogen bond. hydrogen bond covalent bond covalent bond

Water in the liquid and solid states exists as groups in which the water molecules are linked together by hydrogen bonds. 13.8