Intramolecular Forces

1. Intramolecular Forces

2. Is the Molecule Polar? We have already talked about diatomic molecules. 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. Thus giving the molecule A dipole moment. But what about molecules made up of more than two molecules?

3. Molecules with 3 Atoms 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

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

5. 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 than the molecule is polar (HCN; SO2)

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

7. Intermolecular Forces • 1. London Forces (Dispersion Forces) • Dipole-Dipole Interactions • 3. Ion-Dipole Interactions (Salt dissolving in solution) • 4. Hydrogen Bonding

8. Generally, intermolecular forces are much weaker than intramolecular forces. Intermolecular forces are attractive forces between molecules. Intramolecular forces hold atoms together in a 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 11.2

9. Intermolecular forces are feeble; but without them, life as we know it would be impossible. Water would not condense from vapor into solid or liquid forms if its molecules didn't attract each other. Intermolecular forces are responsible for many properties of molecular compounds, including crystal structures (e. g. the shapes of snowflakes), melting points, boiling points, heats of fusion and vaporization, surface tension, and densities. Intermolecular forces pin gigantic molecules like enzymes, proteins, and DNA into the shapes required for biological activity. http://www.nationmaster.com/encyclopedia/Image:Myoglobin.png

10. Intermolecular Forces • 1. London Forces (Dispersion Forces) • Dipole-Dipole Interactions • 3. Ion-Dipole Interactions (Salt dissolving in solution) • 4. Hydrogen Bonding

11. Dispersion Forces Occur between every compound and arise from the net attractive forces which is produced from induced charge imbalances Figure 10-8 Olmsted Williams The magnitude of the Dispersion Forces is dependent upon how easily it is to distort the electron cloud. The larger the molecule the greater it’s Dispersion Forces are. Figure 10-9 Olmsted Williams

12. Orientation of Polar Molecules in a Solid Dipole-Dipole Forces Attractive forces between polar molecules 11.2

13. Dipole Forces occur between molecules containing a dipole moment. The positive end of the dipole moment on one molecule is attracted to the Negative end of the dipole moment on a nearby molecule. Consider 2-methyl propane (left) and acetone (right) Both compounds are about Equal in size and shape therby Having similar dispersion forces, But Acetone contains an Oxygen (red) and causes the Molecule to have a dipole Moment allowing it to have Dipole forces and thus a Higher boiling point Figure 10-11 Olmsted Williams

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

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

16. O O S 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. SO2 SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules. 11.2

17. or … … H H B A A A Hydrogen Bond The hydrogen bond is a special dipole-dipole interaction between they hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N, or F atom. A & B are N, O, or F 11.2

18. Fig 10-16B Pg 444 Crystals of benzoic acid contain pairs of molecules held together head to head by hydrogen bonds. These pairs then stack in planes which are Held together by dispersion forces. Courtesy Stephen Frisch

19. Intermolecular Forces • 1. London Forces (Dispersion Forces) • Dipole-Dipole Interactions • 3. Ion-Dipole Interactions (Salt dissolving in solution) • 4. Hydrogen Bonding These forces affect how molecules will interact with each other and As a general rule as the strength of the force increases the boiling Point of the compound increases