Intermolecular Forces

1. Chemical Bonding, Part 2 Intermolecular Forces Honor Chemistry Only

2. A phase is a homogeneous part of the system in contact with other parts of the system but separated from them by a well-defined boundary. 2 Phases Solid phase - ice Liquid phase - water

3. A Molecular Comparison of Liquids and Solids Recall that we go from a gas to a liquid to a solid, a substance’s molecules become closer to each other. We know this happens by decreasing the temperature, which also decreases the volume. Intermolecular forces are the forces that hold solids and liquids together. These are the forces of attraction between molecules (inter-). (An intramolecular force is one that holds the atoms inside a molecule together via a covalent bond.) Intermolecular forces are much weaker than intramolecular forces. When a substance melts or boils, the intermolecular forces are broken while the covalent bonds (intramolecular) stay in place. Intermolecular forces form when a substance condenses or freezes.

4. 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

5. Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together.

6. Intermolecular Forces They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities.

7. INTRAMOLECUAR BOND Which is a stronger bond? INTERMOLECULAR BOND Generally, intermolecular forces are much weaker than intramolecular forces.

8. Intermolecular forces are feeble; • 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.

9. Intermolecular Forces • Ion-Dipole Interactions (Salt dissolving in solution) • Dipole-Dipole Interactions • Dispersion London Forces • Hydrogen Bonding

10. 1) An ion-dipole force is the interaction of an ion and a polar solvent, such as water. This is the strongest of all intermolecular forces. A larger charge and smaller ion means a larger ion-dipole attraction. Example: When NaCl is dissolved in water, the sodium and chloride ions are pulled apart and kept separated by the water molecules. The sodium cation is attracted to the partially negative end of the water molecule (oxygen). The chloride anion is attracted to the partially positive end of the water molecule (hydrogen).

11. 1) Ion-Dipole Interactions • A ion-dipole interactions are an important force in solutions of ions. • The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents.

12. 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

13. 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.

14. 2) Dipole-dipole forces are the third strongest intermolecular force. Dipole-dipole forces exist between neutral polar molecules. For dipole-dipole forces to be effective, the polar molecules must be close together. There is a mix of repulsion and attraction between the ends of the dipoles. A stronger bond means that there is a higher melting point and boiling point because more energy is needed to break the bond.

15. 2) Dipole-Dipole Interactions • Molecules that have permanent dipoles are attracted to each other. • The positive end of one is attracted to the negative end of the other and vice-versa. • These forces are only important when the molecules are close to each other.

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

17. From experimental data, it was found that the boiling points of compounds with H-F, H-O, and H-N bonds were abnormally high. This means the intermolecular bonds in these molecules had to be stronger than normal; however, the only forces present were dipole-dipole forces. This led to the discovery of a special case of dipole-dipole forces: hydrogen bonding. Hydrogen bonding is the second strongest of all intermolecular forces. Hydrogen bonding requires a hydrogen bonded to a strongly electronegative element (F,O or N usually). In these bonds, the electrons lie closer to the electronegative element. Since hydrogen has only one electron, the slightly positive hydrogen presents an almost bare proton.

18. In many cases, this almost bare proton will attract other (partially) negatively charged atoms besides the one it is bonded to (usually F, O, or N, on another molecule). This will create a “bond” between the hydrogen and the other atom. While this is not a true bond, there is a much stronger electrostatic attraction than normal which affects the properties of the substances involved. δ+ H Hydrogen bonding δ- δ- O O O C C H H CH2

19. 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

20. 3) Hydrogen Bonding • The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. • We call these interactions hydrogen bonds.

21. It is possible for two adjacent neutral molecules to affect each other. The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom), and vise versa. For an instant, the electron clouds become distorted. In that instant, a dipole is formed. One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule or atom. 4) Instantaneous dipoles are called London dispersion forces. These are the weakest of all intermolecular forces, but they exist between all molecules. Polarizability is the ease with which an electron cloud can be deformed. A larger molecule is more polarizable because it has more electrons. Thus, London dispersion forces increase with increasing molar mass.

22. 4) London Dispersion Forces Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1.

23. 4) London Dispersion Forces London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole.

24. 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

25. no yes yes no no yes no yes Interacting molecules or ions Are polar molecules involved? Are polar molecules involved? Are ions involved? Are hydrogen atoms bonded to F, O or N? London dispersion forces only Dipole-dipole forces Hydrogen bonding Ion-dipole forces Ionic bonding Increasing strength