ch13-1web

Chem 5 Chapter 13 Liquids, Solid, and Intermolecular Forces Part 1 December 2, 2002

Condensed phases - Solids and Liquids

Chemical Bonds: • Ionic bond • Covalent bond • Metallic bond

Intermolecular Interactions: • Ion-dipole • Dipole-dipole • Dispersion (London) force or instantaneous dipole • Hydrogen bond

2s 2s Two Li atoms

2s

2s 2s Three Li atoms

Empty delocalized orbitals Half antibonding

responsible for conductivity

Half bonding

Li

Li2

Li3

Small spacing

Band of 1023 delocalized molecular orbitals of slightly different energies

Energy

Band half filled in lithium

2s band is completely filled in beryllium but overlaps with the 2p band. Be is a conductor.

Graphite and Conducting Polymers

2p

Electrons in a 2D box Conductor because band half-filled with electrons

Electrons in a 1D box

Diamond

sp3 hybridization

Conduction and Valence Bands for Diamond Conduction band

Valence band

Diamond is an insulator due to the large band gap.

Bands of semiconductors (Si, for example) formed from equivalent sp3 localized hybrid orbitals Conduction band

Si-Si bond weaker than C-C bond, smaller gap between σ and σ* orbitals

Semiconductors have small band gaps.

Valence band

Band Gaps

Li

Be

Si

Diamond

Doping in semiconductors Si doped with P

Conduction by electrons in the conduction band

Si doped with Al

Conduction by holes in the valence band

Intermolecular Interactions (Van der Waals Forces): • Ion-dipole • Dipole-dipole • Dispersion (London) force • Hydrogen bond

Van der Waals Forces • Dipole-Dipole Interaction - Molecules with dipole moments Examples: HCl, H2S

• Dispersion (London) Force - Instantaneous dipoles Example: He condenses to a liquid at 4K and freezes to a solid under pressure at1K.

Repulsion due to electron interaction

Attraction due to Dispersion force

Dispersion Force

He - He

Ar - Ar

A comparison of the potential energy curves between two Ar atoms and two He atoms. The larger Ar atoms are more tightly held, although the bond energy is still 1/400 that of H-H bond.

Hydrogen Bond A special type of intermolecular attraction between the hydrogen atom in a polar bond (particularly an H-F, H-O, or H-N bond) and an unshared electron pair on a nearby small electronegative atom (usually an F, O, or N atom on another molecule).

.. —N—H | 180o

.. : O— H | H

.. : O—H |

H | : N—H | H

Hydrogen bond energy 15-40kJ/mol

.. : F—H ..

.. : F—H ..

Comparison of boiling points of some hydrides of the elements of groups 14, 15, 16, 17

The values for NH3, H2O and HF are unusually high because of hydrogen bonds.

Hydrogen Bonding in Ice

Movie of Ice and Water Made by Molecular Dynamics Simulation Molecular dynamics simulation is a powerful computational tool to study liquids. Real-time trajectories of atoms are obtained by integrating the equations of motion based on the Newton’s law. These equations describe motions governed by the potential of interacting molecules in the system.

Ice at 100 K Red, O

Silver: H

Water 300K Dashed line: Hydrogen bond

Hydrogen bonds determine the structure of proteins.

Amide planes

The first model by Linus Pauling

Hydrogen bonds are responsible for base pairing in DNA.

Watson (left) and Crick proposed the double helix model of DNA in 1953. Their model was based on data obtained by British chemist Rosalind Franklin. She used the X-ray diffraction technique to show that the DNA has a helical structure. Without her permission, her senior colleague Maurice Wilkins shared some of her data with Waston and Crick. These data together with Waston and Crick’s realization of base pairing by hydrogen bonding allowed Waston and Crick to deduce the double helix model. Four years after Franklin’s death, Watson and Crick shared the Noble Prize with Wilkins for the discovery of DNA’s structure. Franklin might have shared in the Prize had she lived.

"It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." – Waston and Crick

Rosalind Franklin

Surface Tension: the energy required to increase the surface area of a liquid. There is a tendency for liquids to maintain a minimum surface area.

Adhesive force between the water and oil is not enough to spread water

Water wets a clean glass surface but beads up along a surface coated with oil.

Cohesive Forces: intermolecular forces between like molecules Adhesive Forces: intermolecular forces between unlike molecules

Anti-fog Coating Film for Bathroom Mirrors

Small water droplets condensed on the glass cause it to appear foggy. The large adhesive force between the water and the polymer film helps disperse the droplets, clearing the mirror.

Summary