Liquids, Solids and Materials

1. Chapter 12 Liquids, Solids and Materials

2. Gases, Liquids and Solids high temperatures low temperatures Kinetic Energy Attractive Intermolecular Forces

3. + + + + + - - - - Ionic Liquids Ionic Forces Ion-Ion Ion-Dipole Ion-ion forces are very strong and produce high boiling points and melting points. e.g. NaCl dissolved in water e.g. NaCl(s) Ions form strong intermolecular forces with polar molecule water.

4. Intermolecular Interactions • Three important intermolecular interactions: Dispersion Forces (non-polar molecules), Dipole-Dipole Forces (polar molecules) and Hydrogen Bonding Forces (molecules with F-H, O-H, N-H bonds). Dispersion Forces (van der Waals Forces) • The electrons on one atom are attracted to the nucleus on a neighbouring atom. • This creates an “instantaneous” (i.e. temporary) dipole on the first atom. • The instantaneous dipole on the first atom then induces an instantaneous dipole on the second atom. • The two induced dipoles attract each other.

5. Dispersion Forces (van der Waals Forces) • Dispersion Forces are proportional to a molecule’s polarizability. • The polarizability, the ease with which the e- cloud can be deformed, is approximately proportional to the number of electrons and molecular weight. • For example, the noble gases are spherical, non-polar atoms…… • As the molecule (atom) increases in size, the boiling point increases. • Tbp is a good measure of the strength of intermolecular forces. A higher Tbp indicates stronger intermolecular forces. • Dispersion Forces also depend upon the molecular shape, e.g., Tbp(n-pentane) = 36oC and Tbp(neopentane) = 9oC

6. Propane, MW = 44 Acetonitrile, MW = 41  = 3.9 Debye (D) Large Dipole Moment, Tbp = +82 oC   0 No Dipole Moment, Tbp = -42 oC Dipole-Dipole Forces In liquids of polar molecules, oppositely charged ends of the molecules tend to attract each other, causing partial alignment. For molecules of roughly equal MW’s (i.e. with similar Dispersion Forces), the molecule with the higher dipole moment will have a higher boiling point due to greater Dipole-Dipole forces.

7. Hydrogen Bonding • Hydrogen bond is a special intermolecular interaction between the H atom in a polar N-H, O-H or F-H bond and an electronegative O, N or F atom.

8. Properties of Liquids Viscosity • Viscosity is the resistance of a liquid to flowing. • High viscosity liquids (e.g. molasses, motor oil) flow slowly. Low viscosity liquids (e.g. water, gasoline) flow quickly. • When a liquid flows down a tube, the molecules slide over one another. The higher the forces of attraction, the less of a tendency the liquid will have to flow, and the higher will be the viscosity. Surface Tension • A molecule in the bulk liquid is stabilized by the attraction of the other liquid molecules. A molecule on the surface will be at a higher energy than one in the bulk of the liquid. • Surface tension is a measure of the strength of intermolecular attractions which pull on molecules at the surface of a liquid.

9. Surface Tension • The stronger intermolecular forces, the higher surface tension. • Surface tension is responsible for the rise of a liquid such as H2O in a thin glass capillary. This also gives rise to a meniscus when water or an aqueous solution is in a pipette or a burette. Adhesion is an attraction between unlike molecules Cohesion is the attraction between like- molecules • It is because of its high surface tension that water tends to “bead” up on a waxy surface. That’s because a sphere gives the minimum ratio of surface area to volume. • The high surface tension of water also causes molecules on the surface to pack very closely together. This is why some insects can “walk on water”.

10. Amorphous solid e.g. Silica Glass (SiO2) Structure of Solids • A crystalline solidpossesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions • An amorphous solid does not possess a well-defined arrangement and long-range molecular order. Irregular (disordered) arrangement of atoms (or molecules) are found in an amorphous solid. Crystalline solid, e.g. Quartz (SiO2)

11. lattice point unit cell 3-D lattice arrangement The Crystalline Lattice • Lattice is the three dimensional array of points repeating periodically. • Lattice point can be atoms, molecules and ions. • Unit cell is the basic repeating structural unit of a crystalline solid. • The unit cells stacked in 3-D space describe the bulk arrangement of atoms of the crystal. The unit cell is given by its lattice parameters, the length of the cell edges & the angles between them, while the positions of the atoms inside a unit cell are described by the set of atomic positions measured from a lattice point.

12. primitive cell body-centred face-centred The Crystalline Lattice and Type of Cell

13. The 14 Bravais Lattices

14. 2d sinq = nl (Bragg Equation) X-Ray Crystallography BC + CD = 2d sinq = nl

15. Types of Solids • There are four classifications of solids, depending on the type of bonds that are present:, Ionic Solids, Metallic Solids, Molecular Solids and Covalent Network Solids. Ionic Solids • Lattice points occupied by cations and anions held together by electrostatic attraction • Hard, brittle, high melting point, poor conductor of heat and electricity • Examples: All typical salts, e.g. NaCl, Ca(NO3)2, MgBr2 CsCl ZnS CaF2

16. Metallic Solids • Lattice points occupied by metal atoms held together by metallic bond • Soft to very hard, low to very high melting point, excellent thermal and electrical conductivity, malleable and ductile • Examples: All metals, e.g. Cu, Fe, Sn, Au, Ag Bonding due to delocalized valence electrons (shown in blue) Strength of bonding varies between different metals, resulting in wide range of physical properties Molecular Solids • Lattice points occupied by molecules held by intermolecular forces • Fairly soft, moderately low melting point (usually < 200oC), poor thermal and electrical conductivity • Examples: Argon , CH4, CO2, H2O

17. Covalent Network Solids • Lattice points occupied by atoms connected in network of covalent bonds • Hard, high melting point, poor conductor of heat and electricity • Examples: Diamond (C), Quartz (SiO2) Each carbon is connected to 4 others by covalent bonds carbon atoms graphite diamond

18. Dicopper(II) tetracarboxylate building block [Cu3{1, 3, 5-C6H3-(COOH)3}2(H2O)x] Coordination Network Solids • Coordination polymer is a metal coordination compound where a ligand bridges between metal centres, where each metal centre binds to more than one ligand to create an infinite array of metal centres e.g. a polymer. • More conventionally, coordination polymer is reserved for compounds where the metals are bridged by multi-dentate ligands, such as cyanide or carboxylates. • The majority of common halides and oxides are coordination polymers.

19. Crystal Engineering • Crystal engineering is the design and synthesis of molecular solid-state structures with desired properties based on an understanding and exploitation of intermolecular interactions. • Crystal engineering relies on non-covalent bonding to achieve organization of molecules and ions in the solid state. Much of the initial work on purely organic systems focused on the use of hydrogen bonds, though with the more recent extension to inorganic systems, the coordination bond has also emerged as a powerful tool. • Other intermolecular forces such as p…p, halogen…halogen and Au…Au interactions have all been exploited in crystal engineering studies, and ionic interactions can also be important. • Polymorphism is the phenomenon wherein the same chemical compound exists in different crystal forms. It is one of the most exciting branches of the subject partly because polymorphic forms of drugs may be entitled to independent patent protection if they show new and improved properties over the known crystal forms.