Chapter -13 Liquids and Solids Review Gases, liquids and solids: Some generalizations, page 378. Distinguish between different types of forces that exist in liquids and solids. Interionic forces: is the strongest of all the forces that exist between cations and anions that binds them together to form the crystal lattice of solid ionic compounds. Ionic compounds have high melting points due to the strong electrostatic attraction between ions of opposite charges. The greater the strength of Interionic forces, the higher the melting point of ionic compounds. Intramolecular forces are the forces that exist between atoms within a molecule or compound due to covalent bonding. Intermolecular forces or Van der Waals forces: exist between molecules. This force is weaker than intramolecular forces and used to determine the physical properties of molecular substances. DIPOLE FORCES: If there is a substantial difference in electro negativity between atoms that make up molecular compounds such as HCl that contain covalent bonds, due to development of partial + or –ve charges, charge separation occurs that leads to the formation of Dipoles. When molecules with dipole are brought together, the + ve end of one molecule attracts the –ve end of another molecule .these forces are called dipole forces. Hydrogen Bonds: are strong intermolecular forces that exist between polar molecules that have H atoms covalently bonded to a more electronegative F/O/N atom of molecule. Hydrogen bonding accounts for the unusually high boiling points for water, HF and NH3. London /Dispersive forces: The motion of electrons in non-polar molecules produces momentary dipoles that results in an attractive force that acts between the electron rich one end of molecule and the electron poor end of the next molecule. The transient attractive Forces between nonpolar molecules are called London or dispersive forces. London forces become significant in larger nonpolar molecules. The strength of dispersive force decreases as follows: I > Br > Cl > F. The Liquid State: The viscosity of liquid is related to the shape of molecules of liquid. Liquids with low viscosity flow readily as they are made of small , symmetrical molecules with weak intermolecular forces.2 types of intermolecular forces accounting for high viscosity of liquids are 1) London dispersion forces (present in larger nonpolar molecules) and hydrogen bonds present in small unsymmetrical molecules. Viscosity decreases with increasing temperature. Surface Tension: is the force of attraction that causes the surface of liquid to contract and form a bead or spherical drop. Liquids with strong intermolecular forces have higher surface tension than those with weak intermolecular forces. Chemicals that reduce surface tension of water are called surfactants found in laundry detergents. The wetting action is defined as ability of liquid to spread evenly over a surface as a thin film. Vaporization is a process by which molecules of liquid break away to enter into a gaseous phase. The reverse process of vaporization in which the vapor of liquid is converted back into liquid is called condensation. Evaporation is a process in which a liquid with low boiling point (volatile liquid) is converted from liquid phase to gaseous phase. In a closed system, when rate of evaporation of liquid is equal to the rate of condensation of liquid, dynamic equilibrium is achieved. The temperature at which liquid boils when the vapor pressure of liquid becomes equal to the atmospheric pressure (1atm /760 torr) is referred as the normal Boiling point of liquid. Boiling points are used to identify the given unknown liquids. Distillation: is a procedure used to resolve liquid mixtures on the basis of difference in their boiling points. Boil the liquid mixture in round bottom flask to produce vapors. When vapor temperature corresponds to its boiling point, the separated liquid distills out in pure form. Simple distillation is made up of one vaporization and one condensation step. Heat is absorbed from surroundings to vaporize a liquid. The quantity of heat required to vaporize 1 mol of a liquid at constant pressure and temperature is called molar heat of vaporization. Example 13.4, pg: 389. The Solid State: The molecules of solid are closely and tightly packed in definite array and pattern that defines the crystal lattice and three dimensional structure of solid. The degree of freedom of motion for the molecules of solid is very much restricted. The molecules of solid vibrate or oscillate from their almost fixed positions. As temperature increases the vibration of molecules of solid also increases. Solids are broadly classified as Crystalline solids and Noncrystalline solids. Noncrystalline solids: are irregularly shaped and structured amorphous solids. Molecules are not packed in definite order and hence crystal lattice is absent. Amorphous solids do not exhibit sharp melting points and soften gradually when heated. Crystalline Solids: The molecules of solid are closely and tightly packed in definite array and pattern that defines the crystal lattice and three dimensional structure of solid. The three types of crystal lattice found in crystalline solids are shown in fig 13.13, pg. 390. They are simple cubic, body centered cubic and face centered cubic arrangement. Cr, Mn, Fe and Alkali metals have body centered cubic arrangement in their crystal lattice. Some ionic compounds like NaCl, KCl and CaO have face- centered cubic arrangement in their crystal lattice. Classification of solids on the basis of bonding found in them Review the information in detail given in table 13.4 on page 391. Ionic solids have ions in each lattice point. (Hard, high melting, low vapor pressure, soluble in water) Molecular solids have individual, discrete covalent molecules at the lattice points. (Soft, low melting, high vapor pressure, insoluble in water) Covalent network/Macromolecular solids: contain atoms at the lattice points that are connected by covalent bonds leading to extensive network distributed throughout the molecule. These solids are extremely hard and nonvolatile, have extremely high melting points. Examples of such solids are tungsten carbide, Quartz, Silicon carbide and SiO2. When an element is capable of existing in 2 or more different physical forms in the same state, they are referred as Allotropes. The allotropes of carbon are diamond, graphite, nanotube (synthetic allotrope) which also happens to be covalent network solids. The carbon atoms are tetrahedral arranged in diamond connected by covalent bonds (fig 13.14/392). In Graphite the carbon atoms form hexagonal framework in each plane. The different planes are stacked in layers which slide over one another (fig. 13.15/ 393). Metallic solids: refer to pg: 224/ fig. 8.9. Metallic solid has three dimensional arrays of positive ions that remain fixed in the crystal lattice and the loosely held valence electrons cruise freely throughout the crystal like a fluid making the metallic solids as good conductors of heat and electricity. Melting point, Freezing point and Heating curve of water. The temperature at which solid and liquid exist in dynamic equilibrium is called melting point of solid The temperature at which liquid and solid are in dynamic equilibrium is called Freezing point. Molar heat of Fusion: The amount of heat required to convert 1 mol of a solid to a liquid at its melting point is called Molar heat of fusion. Molar heat of vaporization: The amount of heat required to convert 1 mol of a liquid into gas at its boiling point is called Molar heat of Vaporization. Study the figure 13.18, page v395 which displays the heating curve for water. Heat of fusion is less than heat of vaporization because less energy is required ( slope of solid in fig 13.18) to disrupt the molecules of Crystal lattice (solid) to allow the molecules to move around freely in the liquid state. The molecules of liquid still experience intermolecular force of attraction between them. Substantial amount of energy supplied in the form of heat is required (slope of liquid water in fig 13.18) to disrupt and overcome the intermolecular force of attraction experienced between the liquid molecules. This enables the liquid molecules to escape into gaseous phase. Review example 13.1, 13.2, 13.4, 13.5and 13.7 discussed in chapters 13. From this chapter I will just test you on definitions and on energy problems similar to 13.5 or 13.7. HW Assignment 13.2, 13.7, 13.9, 13.11, 13.12, 13.15, 13.53 and 13.65. Due next Thursday.