Liquids and solids

Liquids and solids Chapter 10

Intermolecular forces • The attractive and repulsive force between the atoms in the molecule are called as intermolecular forces. • London forces • London forces exist in nonpolar molecules. • These forces result from temporary charge imbalances. The temporary charges exist because the electrons in a molecule or ion move randomly in the structure. The nucleus of one atom attracts electrons form the neighboring atom. At the same time, the electrons in one particle repel the electrons in the neighbor and create a short lived charge imbalance.

These temporary charges in one molecule or atom attract opposite charges in nearby molecules or atoms. A local slight positive charge d+ in one molecule will be attracted to a temporary slight d- negative charge in a neighboring molecule.

Dipole-dipole interactions Dipole-dipole interactions exist between molecules that are polar. This requires the presence of polar bonds and a un symmetric molecule. These molecules have a permanent separation of positive and negative charge. In the illustration the H end of HCl is permanently slightly positive charge. The Cl end of HCl has a permanent slight negative charge. The "H" in one molecule is attracted to the "Cl" in a neighbor. The intermolecular force is weak compared to a covalent bond. But this dipole-dipole interaction is one of the stronger intermolecular attractions.

Hydrogen bonding • The polar molecules, such as water molecules, have a weak, partial negative charge at one region of the molecule, and a partial positive charge where the hydrogen atoms are. • Thus when water molecules are close together, their positive and negative regions are attracted to the oppositely-charged regions of nearby molecules. This force of attraction, is called a hydrogen bond. Each water molecule is hydrogen bonded to four others.

Liquid structure • Viscosity is a quantity that describes a fluid's resistance to flow. • The higher the viscosity of a liquid, the more slowly it flows; hydrogen bonded liquids typically have high viscosities. Viscosity usually decreases with increasing temperatures.

Surface tension • The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This cause an imbalance of the intermolecular forces at the surface which causes surface tension.

Structures Of Solids • The ordered arrangement of atoms, molecules or ions in a crystalline solid means that we can describe a crystal as being constructed by the repetition of a simple structural unit. • The crystal structure of a material or the arrangement of atoms in a crystal can be described in terms of its unit cell.

Spheres can pack in close-packed and open (non close-packed) structures. • In the cubic crystal system, there are, besides the close-packed structure (face-centered cubic) two important packings;the simple cubic structure and body-centered cubic structure. b.c.c f.c.c Simple cubic

Counting the number of atoms in a unit cell • The atom in a unit cell are counted by determining what fraction of each atom resides within the cell. • The number of atoms in a unit cell is counted by noting how they are shared between neighboring cells.An atom at the center of a cell belongs entirely to that cell. For an fcc structure each of the eight corner atoms is shared by eight cells, so overall they contribute 8x 1/8=1atom to the cell. • Each atom at the center of each of the six faces contributes ½ an atom so jointly they contribute 6x1/2=3 atoms. • Total number of atoms in a fcc unit cell is 1+3=4and the mass of the unit cell is 4 times the mass of one atom.

Atom LocationFraction Inside Unit Cell • Corner 1/8Edge 1/4Face 1/2Anywhere else 1

Class practice • The atomic radius of copper is 128pm,and the density of copper is 8.93g/cm³. Is the copper metal close packed?

Band Theory of solids • In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically.

Doping of semiconductors • The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors. • Impurity atom with 5 valence electrons produce n-type semiconductors by contributing extra electrons. • Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a “hole" or electron deficiency.

An impurity of valence five elements is added to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers.

An N-type semiconductor (N for Negative) is obtained by carrying out doping, that is, by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers. • When the doping material is added, it donates weakly-bound outer electrons to the semiconductor atoms. This type of doping agent is also known as donor material since it gives away some of its electrons.

Consider the case of Si atom. Si atoms have four valence electrons, each of which is covalently bonded with one of four adjacent Si atoms. If an atom with five valence electrons, such as those from group 15 (eg. phosphorus, arsenic, or antimony), is incorporated into the crystal lattice in place of a Si atom, then that atom will have four covalent bonds and one un bonded electron. This extra electron is only weakly bound to the atom and can easily be excited into the conduction band. At normal temperatures, virtually all such electrons are excited into the conduction band.

P-type semiconductor (P for Positive) is obtained by carrying out doping, in order to increase the number of free (in this case positive) charge carriers. When the doping material is added, it takes away (accepts) weakly-bound outer electrons from the semiconductor atoms. This type of doping agent is also known as acceptor material and the semiconductor atoms that have lost an electron are known as holes.

The purpose of P-type doping is to create an abundance of holes. In the case of silicon, a trivalent atom (typically from group IIIA of the periodic table, such as boron or aluminium) is substituted into the crystal lattice. The result is that one electron is missing from one of the four covalent bonds. • Thus the dopant atom can accept an electron from a neighboring atoms' covalent bond to complete the fourth bond. Such dopants are called acceptors. The dopant atom accepts an electron, causing the loss of half of one bond from the neighboring atom and resulting in the formation of a "hole".

Phase changes

A phase change may be written as a chemical reaction. The transition from liquid water to steam, for example, may be written as • H2 (l) H2 (g) The equilibrium constant for this reaction (the vaporization reaction) is • K = Pw • where Pw is the partial pressure of the water in the gas phase when the reaction is at equilibrium. This pressure is often called the vapor pressure. The vapor pressure is literally the partial pressure of the compound in the gas.

The boiling point corresponds to the temperature at which the vapor pressure of the liquid equals the atmospheric pressure. • If the liquid is open to the atmosphere, it is not possible to sustain a pressure greater than the atmospheric pressure, because the vapor will simply expand until its pressure equals that of the atmosphere. • The temperature at which the vapor pressure exactly equals one atm is called the normal boiling point.

In a closed container vapor is formed as the molecules leave the surface of the liquid. As the number of molecules in the vapor phase increases, more of them strike the surface of the liquid. Eventually the number of molecules returning to the liquid matches the number escaping. The liquid is in equilibrium with the vapor. • H2O(l) H2O(g)

The van't Hoff equation provides a relationship between an equilibrium constant and temperature. • ln K = - ΔHvap /R T + ΔSvap/R • For this reaction, the equilibrium constant is simply the vapor pressure, P, which when substituted into the above equation yields the Claussius-Clapeyron equation. • ln P = - ΔHvap /R T + ΔSvap/R • The normal boiling point, Tbpo, corresponds to the temperature at which both the reactant and the product are in the standard state. A pure liquid under 1 atm pressure is in the standard state. A pure gas at 1 atm pressure is also in the standard state. Thus in the standard state P = 1 atm. This relation allows the Claussius-Clapeyron equation to be rewritten as • ln P = - ΔHvap / R (1/T – 1/Tbp⁰) • Tbp⁰= - ΔHvap / ΔSvap

Phase diagram • A phase diagram summarizes the pressures and temperatures at which each phase is most stable. The phase boundaries show the conditions under which two phases can coexist in equilibrium with each other. Three phases coexist at equilibrium at the triple bond. A substance cannot be converted to a liquid by the application of pressure if the temperature above is critical temperature of the substance.

Home work • Page 465 • 10.34, 10.42,10.44, 10.66,10.72

Liquids and solids