Solids and Liquids

1. Solids and Liquids Unit 8

2. Solids – “no translation, occasional rotation, definite vibration” • There are 2 broad categories of solids: 1. crystalline – atoms arranged in a pattern 2. amorphous – no regular pattern • There are 3 main types of crystalline solids: 1. atomic – atoms at the lattice points a. metallic – delocalized e- bonding (metallic bonding), Ex. - Cu, Ag, Fe, Na, Mg b. network – strong, directional covalent bonding, Ex. – diamond, SiC, SiO2

3. Solids Continued 2. Molecular – molecules at the lattice points - intermolecular forces holding particles together Ex. – HCl, SO2, N2, H2O 3. Ionic – ions at the lattice points - particles held together by ionic bonding (electrostatic attraction) Ex. – NaCl, CaCl2, NaNO3

4. Crystal Lattice and Unit Cells • Crystal Lattice – well-organized, repeating pattern that particles are arranged in to form the crystal. • Unit Cell – smallest repeating unit in the crystal lattice which retains the properties of that solid. Lattice Unit Cell

5. Simple-Cubic Coordination number = 6 Number of particles/unit cell = 1

6. Body-Centered Cubic Coordination number = 8 Number of particles/unit cell = 2

7. Face-Centered Cubic Coordination number = 12 Number of particles/unit cell = 4

8. Properties of Solids • Melting Point • Ionic solids  high MP • Metallic solids  MP varies • Molecular solids  low MP • Network solids  very high MP • Conductivity • Ionic solids  good conductor (aq or l) • Metallic solids  good conductor (even as solid) • Molecular solids  nonconductors • Network solids  nonconductors (unless double bond)

9. Regular arrangement of metal cations forms the crystal Delocalized electrons move around between the metal atoms (cations) and act like the glue to hold them together Conductivity in solids

10. Doping: n- and p-semiconductors Doping means the introduction of impurities into a semiconductor crystal to modify the conductivity. Two of the most important materials silicon can be doped with, are boron (3 valence electrons = 3-valent) and phosphorus (5 valence electrons = 5-valent). Other materials are aluminum, indium (3-valent) and arsenic, antimony (5-valent). The impurity is integrated into the lattice structure of the semiconductor crystal, the number of outer electrons define the type of doping. Elements with 3 valence electrons are used for p-type doping, 5-valent elements for n-doping. The conductivity of a deliberately contaminated silicon crystal can be increased by a factor of 106.

11. Doping continued n-doping The 5-valent dopant(impurity) has one outer electron more than the silicon atoms. Four outer electrons combine with every one silicon atom, while the fifth electron is free to move and serves as charge carrier. This free electron requires much less energy to be lifted from the valence band into the conduction band, than the valence electrons of silicon. The dopant, which emits an electron, is known as an electron donor. Doped semimetals whose conductivity is based on free (negative) electrons are n-type.

12. Doped semiconductors are electrically neutral. The terms n- and p-type doped do only refer to the majority charge carriers. Each positive or negative charge carrier belongs to a fixed negative or positive charged dopant. Through the introduction of a dopant with five outer electrons, in n-dopedsemiconductors there is an electron in the crystal which is not bound and therefore can be moved with relatively little energy into the conduction band. Thus in n-doped semiconductors one finds a donator energy level near the conduction band edge, the band gap to overcome is very small.

13. n-doping with phosphorus Arsenic,oreven antimony, may be used instead of P p-doping In contrast to the free electron due to doping with phosphorus, the 3-valent dopant effect is exactly the opposite. The 3-valent dopants can catch an additional outer electron, thus leaving a hole in the valence band of silicon atoms.

14. Therefore the electrons in the valence band become mobile. The necessary energy to lift an electron into the energy level of indium as a dopant, is only 1 % of the energy which is needed to raise a valence electron of silicon into the conduction band. Due to positive holes these semiconductors are called p-conductive or p-doped.

15. p-doping with boron Al, Ga and In are also used as dopants for p-type semiconductors.

16. Substitutional Alloy • atoms of approximately the same size replace each other in the crystal • sterling silver, coins, solder, brass, 18-carat gold, bronze

17. Interstitial Alloy • atoms of smaller size fit into the space (interstices) between the larger atoms • cast iron, steel, stainless steel, surgical steel

18. Allotropes • Allotropes – two or more different molecular forms of the same element in the same physical state • they are composed of atoms of the same element, but have different properties because their structures are different • The element carbon is a good example- • Diamond • Graphite • Buckminsterfullerene (buckyball)

19. Amorphous Solids • Material that appears to be a solid but is not made up of an ordered crystal lattice • No defined MP – tend to soften over a range of temps. as it gets hotter • As it cools it flows more & more slowly (more viscous) • Amorphous solids are called supercooled liquids – they appear hard or rigid like solids but if an external force is applied it will flow and become deformed. • Ex. – plastic, glass, hard candy

20. Solubility What, again? Yes. Rule of thumb? Why? With your partner, or someone you like, discuss/review the 3 parts of solution formation, in terms of energy. • Solubilty (in water) • Ionic solids  usually, but depends on lattice E • Metallic solids  seriously? • Molecular solids  depends on polarity • Network solids  uh, no. For each pair decide which is most soluble: I2 or sucrose; NaOH or hexane; ethanol or butanol; calcium bromide or magnesium chloride

21. Water • Also, because of the different charges water molecules can bond to one another. • This hydrogen bonding gives water many of its unique properties.

22. Properties of Water • Hydrogen bonding causes water to have some unique properties: • high surface tension • high specific heat • low vapor pressure • high heat of vaporization • capillary action

24. Energy Requirements for State Changes • To change state, energy must be supplied in order to overcome the intermolecular forces (IMF). • Solid to a Liquid  Melting • particles overcome IMA and move around & past other particles • Solid to a Gas  Sublimation • occurs only at conditions far from normal MP

25. Energy Requirements for State Changes • Liquid to a Gas  Vaporization • particles are very spread out – requires a lot of energy • evaporation – vaporization at the surface of a liquid • Gas to a Liquid  Condensation

26. Vapor Pressure & Dynamic Equilibrium • Vapor pressure is the pressure exerted by a vapor above the liquid • Vapor pressure increases with temperature • Dynamic equilibrium refers to the point at which the rate of evaporation and rate of condensation are equal. Equilibrium Liquid just poured into open container, little vapor Evaporation faster than Condensation Evaporation as fast as Condensation

27. Boiling Point • Boiling occurs when a liquid turns to a gas inside the liquid • bubbles are produced • Liquid boils when its Vapor Pressure = Atmospheric Pressure • Normal boiling point • Larger IMF = lower vapor pressure = high BP • Weaker IMF = high vapor pressure = lower BP

29. How much energy would be given off if 25.0g of liquid water at 23.70C were cooled to ice at -5.90C? CpH2O(s) =2.06 J/g 0C; ∆Hfus = 334 J/g How many grams of water could be changed from liquid to gas if 69.7 kJ of heat is added to liquid water at 100.0 0C? ∆Hvap = 40.7 kJ/mol

31. Phase Change Diagram • We use these diagrams to relate the process that occur when a substance changes from one phase to another. • Substances are in the following states when in certain locations on the diagram: • Solid – left side of diagram • Liquid – middle of diagram • Gas/Vapor – right side of diagram • When either the temp or pressure is changed, you can identify the process that is taking place and identify the phase change. • Ex (from diagram on last slide) – At 1 atm if you increase the temperature from 90oC to 200oC, the process you are undergoing is vaporization or boiling (liquid to gas).

32. Phase Change Diagram • The change of state occurs right on the equilibrium line. • Triple point identifies the conditions when you have all 3 states in dynamic equilibrium with one another. • Tm  normal melting point • The point at 1 atm or 101.3 kPa when solid turns to liquid. • Tb  normal boiling point • The point at 1 atm or 101.3 kPa when a liquid turns to a vapor • Critical point – you are no longer able to distinguish between gas and liquid phases past this point.