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# Kinetic Theory - PowerPoint PPT Presentation

Kinetic Theory. 1.) All matter is composed of small particles. 2.) These particles are in constant motion. Kinetic Theory. 3.) Collisions between particles are perfectly elastic (no change in total kinetic energy of the system). Physical States. States of Matter Solid Liquid Gaseous

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• 1.) All matter is composed of small particles.

• 2.) These particles are in constant motion.

• 3.) Collisions between particles are perfectly elastic (no change in total kinetic energy of the system).

• States of Matter

• Solid

• Liquid

• Gaseous

• Plasma

• Molecular Substances:

• 1.)General Properties- nonconductors of electricity, often insoluble in water, low melting and boiling points.

• Molecules are relatively easy to separate from each other because intermolecular forces are weak.

• 2.) Dispersion Forces - result from temporary dipoles formed in adjacent molecules.

• Their strength depends on how readily electrons are dispersed and increase with increasing molecular size and mass.

• Ex. Ordinarily the boiling points of molecular substances will increase with increasing molar mass.

• Ex. F2 < Cl2 < Br2 < I2.

• B.P. (in degrees Celsius) are: -188, -34, 59, 184

• 3.) Dipole forces- electrical attractive forces between the + end of a polar molecule and the - end of an adjacent molecule.

http://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804http://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

Molecular Substanceshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Ex. Compare the boiling pts of NO (bp = -151oC), N2 (bp = -196oC), and O2(bp = - 183oC).

Molecular Substanceshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• 4.) Hydrogen bonds - are unusually strong dipole forces. The H atom is very small and differs greatly in electronegativity from F, O or N.

Molecular Substanceshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Compare the boiling points of Group 16 hydrides -

• H2O = 100oC

• H2S = -61oC

• H2Se = -42oC

• H2Te = -2oC

Molecular Substanceshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Note that water has many unusual properties in addition to high boiling point. The open structure of ice, a result of hydrogen bonding, accounts for its low density.

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Network Covalent Solids - ex. C, SiC, SiO2 (see. P. 246, 247)

• High melting pts (covalent bonds must be broken)

• Nonconducting

• Insoluble in water and other common solvents.

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Network Covalent Solids- ex. Compare the properties of diamond and graphite (allotropes)

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Ionic Solids- ex. NaCl, KNO3

• High melting points (due to strong attractive forces between oppositely charged ions).

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Ionic Solids-

• Nonconducting as solids, conducting when molten

• Often water soluble (depends on between attractive forces for each other versus for water)

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Ionic Bond Strength-

• Remember Coulomb’s Law - ex. CaO melts at 2927oC vs NaCl at 801oC

• Ex NaCl slightly higher than KBr (internuclear distance)

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Metallic Crystal/Solids-

• “Electron-sea” model (d-orbital overlap); cations in mobile sea of electrons.

• Conduct electricity, heat

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Metallic Crystal/Solids-

• Electrical conductivity is enhanced by the mobility of electrons

• Heat is carried by the collision of electrons (which is frequent in metals)

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Metallic Crystal/Solids-

• Ductile, maleable.

• Wide range of melting points, depending on the number of valence electrons.

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Metallic Crystal/Solids-

• Electrons act as a flexible glue holding the atomic nuclei together

• Lowest melting pt from +1 cations

Nonmolecular Solidshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Metallic Crystal/Solids-

• Insoluble in water and other common solvents

• Cations cannot dissolve by themselves

Crystal Structureshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Unit Cells in Metals-

• Unit Cell: smallest unit which, repeated over and over again, generates the crystal.

Crystal Structureshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Simple Cubic - unit cell consists of eight atoms at the corners of a cube.

• 2r = s

• r = atomic radius, s = edge length

Crystal Structureshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Face centered cubic - atoms at the corners of a cube and in the center of each face. Atoms touch along the face diagonally.

• 4r = s(2)1/2

Crystal Structureshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Body-centered cubic: atoms at corner of cube and at center. Atoms touch along the body diagonally.

• 4r = s(3)1/2

Crystal Structurehttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

Sodium crystallizes into a BCC structure; the unit cell has a length of 0.429nm. What is the atomic radius?

r = s(3)1/2/4 = 0.186nm

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Vapor Pressure - when a liquid is introduced into a closed container, it establishes a dynamic equilibrium with its vapor.

• Liquid <--> vapor

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• The pressure of the vapor at equilibrium is referred to as the vapor pressure of the liquid.

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Vapor Pressure is independent of the volume of the container.

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Add 0.0100 mol of liquid benzene to 1.00L flask at 25oC (vp benzene = 92 mm Hg). How much of the benzene vaporizes?

• n vap = 0.0050 mol, n liq = 0.0050 mol

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Add 0.0100 mol of liquid benzene to 2.00L flask at 25oC (vp benzene = 92 mm Hg). How much of the benzene vaporizes?

• all; no equilibrium is established

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Temperature dependence of vapor pressure: the vapor pressure of a liquid is always increases as temperature does.

Vapor pressure http://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

vs

Temperature

(degrees C)

Log VP vs

1/T (K)

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Water evaporates most readily on hot, dry days.

• Stoppers pop out of bottle of volatile liquids as temperature rises.

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• (to get a linear relationship) Use the Clausius-Clapeyron equation:

• ln P2/P1 = Hvap (1/T1 - 1/T2)/R

• Hvap = heat of vaporization in joules per mole

• R = 8.31 J/mol*K

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Vapor pressure of water:

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

Take the heat of vaporization of benzene to be 30.8 kJ/mol, vp benzene at 25oC = 92 mm Hg. What is the vapor pressure of benzene at 50oC?

241 mm Hg

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• NOTE: the pressure more than doubles when T rises from 25 to 50oC, reflecting the fact that more benzene vaporizes.

• The pressure of an ideal gas only increases by 10%.

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Boiling Point = temperature at which vapor bubbles form in liquid. Hence, boiling point varies with applied pressure.

• When the pressure is 760 mm Hg, water boils at 100oC, at 1075 mm Hg, it boils at 110oC, at 5 mm Hg, it boils at 0oC.

BOILINGhttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

POINT

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Critical temperature: temperature above which liquid cannot exist.

• Critical pressure: vapor pressure at critical T.

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Ex. Since critical T of oxygen is -119oC, liquid oxygen cannot exist at room temperature, regardless of pressure.

• Critical T of propane is 97oC; propane is stored as a liquid under pressure at room T.

Phase Diagramshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• Phase diagram - a graph showing temperatures and pressures at which liquid, solid, and vapor phases of a substance can exist.

Phasehttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

diagram

of

water

Phase Diagramshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• AB = vp curve, liquid - b p line

• AC = vp curve, solid - sublimation

• AD = melting point line

• A = triple point

• B = normal BP

Phase Diagramshttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• If AD slopes toward the P axis, mp decreases as P increases. This is true for water, where the liquid is the denser phase.

• More often the solid is denser. AD slopes away from P axis & mp increases with pressure.

Liquid more densehttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

- water

Solid more dense

Liq-Vap Equilibriahttp://www.wisc-online.com/objects/index_tj.asp?objID=GCH6804

• NOTE: solid sublimes below the triple point

• If the line AD slopes toward P axis, melting point decreases as P increases. This behavior is observed for water, where the liquid is the more dense phase. More often, AD tilts away from the P axis, and the melting point increases with pressure.