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Gases, Liquids, and Solids

Chapter 8

Chapter Overview

• States of matter and their changes• Kinetic Theory of gases• Relationships between pressure, volume and

temperature• Intermolecular Forces• Liquids and Solids

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Change of State Terminology• Changes from solid to liquid to gas are

endothermic; the opposite processes areexothermic

Kinetic Theory of Gases• Molecules of a gas are in constant motion and are

widely separated from each other– Most of a gas is empty space– Gases are compressible

• Collisions of the molecules with each other and thewall of the container are elastic– No energy is lost upon collisions

• There are no attractions or repulsions between gasmolecules– Each gas molecule acts independently of the others

• The average kinetic energy of the gas moleculesincreases with increasing temperature

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Gases• Because there is no attraction between molecules, gases

take on the shape and volume of the container• Because the molecules collide with the walls of the

container, gases exert a pressure– Pressure- the force per unit are exerted by a gas on the walls of the

container• Units of pressure P = F (force)/ A (unit area)

– mm Hg (torr)– Standard atmosphere (atm) 1 atm = 760 mm Hg– Pounds per square inch (psi) 1 atm = 14.7 psi– Pascal (Pa) - SI unit of pressure 1 atom = 101.3 kPa

• Standard Temperature and Pressure (STP) = 1 atm, 273K

Practical Application of Pressure• Measuring blood pressure

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Boyle’s Law• The volume of a fixed amount of gas is

inversely proportional to the pressure atconstant temperature.– Increase pressure, decrease the volume– Increase volume, decrease the pressure

• P1V1 = P2V2 for a fixed amount of gas

Practical Application of Boyle’s Law

• Breathing– If your diaphragm contracts, the volume of the thoracic

cavity expands, Pressure in the lungs is lower than theoutside pressure, so air flows into the lungs

– If your diaphragm expands, the volume of the thoraciccavity decreases, Pressure in the lungs exceeds theexternal pressure, so you expel the air

• SCUBA diving– You cannot hold you breathe while ascending from the

depths why?

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Henry’s Law

• Henry’s Law- The solubility of gas isdirectly proportional to the pressure of thegas– Explains why your champagne, beer and soft

drinks go flat after opening– Explains “rapture of the depths”- nitrogen

narcosis– Increase oxygen level of someone with

emphysema by administering air containing ahigher percentage of oxygen

Charles’s Law• The volume of a fixed amount of gas is directly

proportional to the temperature when the pressure is heldconstant

• V1/T1 = V2/T2

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Gay-Lussac’s Law• The pressure for a fixed amount of gas is directly

proportional to its temperature• P1/T1 = P2/T2

Combined Gas Law

• Since PV, V/T and P/V have constantvalues for a fixed amount of gas, theserelationships can be merged into acombined gas law

• P1V1/T1 = P2V2/T2

• Therefore if 5 of the components are knownthe 6th can be determined

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Avogadro’s Law and STP

• Avogadro’s Law - The volume of a gas is directlyproportional to its molar amount at a constantpressure and temperature

• V1/n1 = V2n2 where n = number of moles• Standard temperature and pressure (STP) is defined

as 0oC (273.15 K) and 1 atm(760 mm Hg)

• Standard molar volume - 1 mole of any gas willoccupy 22.4 L at STP

Ideal Gas Law

• Ideal Gas Law - The relationships betweenthe four gas variables (P, V, T, n) can becombined into a single expression

• PV = nRT, where R is the gas constant• R = 0.0821 L.atm/mol.K• If you know 3 of the 4 variables you can

determine the 4th

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Dalton’s Law of Partial Pressure• In a mixture of gases, each gas behaves

independently of the other components• The pressure of one component in a mixture

of gases is called the partial pressure of thatcomponent

• Dalton’s Law of Partial Pressures - the totalpressure in a mixture of gases is equal to thesum of the components of the mixture

• Ptotal = Pgas1 + Pgas2 + Pgas3 + …

Intermolecular Forces

• Intermolecular Forces - forces that actbetween different molecules– Determines physical properties of molecular

compounds, I.e., liquid or solid at a giventemperature

• Three main forces– Dipole-dipole forces– London Dispersion forces– Hydrogen bonding

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Dipole-dipole forces• Molecules containing polar covalent bonds may have a net

molecular polarity• Those that do have positive and negative ends that are

attracted to other molecules• Relatively weak force- 1 kcal/mol

London Dispersion Forces• Due to constant motion of electrons there are short-lived

regions of polarity.• All molecular compounds experience this• Strength of force increases with molecular weight, 0.5 - 2.5

kcal/mol and surface contact

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Hydrogen Bonds

• Hydrogen bonds - a special dipole-dipole interactionbetween an unshared electron pair attached to O, N, or Fand a hydrogen attached to another O, N or F

• Strongest of the 3 intermolecular forces- up to 10 kcal/mol

Hydrogen Bond Effects

• Hydrogenbonds affectboiling points

• Hydrogenbond networkextensive inwater andbiomolecules

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Liquids• Molecules are in motion in a liquid• Evaporation- molecules with enough energy can

escape the liquid state and enter the gas state (vapor)• Vapor pressure - In a closed system gas molecules

make contribution to total pressure (Dalton’s Law)

Boiling Points• Vapor pressure increases with temperature• Boiling point- vapor pressure equal to atmospheric

pressure• Lower atmospheric pressure, lower b.p.

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Water- Solvent of Life• Highest specific heat of any liquid• High heat of vaporization (540 cal/g)• Density decreases when solid

Solids• Crystalline solids- ordered arrangement of atoms,

ions or molecules– Ionic solids (ionic bonds, NaC:)– Molecular solids (intermolecular forces, ice)– Covalent network solids- (covalent bonds, diamond)– Metallic solids (metal atoms in a sea of electrons, Au)

• Amorphous solids- randomly arranged particlestypically due to cooling before internal orderestablished

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Change of State• Heat of fusion- energy required to completely melt a

substance once it has reached its melting point• Heat of vaporization- energy required to completely

vaporize a liquid once it has reached its boiling point• Both dependent of types of forces involved