Chapter 11 States of Matter and Intermolecular Forces General Chemistry: An Integrated Approach...

Chapter 11States of Matter and

Intermolecular Forces

General Chemistry: An Integrated Approach

Hill, Petrucci, 4th Edition

Mark P. HeitzState University of New York at Brockport

© 2005, Prentice Hall, Inc.

Chapter 11: States of Matter and Intermolecular Forces 2

Intermolecular Forces

Intramolecular forces determine such molecular properties as molecular geometries and dipole moments

EOS

Intermolecular forces determine the macroscopic physical properties of liquids and solids


Intermolecular ForcesGases – fill container, random rapid motion, never coming to rest or clumping togetherMotion is mainly translational

Liquids – fixed volume, flow and assume shape of container, only slightly compressible, stronger forces hold molecules togetherMotion is mainly translational

EOS

Solids – fixed volume, definite shape, generally less compressible than liquids, forces hold particles in a fixed shapeMotion is mainly vibrational


Vaporization and CondensationVaporization is the conversion of a liquid to a gas

The enthalpy of vaporization (Hvapn) is the quantity of heat that must be absorbed to vaporize a given amount of liquid at a constant temperature H2O(l) H2O(g)

= +44 kJ mol–1

EOS

Condensation (Hcondn) is the change of a gas to a liquid

H2O(g) H2O(l) = –44 kJ mol–1


Enthalpies of VaporizationBecause enthalpy is a function of state, the total enthalpy change between vaporization and condensation at constant temperature must be zero

EOS


Vapor PressureThe vapor pressure of a liquid is the partial pressure exerted by the vapor when it is in dynamic equilibrium with a liquid at a constant temperature.

EOS

As T vapor pressure


Vapor Pressure of Water

EOS


Vapor Pressure as a Function of Temperature

EOS

Illustration


Boiling PointThe boiling point of a liquid is the temperature at which its vapor pressure becomes equal to the external pressure

EOS

The normal boiling point is the boiling point at 1 atm – e.g., water boils at 100 oC

At reduced pressure, the normal boiling temperature lowers


Critical PointThe critical temperature, Tc, is the highest temperature at which a liquid and vapor can coexist in equilibrium as physically distinct states of matter

The critical pressure, Pc, is the vapor pressure at the critical temperature

The condition corresponding to a temperature of Tc and a pressure of Pc is called the critical point

EOS


The Critical Point

EOS


Critical Temperature and Pressure of Various Substances

Substances existing beyond the critical point are termed supercritical fluids

EOS


Phase Changes Involving SolidsThe conversion of a solid to a liquid is called melting, or fusion, and the temperature at which a solid melts is its melting point

The enthalpy of fusion, Hfusion, is the quantity of heat required to melt a given amount of solid

Sublimation is the process of a molecule’s passing directly from the solid to the vapor state

EOS

Hsubln = Hfusion + Hvap


Some Enthalpies of Fusion

EOS


Heating and Cooling Curves

EOS


A phase diagram is a graphical representation of the temperature and pressure conditions under which a substance exists as a solid, liquid, gas, or some combination of these in equilibrium

Phase Diagrams

fusion curve

triple point

EOS

critical point

vapor pressure curve

sublimation curve


Phase Diagram for HgI2

EOS


Phase Diagram for CO2

EOS


Phase Diagram for H2O

EOS


Dispersion ForcesDispersion forces are forces of attraction between an instantaneous dipole and an induced dipole

… also called London forces after Fritz London who offered a theoretical explanation in 1928

The polarizability of an atom or molecule is a measure of the ease with which electron charge density is distorted by an external electrical field

EOS

Dipoles can be induced in molecules


Dispersion ForcesThe greater the polarizability of molecules, the stronger the intermolecular forces between them

EOS


Dipole–Dipole ForcesDipole–dipole forces arise when permanent dipoles align themselves with the positive end of one dipole directed toward the negative ends of neighboring dipoles

EOS

A permanent dipole in one molecule can induce a dipole in a neighboring molecule, giving rise to a dipole–induced dipole force


Predicting Physical Properties of Molecular Substances

Dispersion forces (always present) become stronger with increasing molar mass and elongation of moleculesIn comparing nonpolar substances, molar mass and molecular shape are the essential influencing factors

EOS

Dipole–dipole and dipole–induced dipole forces are found in polar substances. The more polar the substance (dipole moment), the greater the intermolecular force is expected to be


Hydrogen Bonds

A hydrogen bond is an intermolecular force in which a hydrogen atom covalently bonded to a nonmetal atom in one molecule is simultaneously attracted to a nonmetal atom of a neighboring molecule

EOS

The strongest hydrogen bonds are formed if the nonmetal atoms are small and highly electronegative – e.g., N, O, F


Hydrogen Bonding in Ice

EOS


Hydrogen Bonding EffectsSolid water is less dense than liquid water due to hydrogen bonding

Hydrogen bonding is also the reason for the unusually high boiling point of water

EOS

The structures of proteins, substances essential to life, are determined partly by hydrogen bonding Proteins


Hydrogen Bonding EffectsMany organic acids can form dimers due to hydrogen bonding

Certain organic molecules can also form an intramolecular hydrogen bond

EOS


Surface Tension

Surface tension () is the amount of work required to extend a liquid surface and is usually expressed in unit of J m–2

EOS

Adhesive forces are intermolecular forces between unlike moleculesCohesive forces are intermolecular forces between like molecules


Another Surface PhenomenonA meniscus is the interface between a liquid and the air above it

EOS

Capillary action results from intermolecular interactions


Viscosity

Liquids which flow easily are said to be mobile

EOS

Viscosity units are poise (P)

Typical liquids have viscosities measured in cP

Viscosity is a measure of a liquid’s resistance to flow


Characteristics of Crystalline Solids

EOS


Network Covalent Solids

These substances contain a network of covalent bonds that extend throughout a crystalline solid, holding it firmly together

Allotropes are two or more forms of an element that differ in basic molecular structure

EOS

Sulfur and carbon are examples of elements that form allotropes


Allotropes of CarbonDiamond has each carbon bonded to four other carbons in a tetrahedral arrangement using sp3 hybridization

EOS

Graphite has each carbon bonded to three other carbons in the same plane using sp2 hybridization


Crystal Lattices

The repeating unit of the lattice is called the unit cell

EOS

Crystals have repeating units …

… in multidimensional space


Unit CellsThe simple cubic cell is the simplest unit cell and has structural particles centered only at its corners

The body-centered cubic (bcc) structure has an additional structural particle at the center of the cube

EOS

The face-centered cubic (fcc) structure has an additional structural particle at the center of each face


Unit Cells – Coordination NumberThe coordination number is the number of nearest neighbor particles to a single particle in the crystal

EOS

In a simple cubic structure, the coordination number is 6


Packing in Crystals

“Open” packing has larger voids in between particles compared to close-packed crystals

EOS


Close-Packed StructuresTetrahedral holes are located above a sphere in the bottom layer

EOS

Octahedral holes are located above a void in the bottom layer


Close-Packed StructuresHexagonal close-packed (hcp) arrangements occur when the third layer covers the tetrahedral holes. These produce two-layer repeating units

EOS

Cubic close-packed (ccp) arrangements occur when the third layer covers the octahedral holes. These produce three-layer repeating units


Ionic Crystal StructuresSmaller cations can fill the voids between the larger anions

Tetrahedral hole filling occurs when the radii ratio is:

0.225 < rc/ra < 0.414

Octahedral hole filling occurs when the radii ratio is:

0.414 < rc/ra < 0.732

EOS

The arrangement is cubic if rc/ra > 0.732


Two Examples

EOS


Experimental Determinationof Crystal Structures

EOS

n = 2d sin


Summary of Concepts• Vaporization is the passage of molecules from the

liquid to the gaseous state• A phase diagram is a plot showing the phases of a

substance at various temperatures and pressures• Fluctuations in electrical charge produce an

instantaneous dipole, which induces a dipole in neighboring atoms and are called dispersion forces

EOS

• A strong intermolecular force called the hydrogen bond is formed when a hydrogen atom is attracted to a N, O, or F atom on a neighboring molecule


Summary of Concepts• The intermolecular forces in a liquid determine its

viscosity, or resistance to flow• In some solids, covalent bonds extend throughout a

crystal• In ionic solids, interionic attractions bind all the

ions together in a crystal• The structure of a crystal is described by a three-

dimensional pattern called a lattice

EOS

• Unit cell properties and dimensions can be used to find atomic radii and the densities of crystalline substances

Chapter 11 States of Matter and Intermolecular Forces General Chemistry: An Integrated Approach...

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Transcript of Chapter 11 States of Matter and Intermolecular Forces General Chemistry: An Integrated Approach...