Chemistry: A Molecular Approach

, 2nd Ed.

Nivaldo Tro

Chapter 11 Liquids, Solids, and Intermolecu lar Forces

April Senger MALMSTROM Park University Great Falls Montana

Properties of the three Phases of Matter

State Solid Liquid Gas Shape

fixed

Volume

fixed indefinite fixed indefinite indefinite

C om pr es W sib Density ill le it F ?

A lo ttr w?

ac In te tio rm ns S tre ole ng cu th la r o f

high high low No No No Yes Yes Yes very strong intermediate weak • Fixed = keeps shape when placed in a container • Indefinite = takes the shape of the container Tro: Chemistry: A Molecular Approach, 2/e 2

Three Phases of Water

Notice that the densities of ice and liquid water are much larger than the density of steam Notice that the densities and molar volumes of ice and liquid water are much closer to each other than to steam Notice that the densities of ice is larger than the density of liquid water. This is not the norm, but is vital to the development of life as we know it.

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Degrees of Freedom

• Particles may have one or several types of freedom of motion – and various degrees of each type • Translational freedom is the ability to move from one position in space to another • Rotational freedom is the ability to reorient the particles direction in space • Vibrational freedom is the ability to oscillate about a particular point in space Tro: Chemistry: A Molecular Approach, 2/e 4

Solids

• The particles in a solid are packed close together and are fixed in position – though they may vibrate • The close packing of the particles results in solids being incompressible • The inability of the particles to move around results in solids retaining their shape and volume when placed in a new container, and

Solids

• Some solids have their particles arranged in an orderly geometric pattern – we call these

crystalline solids

– salt and diamonds • Other solids have particles that do not show a regular geometric pattern over a long range – we call these

amorphous solids

– plastic and glass Tro: Chemistry: A Molecular Approach, 2/e 6

Liquids

• The particles in a liquid are closely packed, but they have some ability to move around • The close packing results in liquids being incompressible • But the ability of the particles to move allows liquids to take the shape of their container and to flow – however, they don ’ t have enough freedom to escape or expand to fill the container Tro: Chemistry: A Molecular Approach, 2/e 7

Gases

• In the gas state, the particles have complete freedom of motion and are not held together • The particles are constantly flying around, bumping into each other and the container • There is a large amount of space between the particles – compared to the size of the particles – therefore the molar volume of the gas state of a material is much larger than the molar volume of

Gases

• Because there is a lot of empty space, the particles can be squeezed closer together – therefore gases are compressible • Because the particles are not held in close contact and are moving freely, gases expand to fill and take the shape of their container, and will flow Tro: Chemistry: A Molecular Approach, 2/e 9

Compressibility

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Kinetic – Molecular Theory

• What state a material is in depends largely on two major factors 1. the amount of kinetic energy the particles possess 2. the strength of attraction between the particles • These two factors are in competition with each other Tro: Chemistry: A Molecular Approach, 2/e 11

States and Degrees of Freedom

• The molecules in a gas have complete freedom of motion – their kinetic energy overcomes the attractive forces between the molecules • The molecules in a solid are locked in place, they cannot move around – though they do vibrate, they don’t have enough kinetic energy to overcome the attractive forces • The molecules in a liquid have limited freedom – they can move around a little within the structure of the liquid – they have enough kinetic energy to overcome some of the attractive forces, but not enough to escape each other Tro: Chemistry: A Molecular Approach, 2/e 12

Kinetic Energy

• Increasing kinetic energy increases the motion energy of the particles • The more motion energy the molecules have, the more freedom they can have • The average kinetic energy is directly proportional to the temperature – KE avg = 1.5 kT Tro: Chemistry: A Molecular Approach, 2/e 13

Attractive Forces

• The particles are attracted to each other by electrostatic forces • The strength of the attractive forces varies, some are small and some are large • The strength of the attractive forces depends on the kind(s) of particles • The stronger the attractive forces between the particles, the more they resist moving – though no material completely lacks particle motion Tro: Chemistry: A Molecular Approach, 2/e 14

Kinetic –Molecular Theory of Gases

• When the kinetic energy is so large it overcomes the attractions between particles, the material will be a gas • In an ideal gas, the particles have complete freedom of motion – especially translational • This allows gas particles to expand to fill their container – gases flow • It also leads to there being large spaces between the particles

Gas Structure

Gas molecules are rapidly moving in random straight lines, and are free from sticking to each other.

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Kinetic –Molecular Theory of Solids

• When the attractive forces are strong enough so the kinetic energy cannot overcome it at all, the material will be a solid • In a solid, the particles are packed together without any translational or rotational motion – the only freedom they have is 17

Kinetic –Molecular Theory of Liquids

• When the attractive forces are strong enough so the kinetic energy can only partially overcome them, the material will be a liquid • In a liquid, the particles are packed together with only very limited translational or rotational freedom Tro: Chemistry: A Molecular Approach, 2/e 18

Explaining the Properties of Liquids

• Liquids have higher densities than gases and are incompressible because the particles are in contact • They have an indefinite shape because the limited translational freedom of the particles allows them to move around enough to get to the container walls • It also allows them to flow • But they have a definite volume because the limit on their freedom keeps the particles from Tro: Chemistry: A Molecular Approach, 2/e escaping each other 19

Phase Changes

• Because the attractive forces between the molecules are fixed, changing the material’s state requires changing the amount of kinetic energy the particles have, or limiting their freedom • Solids melt when heated because the particles gain enough kinetic energy to partially overcome the attactive forces • Liquids boil when heated because the particles gain enough kinetic energy to completely overcome the attractive forces – the stronger the attractive forces, the higher you will need to raise the temperature • Gases can be condensed by decreasing the temperature and/or increasing the pressure – pressure can be increased by decreasing the gas volume Tro: Chemistry: A Molecular Approach, 2/e 20 – reducing the volume reduces the amount of translational

Phase Changes

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Intermolecular Attractions

• The strength of the attractions between the particles of a substance determines its state • At room temperature, moderate to strong attractive forces result in materials being solids or liquids • The stronger the attractive forces are, the higher will be the boiling point of the liquid and the melting point of the solid – other factors also influence the melting point Tro: Chemistry: A Molecular Approach, 2/e 22

Why Are Molecules Attracted to Each Other?

• Intermolecular attractions are due to attractive forces between opposite charges – + ion to − ion – + end of polar molecule to − end of polar molecule • H-bonding especially strong – even nonpolar molecules will have temporary charges • •

Larger charge = stronger attraction Longer distance = weaker attraction

• However, these attractive forces are small relative to the bonding forces between atoms – generally smaller charges Tro: Chemistry: A Molecular Approach, 2/e 23 – generally over much larger distances

Trends in the Strength of Intermolecular Attraction

• • The stronger the attractions between the atoms or molecules, the more energy it will take to separate them • Boiling a liquid requires we add enough energy to overcome all the attractions between the particles – However, not breaking the covalent bonds

The higher the normal boiling point of the liquid, the stronger the intermolecular

24

Kinds of Attractive Forces

• Temporary polarity in the molecules due to unequal electron distribution leads to attractions called

dispersion forces

• Permanent polarity in the molecules due to their structure leads to attractive forces called

dipole –dipole attractions

• An especially strong dipole–dipole attraction results when H is attached to an extremely electronegative atom. These are called

hydrogen bonds .

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Dispersion Forces

• Fluctuations in the electron distribution in atoms and molecules result in a temporary dipole – region with excess electron density has partial (─) charge – region with depleted electron density has partial (+) charge • The attractive forces caused by these temporary dipoles are called

dispersion forces

– aka London Forces • All molecules and atoms will have them • As a temporary dipole is established in one Tro: Chemistry: A Molecular Approach, 2/e 26 molecule, it induces a dipole in all the

Effect of Molecular Size on Size of Dispersion Force

Therefore the strength of the dispersion forces increases.

The stronger the attractive forces between the molecules, the higher the boiling point will be.

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Boiling Points of

n

-Alkanes

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Alkane Boiling Points

• Branched chains have lower BPs than straight chains • The straight chain isomers have more surface-to surface 29

Practice – Choose the Substance in Each Pair with the Higher Boiling Point

a) CH 4 C 4 H 10 b) C 6 H 12 C 6 H 12 Tro: Chemistry: A Molecular Approach, 2/e 30

Practice – Choose the Substance in Each Pair with the Higher Boiling

a) CH 4

Point

CH 3 CH 2 CH 2 CH 3 Both molecules are nonpolar larger molar mass b) CH 3 CH 2 CH=CHCH 2 CH 3 cyclohexane Both molecules are nonpolar, but the flatter ring molecule has larger surface-to surface contact Tro: Chemistry: A Molecular Approach, 2/e 31

Dipole –Dipole Attractions

• Polar molecules have a permanent dipole – because of bond polarity and shape – dipole moment – as well as the always present induced dipole • The permanent dipole adds to the attractive forces between the molecules – raising the boiling and melting points relative to nonpolar molecules of similar size and shape Tro: Chemistry: A Molecular Approach, 2/e 32

Example 11.1b: Determine if dipole – dipole attractions occur between

Given: Find:

CH

2

Cl

2

molecules

CH 2 Cl 2 , EN C = 2.5, H = 2.1, Cl = 3.0

If there are dipole –dipole attractions

Conceptual Plan: Relationships:

Formula Lewis Structure Bond Polarity EN Difference Shape Molecule Polarity molecules that have dipole –dipole attractions must be polar

Solution:

Cl —C 3.0−2.5 = 0.5

no lone pairs = C —H 2.5−2.1 = 0.4

nonpolar polar bonds and tetrahedral shape = polar molecule polar molecule; therefore dipole – dipole attractions Tro: Chemistry: A Molecular Approach, 2/e 33

a)

Practice – Choose the substance in each pair with the higher boiling point

CH 2 FCH 2 F CH 3 CHF 2 b) or Tro: Chemistry: A Molecular Approach, 2/e 34

Practice – Choose the substance in each pair with the higher boiling point

a) CH 2 FCH 2 F CH 3 CHF 2 more polar b) polar or Tro: Chemistry: A Molecular Approach, 2/e 35 nonpolar

Hydrogen Bonding

• When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it – O ─ H, N ─ H, or F ─ H • Because hydrogen has no other electrons, when its electron is pulled away, the nucleus becomes deshielded – exposing the H proton • The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules Tro: Chemistry: A Molecular Approach, 2/e 36

H-Bonding

Tro: Chemistry: A Molecular Approach, 2/e 37 HF

H-Bonding in Water

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H-Bonds

• Hydrogen bonds are very strong intermolecular attractive forces – stronger than dipole–dipole or dispersion forces • Substances that can hydrogen bond will have higher boiling points and melting points than similar substances that cannot • But hydrogen bonds are not nearly as strong as chemical bonds – 2 to 5% the strength of covalent bonds Tro: Chemistry: A Molecular Approach, 2/e 39

Attractive Forces and Solubility

• Solubility depends, in part, on the attractive forces of the solute and solvent molecules – –

like dissolves like miscible liquids will always dissolve in each other

• Polar substances dissolve in polar solvents –

hydrophilic groups = OH, CHO, C=O, COOH, NH 2 , Cl

• Nonpolar molecules dissolve in nonpolar solvents –

hydrophobic groups = C-H, C-C

• Many molecules have both hydrophilic and hydrophobic parts – solubility in water becomes a competition between the attraction

Pentane, C 5 H 12

Immiscible Liquids

is a nonpolar molecule.

Water is a polar molecule.

The attractive forces between the water molecules is much stronger than their attractions for the pentane molecules. The result is the liquids are immiscible.

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Ion –Dipole Attraction

• In a mixture, ions from an ionic compound are attracted to the dipole of polar molecules • The strength of the ion–dipole attraction is one of the main factors that determines the solubility of ionic compounds in water Tro: Chemistry: A Molecular Approach, 2/e 42

Summary

• Dispersion forces are the weakest of the intermolecular attractions • Dispersion forces are present in all molecules and atoms • The magnitude of the dispersion forces increases with molar mass • Polar molecules also have dipole–dipole attractive forces Tro: Chemistry: A Molecular Approach, 2/e 43

Summary (cont’d)

• Hydrogen bonds are the strongest of the intermolecular attractive forces – a pure substance can have • Hydrogen bonds will be present when a molecule has H directly bonded to either O , N, or F atoms – only example of H bonded to F is HF • Ion–dipole attractions are present in mixtures of ionic compounds with polar molecules.

• Ion–dipole attractions are the strongest intermolecular attraction • Ion–dipole attractions are especially important Tro: Chemistry: A Molecular Approach, 2/e 44 in aqueous solutions of ionic compounds

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Liquids

Properties & Structure

Surface Tension

• Surface tension is a property of liquids that results from the tendency of liquids to minimize their surface area • To minimize their surface area, liquids form drops that are spherical – as long as there is no gravity Tro: Chemistry: A Molecular Approach, 2/e 47

Surface Tension

• The layer of molecules on the surface behave differently than the interior – because the cohesive forces on the surface molecules have a net pull into the liquid interior • The surface layer acts like an elastic skin – allowing you to “float” a paper clip even though steel is denser than water Tro: Chemistry: A Molecular Approach, 2/e 48

Surface Tension

• Because they have fewer neighbors to attract them, the surface molecules are less stable than those in the interior – have a higher potential energy • The surface tension of a liquid is the energy required to increase the surface area a given amount – surface tension of H 2 O = 72.8 mJ/m 2 • at room temperature – surface tension of C 6 H 49 6 = 28

Factors Affecting Surface Tension

• The stronger the intermolecular attractive forces, the higher the surface tension will be • Raising the temperature of a liquid reduces its surface tension – raising the temperature of the liquid increases the average kinetic energy of the molecules – the increased molecular motion makes it easier to stretch the surface Tro: Chemistry: A Molecular Approach, 2/e 50

•

Viscosity

Viscosity

is the resistance of a liquid to flow – 1 poise = 1 P = 1 g/cm∙s – often given in centipoise, cP • H 2 O = 1 cP at room temperature • Larger intermolecular attractions = larger viscosity Tro: Chemistry: A Molecular Approach, 2/e 51

Factors Affecting Viscosity

• The stronger the intermolecular attractive forces, the higher the liquid’s viscosity will be • The more spherical the molecular shape, the lower the viscosity will be – molecules roll more easily – less surface-to-surface contact lowers attractions • Raising the temperature of a liquid reduces its viscosity – raising the temperature of the liquid increases the average kinetic energy of the molecules – the increased molecular motion makes it easier to

Capillary Action

•

Capillary action

is the ability of a liquid to flow up a thin tube against the influence of gravity – the narrower the tube, the higher the liquid rises • Capillary action is the result of two forces working in conjunction, the cohesive and adhesive forces – cohesive forces hold the liquid molecules together – adhesive forces attract the outer liquid molecules to the tube’s surface

Capillary Action

• The adhesive forces pull the surface liquid up the side of the tube, and the cohesive forces pull the interior liquid with it • The liquid rises up the tube until the force of gravity counteracts the capillary action forces • The narrower the tube diameter, the higher the liquid will rise up the tube Tro: Chemistry: A Molecular Approach, 2/e 54

Meniscus

• The curving of the liquid surface in a thin tube is due to the competition between adhesive and cohesive forces • The meniscus of water is concave in a glass tube because its adhesion to the glass is stronger than its cohesion for itself • The meniscus of mercury is convex in a glass tube because its cohesion for itself is stronger than its adhesion for the glass – metallic bonds are stronger than intermolecular attractions

The Molecular Dance

• Molecules in the liquid are constantly in motion – vibrational, and limited rotational and translational • The

average

kinetic energy is proportional to the temperature • However, some molecules have more kinetic energy than the average, and others have less Tro: Chemistry: A Molecular Approach, 2/e 56

Vaporization

• If these high energy molecules are at the surface, they may have enough energy to overcome the attractive forces – therefore –

the larger the surface area, the faster the rate of evaporation

• This will allow them to escape the liquid and become a

vapor

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Condensation

• Some molecules of the vapor will lose energy through molecular collisions • The result will be that some of the molecules will get captured back into the liquid when they collide with it • Also some may stick and gather together to form droplets of liquid – particularly on surrounding surfaces • We call this process

condensation

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Evaporation vs. Condensation

• Vaporization and condensation are opposite processes • In an open container, the vapor molecules generally spread out faster than they can condense • The net result is that the rate of vaporization is greater than the rate of condensation, and there is a net loss of liquid • However, in a closed container, the vapor is not allowed to spread out indefinitely • The net result in a closed container is that at some time the rates of vaporization and condensation will Tro: Chemistry: A Molecular Approach, 2/e be equal 59

Effect of Intermolecular Attraction on Evaporation and

molecules, the less energy they will need to vaporize • Also, weaker attractive forces means that more energy will need to be removed from the vapor molecules before they can condense • The net result will be more molecules in the vapor phase, and a liquid that evaporates faster –

the weaker the attractive forces, the faster the rate of evaporation

• Liquids that evaporate easily are said to be

volatile

liquids that do not evaporate easily are called

Energetics of Vaporization

• When the high energy molecules are lost from the liquid, it lowers the average kinetic energy • If energy is not drawn back into the liquid, its temperature will decrease – therefore,

vaporization is an endothermic process

– and condensation is an exothermic process • Vaporization requires input of energy to overcome the attractions between molecules Tro: Chemistry: A Molecular Approach, 2/e 61

Heat of Vaporization

 • The amount of heat energy required to vaporize one mole of the liquid is called the

heat of vaporization,

D

H vap

– sometimes called the enthalpy of vaporization • Always endothermic, therefore D H vap is + • Somewhat temperature dependent D H condensation = − D H vaporization Tro: Chemistry: A Molecular Approach, 2/e 62

Example 11.3: Calculate the mass of water that can be vaporized with 155

Given:

kJ of heat at 100 °C

155 kJ

Find:

g H 2 O

Conceptual Plan:

kJ mol H 2 O g H 2 O

Relationships:

1 mol H 2 O = 40.7 kJ, 1 mol = 18.02 g

Solution: Check:

because the given amount of heat is almost 4x the D H vap , the amount of water makes sense Tro: Chemistry: A Molecular Approach, 2/e 63

Practice – Calculate the amount of heat needed to vaporize 90.0 g of C 3 H 7 OH at its boiling point

Given: Find:

90.0 g kJ

Conceptual Plan:

g mol kJ

Relationships:

1 mol C 3 H 7 OH = 39.9 kJ, 1 mol = 60.09 g

Solution: Check:

because the given amount of C 3 H 7 OH is more than 1 mole the amount of heat makes sense Tro: Chemistry: A Molecular Approach, 2/e 64

Dynamic Equilibrium

• In a closed container, once the rates of vaporization and condensation are equal, the total amount of vapor and liquid will not change • Evaporation and condensation are still occurring, but because they are opposite processes, there is no net gain or loss of either vapor or liquid • When two opposite processes reach the same rate so that there is no gain or loss of material, we call it a

dynamic equilibrium

– this does

not

mean there are equal amounts of vapor and liquid – it means that they are

changing

by equal amounts

Dynamic Equilibrium

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Vapor Pressure

• The pressure exerted by the vapor when it is in dynamic equilibrium with its liquid is called the

vapor pressure

– remember using Dalton’s Law of Partial Pressures to account for the pressure of the water vapor when collecting gases by water displacement?

• The weaker the attractive forces between the molecules, the more molecules will be in the vapor • Therefore,

the weaker the attractive forces, the higher the vapor pressure

liquid

Vapor –Liquid Dynamic

decrease the pressure of the vapor inside the chamber – at that point, there are fewer vapor molecules in a given volume, causing the rate of condensation to slow • Therefore, for a period of time, the rate of vaporization will be faster than the rate of condensation, and the amount of vapor will increase • Eventually enough vapor accumulates so that the rate of the condensation increases to the point where it is once again as fast as evaporation – equilibrium is reestablished • At this point, the vapor pressure will be the same as Tro: Chemistry: A Molecular Approach, 2/e it was before 68

Changing the Container’s Volume Disturbs the Equilibrium

Initially, the rate of vaporization and condensation are equal and the system is in dynamic equilibrium Tro: Chemistry: A Molecular Approach, 2/e When the volume is increased, the rate of vaporization becomes faster than the rate of condensation 69 When the volume is decreased, the rate of vaporization becomes slower than the rate of condensation

Dynamic Equilibrium •

• A system in dynamic equilibrium can respond to changes in the conditions

When conditions change, the system shifts its position to relieve or reduce the effects of the change

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Vapor Pressure vs. Temperature

• Increasing the temperature increases the number of molecules able to escape the liquid • The net result is that

as the temperature increases, the vapor pressure increases

• Small changes in temperature can make big changes in vapor pressure – the rate of growth depends on strength of the intermolecular forces Tro: Chemistry: A Molecular Approach, 2/e 71

Practice – Which of the following is the most volatile?

b) TiCl 4 c) ether Tro: Chemistry: A Molecular Approach, 2/e 72

Practice – Which of the following has the strongest Intermolecular attractions?

b) TiCl 4 c) ether Tro: Chemistry: A Molecular Approach, 2/e 73

Boiling Point

• When the temperature of a liquid reaches a point where its vapor pressure is the same as the external pressure, vapor bubbles can form anywhere in the liquid – not just on the surface • This phenomenon is what is called

boiling

and the temperature at which the vapor pressure = external pressure is the

boiling

74

Boiling Point

•

The normal boiling point is the temperature at which the vapor pressure of the liquid = 1 atm

• The lower the external pressure, the lower the boiling point of the liquid Tro: Chemistry: A Molecular Approach, 2/e 75

Practice – Which of the following has the highest normal boiling point?

b) TiCl 4 c) ether Tro: Chemistry: A Molecular Approach, 2/e 76

Heating Curve of a Liquid

• As you heat a liquid, its temperature increases linearly until it reaches the boiling point –

q

= mass liquid x

C

stays constant

s

x D T • Once the temperature reaches the boiling point, all the added heat goes into boiling the – the temperature • Once all the liquid has been turned into gas, the temperature can again start to rise 77 Tro: Chemistry: A Molecular Approach, 2/e

Sublimation and Deposition

• Molecules in the solid have thermal energy that allows them to vibrate • Surface molecules with sufficient energy may break free from the surface and become a gas – this process is called called

deposition

– therefore,

sublimation

• The capturing of vapor molecules into a solid is • The solid and vapor phases exist in dynamic equilibrium in a closed container – at temperatures below the melting point

molecular solids have a vapor pressure

sublimation solid gas deposition Tro: Chemistry: A Molecular Approach, 2/e 78

Sublimation

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Melting = Fusion

• As a solid is heated, its temperature rises and the molecules vibrate more vigorously • Once the temperature reaches the melting point, the molecules have sufficient energy to overcome some of the attractions that hold them in position and the solid melts (or fuses) • The opposite of melting is freezing Tro: Chemistry: A Molecular Approach, 2/e 80

Heating Curve of a Solid

• As you heat a solid, its temperature increases linearly until it reaches the melting point –

q

= mass x

C s

x D T • Once the temperature reaches the melting point, all the added heat goes into melting the solid – the temperature stays constant • Once all the solid has been turned into liquid, the temperature can again start to rise – ice/water will always have a Tro: Chemistry: A Molecular Approach, 2/e temperature of 0 °C 81

Energetics of Melting

• When the high energy molecules are lost from the solid, it lowers the average kinetic energy • If energy is not drawn back into the solid its temperature will decrease – therefore,

melting is an endothermic process

– and freezing is an exothermic process • Melting requires input of energy to overcome the attractions between molecules Tro: Chemistry: A Molecular Approach, 2/e 82

Phase Diagrams

• Phase diagrams describe the different states and state changes that occur at various temperature/pressure conditions • Regions represent states • Lines represent state changes – liquid/gas line is vapor pressure curve – both states exist simultaneously – critical point is the furthest point on the vapor pressure curve • Triple point is the temperature/pressure condition where all three states exist simultaneously • For most substances, freezing point increases Tro: Chemistry: A Molecular Approach, 2/e 83 as pressure increases

Phase Diagram of Water

critical point 374.1 °C 217.7 atm

Water

1 atm

Ice

normal melting pt.

0 °C triple point 0.01 °C 0.006 atm

Steam

normal boiling pt.

100 °C Tro: Chemistry: A Molecular Approach, 2/e Temperature 84

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Water – An Extraordinary Substance

– most molecular substances with similar molar masses are gases at room temperature • e.g. NH 3 , CH 4 – due to H-bonding between molecules • Water is an excellent solvent – dissolving many ionic and polar molecular substances – because of its large dipole moment – even many small nonpolar molecules have some solubility in water • e.g. O substance 2 – about 9% , CO 2 • Water has a very high specific heat for a molecular – moderating effect on coastal climates • Water expands when it freezes • at a pressure of 1 atm Tro: Chemistry: A Molecular Approach, 2/e 86 – making ice less dense than liquid water

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Solids

Properties & Structure

Crystal Lattice

• When allowed to cool slowly, the particles in a liquid will arrange themselves to give the maximum attractive forces – therefore minimize the energy • The result will generally be a crystalline solid • The arrangement of the particles in a crystalline solid is called the

crystal lattice

• The smallest unit that shows the pattern of arrangement for all the particles is called the

unit cell

88

Unit Cells

• Unit cells are 3-dimensional – usually containing 2 or 3 layers of particles • Unit cells are repeated over and over to give the macroscopic crystal structure of the solid • Starting anywhere within the crystal results in the same unit cell • • Each particle in the unit cell is called a

lattice point Lattice planes

are planes connecting equivalent points in unit cells throughout the lattice Tro: Chemistry: A Molecular Approach, 2/e 89

c b a Cubic a = b = c all 90 ° c b a Hexagonal a = c < b 2 faces 90 ° 1 face 120 °

7 Unit Cells

c c b b a Tetragonal a = c < b all 90 ° a Orthorhombic a  b  c all 90 ° c b a Rhombohedral a = b = c no 90 ° 90 a b Monoclinic a  b  c 2 faces 90 ° c b a Triclinic a  b  c no 90 ° c

Molecular Solids

• The lattice sites are occupied by molecules – CO 2 , H 2 O, C 12 H 22 O 11 • The molecules are held together by intermolecular attractive forces – dispersion forces, dipole–dipole attractions, and H-bonds • Because the attractive forces are weak, they tend to have low melting points – generally < 300 °C Tro: Chemistry: A Molecular Approach, 2/e 91

Ionic Solids

• Lattice sites occupied by ions • Held together by attractions between oppositely charged ions – nondirectional – therefore every cation attracts all anions around it, and vice versa • The coordination number represents the number of close cation –anion interactions in the crystal • The higher the coordination number, the more stable the solid – lowers the potential energy of the solid • The coordination number depends on the relative sizes of the cations and anions that maintains charge balance

Ionic Crystals

CsCl coordination number = 8 Cs + Cl ─ = 167 pm = 181 pm Tro: Chemistry: A Molecular Approach, 2/e 93 NaCl coordination number = 6 Na + Cl ─ = 97 pm = 181 pm