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Liquids, Solids, and Intermolecular Forces

The Three Phases of Matter(A Macroscopic Comparison)

State of Matter Shape and volume Compressibility Ability to Flow

Solid Retains its own shape and volume very low none

LiquidConforms to shape of container, but not

to volumelow moderate

GasConforms to shape

and volume of container

high high

Particles packed close together and are fixed in position

(They may vibrate) Noncompressible

Retain shape and volume when placed in a new

container Do not flow

Particles closely packed Particles have some ability

to move around Noncompressible

Take the shape of their container

Flow, but don’t have enough freedom to escape or

expand

Particles have complete freedom of motion

There is a large amount of space between the

particles

Molecular View of Phases of Matter

An important distinction !!

Kinetic–Molecular Theory

Gases Kinetic energy overcomes attractions between particles.

Particles have complete freedom of motion.

Solids Kinetic energy cannot overcome attractive forces at all.

No translational or rotational motion. Particles may vibrate.

Liquids Kinetic energy can only partially overcome attractive forces. Particles have limited translational and rotational motion.

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.

Gas phase particles

Attractive Forces

Particles are attracted to each other by electrostatic forces.

The strength of the attractive forces varies.

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.

Phase Changes

Gases can be condensed.

The amount of kinetic energy the particles have determines the state of matter.

Solids melt when heated.

Liquids boil when heated.

Liquids can be condensed.

Special Properties of Liquids

Surface tension

Viscosity

Capillary Action

Vaporization

Molecules in a liquid are constantly in motion; some molecules have more kinetic energy than

others.

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.

Condensation-The Opposite of Vaporization

Molecules of the vapor will lose energy through molecular collisions

Molecules get captured back into the liquid

Some may stick and gather together to form droplets of liquid

Effect of Intermolecular Attraction on Evaporation and Condensation

Weaker attractive forces ➡ less energy needed to vaporize

Weaker attractive forces ➡ more energy will need to be removed from the vapor molecules before condensation

Weak attractive forces ➡ the faster the evaporation

Liquids that evaporate easily are said to be volatile.

Liquids that do not evaporate easily are called nonvolatile.

Liquid FormulaNormal Boilng

Point(ºC)

Heat of Vaporization

(kJ/mol)

water H2O 100.0 40.7Isopropyl alcohol C3H8O3 82.3 39.9

acetone C3H6O 56.1 29.1

ethyl ether C4H10O 34.5 26.5

The amount of heat energy required to vaporize one mole of the liquid is called the heat of vaporization, ΔHvap, or the

enthalpy of vaporization.

Energetics of Vaporization

1. Calculate the amount of heat needed to vaporize 90.0 g of C3H7OH at its boiling point (ΔHvap = 39.9 kJ/mol)

g mol kJ

39.9 kJ + C3H8O (liquid) C3H8O (gas)

2. Calculate the mass of water that can be vaporized with 155 kJ of heat at 100 ° (ΔHvap = 40.7 kJ/mol)

kJ mol H2O g H2O

40.7 kJ + H2O (liquid) H2O (gas)

What happens when you heat up a liquid ??

Boiling As a liquid is heated, its temperature rises and the molecules move past each other more vigorously.

Once the temperature reaches the boiling point, the molecules have sufficient energy to overcome the attractions that hold them in contact with other

molecules and the liquid boils.

ΔT

Heating Curve of a Liquid

As you heat a liquid, its temperature increases linearly until it reaches the boiling point q = mass x Cs x ΔT

Once the temperature reaches the boiling point, all the added heat goes into boiling the liquid – the temperature stays constant

Once all the liquid has been turned into gas, the temperature can again start to rise. q = mass x Cs x ΔT

ΔT

What happens when you heat up a solid ??

Melting 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.

ΔT

Heating Curve of a Solid

Once all the solid has been turned into liquid, the temperature can again start to rise. q = mass x Cs x ΔT

As you heat a solid, its temperature increases linearly until it reaches the melting pointq = mass x Cs x ΔT

Once the temperature reaches the melting point, all the added heat goes into melting the solid – the temperature stays constant ΔT

Energetics of MeltingThe amount of heat energy required to melt one mole of the solid is

called the Heat of Fusion, ΔHfus or the enthalpy of fusion

Generally much less than ΔHvap

Liquid FormulaNormal

Melting Point(ºC)

Heat of Fusion(kJ/mol)

water H2O 0.0 6.02Isopropyl alcohol C3H8O3 -89.5 5.37

acetone C3H6O -94.8 5.69

ethyl ether C4H10O -116.3 7.27

Quantitative Aspects of Phase Changes

Within a phase, a change in heat is accompanied by a change in temperature which is associated with a change in average kinetic energy of the molecules.

q = ( )(molar heat capacity)(∆T)quantity

ofmatter

J = g x x ºCJg ºC

Quantitative Aspects of Phase Changes

During a phase change, a change in heat occurs at a constant temperature, which is associated with a change in average

rotational and translational energy of the molecules, as the average distance between molecules changes.

q = ( )(enthalpy of the phase change)quantity

ofmatter

kJ = mol x kJmol

No ΔT !!

How much heat energy is required to raise the temperature of 1.0 mol of water

from -25ºC to 125ºC ??

Heating Curve of Water

Segment 1

Heating 1.00 mole of ice at −25.0 °C up to the melting point, 0.0 °C

q = mass x Cs x ΔTmass of 1.00 mole of ice = 18.0 g Cs = 2.09 J/g·°C

Segment 2

Melting 1.00 mole of ice at the melting point, 0.0 °Cq = n·ΔHfus

n = 1.00 mole of ice ΔHfus = 6.02 kJ/mol

Segment 3

Heating 1.00 mole of water at 0.0 °C up to the boiling point, 100.0 °C

q = mass x Cs x ΔTmass of 1.00 mole of water = 18.0 g Cs = 4.18 J/g·°C

Segment 4

Boiling 1.00 mole of water at the boiling point, 100.0 °C

q = n·ΔHvapn = 1.00 mole of water ΔHvap = 40.7 kJ/mol

Segment 5

Heating 1.00 mole of steam at 100.0 °C up to 125.0 °Cq = mass x Cs x ΔT

mass of 1.00 mole of water = 18.0 g Cs = 2.01 J/g·°C

How much heat energy is required to raise the temperature of 1.0 mol of water

from -25ºC to 125ºC ??

0.941 6.02 7.5240.7

0.904 56.085

56.1 kJ

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Attractive Forces

Particles are attracted to each other by electrostatic forces

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

The strength of the attractions between particles of a substance determines its state.

Kinds of Attractive Forces

Hydrogen Bonds between Molecules An especially strong dipole–dipole attraction resulting

from the attachment of H to an extremely electronegative atom

Dispersion Forces between Molecules Temporary polarity in molecules due to

unequal electron distribution

Dipole–Dipole Attractions between Molecules Permanent polarity in molecules due to their structure

Ion–Dipole Attractions - Not Intermolecular Between mixtures of ionic compounds and polar

compounds (esp. aqueous solutions)

Some molecules are considered nonpolar because of the atoms which they contain and the

arrangement of these atoms in space.

CH4 BH3 C2H2 CO2

Nonpolarizedelectronclouds

But these molecules can all be “condensed.” !!

Origin of Instantaneous Dipoles

δδ-δδ+

The δδ- charge repels electrons.

The δδ+ charge attracts electrons.

Magnitude of the Induced DipoleThe magnitude of the induced dipole depends on several factors:

Polarizability of the electrons

Volume of the electron cloud

larger molar mass ⇒ more electrons ⇒ larger electron cloud ⇒ increased polarizability ⇒ stronger attractions

Larger molecules have more

electrons, leading to increased

polarizability.

Gas Radius Molar Mass B.P.(K)

He 31 4 4.2

Ne 38 20 27

Ar 71 40 87

Kr 88 84 120

Xe 108 131 165

Rn 120 222 211

Effect of Molecular Sizeon Magnitude of Dispersion Force

As the molar mass increases, the number of

electrons increases. Therefore, the strength of

the dispersion forces increases.

The stronger the attractive forces

between the molecules, the

higher the boiling point.

Boiling Points of Straight Chain Alkanes NonPolar Molecules

Size of the Induced DipoleShape of the molecule

more surface-to-surface contact ⇒ larger induced dipole

⇒ stronger attraction

Molecules that are flat have more surface

interaction than spherical ones.

Effect of Molecular Shapeon Size of Dispersion Force

n-pentane molar mass=72.15

b.p = 36.1 ºC

2-methylbutane molar mass=72.15

b.p = 27.9 ºC

2,2-dimethylpropane molar mass=72.15

b.p = 9.5 ºC

A larger surface-to-surface contact between molecules results in stronger dispersion force attractions and a

higher boiling point.

Kinds of Attractive Forces

Hydrogen Bonds between Molecules An especially strong dipole–dipole attraction resulting

from the attachment of H to an extremely electronegative atom

Dispersion Forces between Molecules Temporary polarity in molecules due to

unequal electron distribution

Dipole–Dipole Attractions between Molecules Permanent polarity in molecules due to their structure

Ion–Dipole Attractions - Not Intermolecular Between mixtures of ionic compounds and polar

compounds (esp. aqueous solutions)

Some molecules are inherently polar because of the atoms which they contain and the

arrangement of these atoms in space.

H2O NH3 CH2O HCl

δ− δ+ A crude representation of a polar molecule

Dipole–Dipole Attractions

Polar molecules have a permanent dipole because of bond polarity and shape

1) dipole moment 2) as well as the always present induced dipole

The permanent dipole adds to the attractive forces between the molecules

Name Formula Molar mass Structure Structure b.p. m.p.

formaldehyde CH2O 30.03 -19.5º -92º

ethane C2H6 30.07 -88º -172º

HH

H

H

HH

C

C

Effect of Dipole–Dipole Attraction on Boiling and Melting Points

Hydrogen Bonding

When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it:

O─H, N─H, 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.

Name Formula Molar mass Structure Structure b.p. m.p.

ethanol C2H6O 46.07 78.2º -114.1º

dimethyl ether C2H6O 46.07 -22º -138.5º

Effect of Hydrogen-Bonding on Boiling and Melting Points

H-Bonds

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.

One of these compounds is a liquid at room temperature (the others are gases). Which one and why?

MM = 30.03PolarNo H-Bonds

MM = 34.03PolarNo H-Bonds

MM = 34.02PolarH-Bonds

Because only hydrogen peroxide has the additional very strong H-bond additional attractions, its intermolecular attractions will be the strongest. We therefore expect hydrogen peroxide to be the liquid.

-19ºC -78ºC +150ºC b.p.

All Molecules

Polar Molecules

Molecules containing O-H, N-H, or F-H

Bonds

Dispersion forces

Dipole forces

H-bonding

Hierarchy of Intermolecular Forces

Comparison of Intermolecular Forces

H2, b.p. -253ºCweak attractions between molecules

HCl, b.p. -85ºCstrong attractions between

molecules

HF, b.p. +20 ºCvery strong attractions

between molecules

Dispersion forces:

Dipole-dipole forces:

Hydrogen bonding:

0.05-40.0 kJ/mol

5-25 kJ/mol

10-40 kJ/mol

40-600 kJ/mol

Non-Bonding (Inter-Molecular) Forces

Bonding Molecular Forces

kJ/mol

Intermolecular Forces

Chemical Bonds

0 4000

Comparison of Strengths of Inter-Particle Forces

Summary

Dispersion forces are the weakest of the intermolecular attractions. Present in all molecules and atoms Magnitude increases with molar mass

Dipole–dipole attractive forces - Polar molecules

Hydrogen bonds - the strongest of the intermolecular attractive forces

H directly bonded to either O , N, or F atoms

Ion–dipole attractions are present in mixtures of ionic compounds with polar molecules.

Ion–dipole attractions are especially important in aqueous solutions