intermolecular forces. tro's introductory chemistry, chapter 123 many of the phenomena we...

Intermolecular Forces

Tro's Introductory Chemistry, Chapter 12 3

Many of the phenomena we observe are related to interactions between molecules that do not involve a chemical reaction. Your taste and smell organs work because

molecules in the thing you are sensing interact with the receptor molecule sites in your tongue and nose.

In this chapter, we examine the physical interactions between molecules and the factors that effect and influence them.


Matter can be classified as solid, liquid, or gas based on what properties it exhibits.

State Shape Volume Compress Flow

Solid Fixed Fixed No No

Liquid Indef. Fixed No Yes

Gas Indef. Indef. Yes Yes

•Fixed = Keeps shape when placed in a container. •Indefinite = Takes the shape of the container.


The atoms or molecules have different structures in solids, liquids, and gases, leading to different properties.


Low densities compared to solids and liquids.

Fluid. The material exhibits a smooth, continuous

flow as it moves. Take the shape of their container(s). Expand to fill their container(s). Can be compressed into a smaller

volume.


In the gas state, the particles have complete freedom from each other.

The particles are constantly flying around, bumping into each other and their container(s).

In the gas state, there is a lot of empty space between the particles. On average.


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(s), and will flow.


High densities compared to gases. Fluid.

The material exhibits a smooth, continuous flow as it moves.

Take the shape of their container(s). Keep their volume, do not expand to fill

their container(s). Cannot be compressed into a smaller

volume.


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 and expand to fill the container(s).


High densities compared to gases and liquids.

Nonfluid. They move as an entire “block” rather than a

smooth, continuous flow. Keep their own shape, do not take the shape

of their container(s). Keep their own volume, do not expand to fill

their container(s). Cannot be compressed into a smaller

volume.


The particles in a solid are packed close together and are fixed in position. Though they are vibrating.

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 prevents the particles from flowing.


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.


The state a material exists in depends on the attraction between molecules and their ability to overcome the attraction.

The attractive forces between ions or molecules depends on their structure. The attractions are electrostatic. They depend on shape, polarity, etc.

The ability of the molecules to overcome the attraction depends on the amount of kinetic energy they possess.


Phase Density Shape Volume

Relative strength of attractive forces

Gas Low Indefinite Indefinite Weakest

Liquid High Indefinite Definite Moderate

Solid High Definite Definite Strongest


Generally, we convert a material in the solid state into a liquid by heating it.

Generally, we convert a material in the liquid state to a gas by heating it.

In both cases adding heat energy increases the amount of kinetic energy of the molecules in the solid.

Eventually, they acquire enough energy to partially overcome the attractive forces holding them in place.

This allows the molecules enough extra freedom to move around a little and rotate.


Liquids tend to minimize their surface—a phenomenon we call surface tension.

This tendency causes liquids to have a surface that resists penetration.

The stronger the attractive force between the molecules, the larger the surface tension.


Molecules in the interior of a liquid experience attractions to surrounding molecules in all directions.

However, molecules on the surface experience an imbalance in attractions, effectively pulling them in.

To minimize this imbalance and maximize attraction, liquids try to minimize the number of molecules on the exposed surface by minimizing their surface area.

Stronger attractive forces between the molecules = larger surface tension.


Some liquids flow more easily than others.

The resistance of a liquid’s flow is called viscosity.

The stronger the attractive forces between the molecules, the more viscous the liquid is.

Also, the less round the molecule’s shape, the larger the liquid’s viscosity. Some liquids are more viscous

because their molecules are long and get tangled in each other, causing them to resist flowing.


The process of molecules of a liquid breaking free from the surface is called evaporation. Also known as vaporization.

Evaporation is a physical change in which a substance is converted from its liquid form to its gaseous form. The gaseous form is called a

vapor.


Over time, liquids evaporate—the molecules of the liquid mix with and dissolve in the air.

The evaporation happens at the surface. Molecules on the surface experience a

smaller net attractive force than molecules in the interior.

All the surface molecules do not escape at once, only the ones with sufficient kinetic energy to overcome the attractions will escape.

24

The average kinetic energy is directly proportional to the Kelvin temperature.

Not all molecules in the sample have the same amount of kinetic energy.

Those molecules on the surface that have enough kinetic energy will escape. Raising the temperature increases the number of

molecules with sufficient energy to escape.


Since the higher energy molecules from the liquid are leaving, the total kinetic energy of the liquid decreases, and the liquid cools.

The remaining molecules redistribute their energies, generating more high energy molecules.

The result is that the liquid continues to evaporate .


Liquids that evaporate quickly are called volatile liquids, while those that do not are called nonvolatile.

Increasing the surface area increases the rate of evaporation. More surface molecules.

Increasing the temperature increases the rate of evaporation. Raises the average kinetic energy, resulting in

more molecules that can escape. Weaker attractive forces between the

molecules = faster rate of evaporation.


When a liquid evaporates in a closed container, the vapor molecules are trapped.

The vapor molecules may eventually bump into and stick to the surface of the container or get recaptured by the liquid. This process is called condensation. A physical change in which a gaseous form

is converted to a liquid form.


Evaporation and condensation are opposite processes.

Eventually, the rate of evaporation and rate of condensation in the container will be the same.

Opposite processes that occur at the same rate in the same system are said to be in dynamic equilibrium.


Eventually, the condensation and evaporation reach the same speed. The air in the flask is now saturated with water vapor. We have reached the dynamic equilibrium

Shortly, the waterstarts to evaporate.Initially the rateof evaporation is much faster than rate of condensation

When water is justadded to the flask and it is capped, all the water molecules are in the liquid.


Once equilibrium is reached, from that time forward, the amount of vapor in the container will remain the same. As long as you don’t change the conditions.

The partial pressure exerted by the vapor is called the vapor pressure.

The vapor pressure of a liquid depends on the temperature and strength of intermolecular attractions.


In an open container, as you heat a liquid the average kinetic energy of the molecules increases, giving more molecules enough energy to escape the surface. So the rate of evaporation

increases. Eventually, the temperature is

high enough for molecules in the interior of the liquid to escape. A phenomenon we call boiling.


The temperature at which the vapor pressure of the liquid is the same as the atmospheric pressure is called the boiling point. The normal boiling point is the temperature

required for the vapor pressure of the liquid to be equal to 1 atm.

The boiling point depends on what the atmospheric pressure is. The temperature of boiling water on the top

of a mountain will be cooler than boiling water at sea level.


As you heat a liquid, its temperature increases until it reaches its boiling point.

Once the liquid starts to boil, the temperature remains the same until it all turns to a gas.

All the energy from the heat source is being used to overcome all of the attractive forces in the liquid.


As it loses its high energy molecules through evaporation, the liquid cools.

Then the liquid absorbs heat from its surroundings to raise its temperature back to the same as the surroundings.

Processes in which heat flows into a system from the surroundings are said to be endothermic.

As heat flows out of the surroundings, it causes the surroundings to cool. As alcohol evaporates off your skin, it causes

your skin to cool.


As it gains the high energy molecules through condensation, the liquid warms.

Then the liquid releases heat to its surroundings to reduce its temperature back to the same as the surroundings.

Processes in which heat flows out of a system into the surroundings are said to be exothermic.

As heat flows into the surroundings, it causes the surroundings to warm.


The amount of heat needed to vaporize one mole of a liquid is called the heat of vaporization. Hvap

It requires 40.7 kJ of heat to vaporize one mole of water at

100 °C. Always endothermic.

▪ Number is +. Hvap depends on the initial temperature. Since condensation is the opposite process to

evaporation, the same amount of energy is transferred but in the opposite direction. Hcondensation = −Hvaporization


LiquidChemicalformula

Normal boiling

point, °C

Hvap at boiling point,

(kJ/mol)

Hvap at 25 °C,

(kJ/mol)

Water H2O 100 +40.7 +44.0

Isopropyl alcohol

C3H7OH 82.3 +39.9 +45.4

Acetone C3H6O 56.1 +29.1 +31.0

Diethyl ether

C4H10O 34.5 +26.5 +27.1

Since the given amount of heat is almost 4x the Hvap, the amount of water makes sense.

1 mol H2O = 40.7 kJ, 1 mol = 18.02 g

155 kJg H2O

Check:

Solution:

Solution Map:

Relationships:

Given:Find:

kJ 40.7

mol 1

OH g 8.66 mol 1

g 8.021

kJ 40.7

OH mol 1 kJ 551 2

2

kJ mol H2O g H2O

mol 1

g 02.18


Example 12.1:Calculate the amount of water in grams that can be

vaporized at its boiling point with 176 kJ of heat.

# 47, 51, 59, 57


As you heat a solid, its temperature increases until it reaches its melting point.

Once the solid starts to melt, the temperature remains the same until it all turns to a liquid.

All the energy from the heat source is being used to overcome some of the attractive forces in the solid that hold them in place.


When a solid melts, it absorbs heat from its surroundings, it is endothermic.

As heat flows out of the surroundings, it causes the surroundings to cool. As heat flows out of your drink into the ice cubes

(causing them to melt), the liquid gets cooler. When a liquid freezes, it releases heat into

its surroundings, it is exothermic. As heat flows into the surroundings, it

causes the surroundings to warm. Orange growers often spray their oranges with

water when a freeze is expected. Why?


The amount of heat needed to melt one mole of a solid is called the heat of fusion. Hfus

Fusion is an old term for heating a substance until it melts, it is not the same as nuclear fusion.

Since freezing (crystallization) is the opposite process of melting, the amount of energy transferred is the same, but in the opposite direction. Hcrystal = -Hfus

In general, Hvap > Hfus because vaporization requires breaking all attractive forces.


LiquidChemicalformula

Melting point, °C

Hfusion, (kJ/mol)

Water H2O 0.00 6.02

Isopropyl alcohol C3H7OH -89.5 5.37

Acetone C3H6O -94.8 5.69

Diethyl ether C4H10O -116.3 7.27

47

Since the given amount of heat is almost 4x the Hvap, the amount of water makes sense.

1 mol H2O = 6.02 kJ, 1 mol = 18.02 g

237 kJg H2O

Check:

Solution:

Solution Map:

Relationships:

Given:Find:

kJ 6.02

mol 1

OH g 097 mol 1

g 8.021

kJ 6.02

OH mol 1 kJ 372 2

2

kJ mol H2O g H2O

mol 1

g 02.18

49

Since the given mass is more than one mole, the answer being greater than Hvap makes sense.

1 mol C3H6O = 5.69 kJ at -94.8 C, 1 mol = 58.08 g

87 g C3H6OkJ

Check:

Solution:

Solution Map:

Relationships:

Given:Find:

g 58.08

mol 1

kJ .58 OHC mol 1

kJ .695

g 58.08

OHC mol 1 OHC g 78

63

6363

g C3H6O mol C3H6O kJ

mol 1

kJ .695


Sublimation is a physical change in which the solid form changes directly to the gaseous form. Without going through the

liquid form. Like melting, sublimation

is endothermic.


Intermolecular attractions are a result of attractive forces between opposite charges.

+ ion to – ion. + end of one polar molecule to − end of

another polar molecule. H-bonding is especially strong. Even nonpolar molecules will have temporary

induced dipoles. Larger charge = stronger attraction=

higher melting and boiling ponts.


Also known as London forces or instantaneous dipoles.

Caused by distortions in the electron cloud of one molecule inducing distortion in the electron cloud on another.

Distortions in the electron cloud lead to a temporary dipole.

The temporary dipoles lead to attractions between molecules—dispersion forces.

All molecules have attractions caused by dispersion forces.


Depends on how easily the electrons can move, or be polarized.

The more electrons and the farther they are from the nuclei, the larger the dipole that can be induced.

Strength of the dispersion force gets larger with larger molecules.


Noble Gas Molar Mass (g/mol)

Boiling Point (K)

He 4.00 4.2

Ne 20.18 27

Ar 39.95 87

Kr 83.80 120

Xe 131.29 165


-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

1 2 3 4 5 6

Boi

ling

Poi

nt,

°C

Period

Relationship Between Dispersion Force and Molecular Size

BP, Noble Gas

BP, Halogens

BP, XH4


CH4 or C3H8

BF3 or BCl3

CO2 or CS2

• CH4 or C3H8.

• BF3 or BCl3.

• CO2 or CS2.


Because of the kinds of atoms that are bonded together and their relative positions in the molecule, some molecules have a permanent dipole. Polar molecules.

The size of the molecule’s dipole is measured in debyes, D.


Polar molecules have a permanent dipole. A + end and a – end.

The + end of one molecule will be attracted to the – end of another.


Molar Mass (g/mol)

Boiling Point, °C

Dipole size, D

CH3CH2CH3 44 -42 0

CH3-O-CH3 46 -24 1.3

CH3 - CH=O 44 20.2 2.7

CH3-CN 41 81.6 3.9


+ - + - + - + -

+++

+

____

Dispersion forces—All molecules.

Dipole-to-dipole forces—Polar molecules.


All molecules are attracted by dispersion forces.

Polar molecules are also attracted by dipole-dipole attractions.

Therefore, the strength of attraction is stronger between polar molecules than between nonpolar molecules of the same size.


• CS2 Nonpolar bonds = nonpolar molecule.

• CH2F2 Polar bonds and asymmetrical = polar molecule.

• CF4 Polar bonds and symmetrical shape = nonpolar molecule.

CS2

CH2F2

CF4

C

F

FFF

C

H

HFF

SCS


Like dissolves like. Miscible = Liquids that do not separate, no matter

what the proportions. Polar molecules dissolve in polar solvents.

Water, alcohol, CH2Cl2. Molecules with O or N higher solubility in H2O due

to H-bonding with H2O. Nonpolar molecules dissolve in nonpolar

solvents. Ligroin (hexane), toluene, CCl4.

If molecule has both polar and nonpolar parts, then hydrophilic-hydrophobic competition.


When liquid pentane, a nonpolar substance, is mixed with water, a polar substance, the two liquids separate because they are more attracted to their own kind of molecule than to the other.


HF, or molecules that have OH or NH groups have particularly strong intermolecular attractions. Unusually high melting

and boiling points. Unusually high solubility in

water. This kind of attraction is

called a hydrogen bond.


Name FormulaMolar mass

(g/mol)Structure

Boiling point,

°C

Melting point,

°C

Solubility in water

Ethane C2H6 30.0 -88 -172 Immiscible

Ethanol CH4O 32.0 64.7 -97.8 Miscible

H C

H

H

C H

H

H

H C

H

H

O H


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

Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes deshielded. Exposing the proton.

The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules.


Hydrogen bonds are not chemical bonds.

Hydrogen bonds are attractive forces between molecules.

Chemical bonds are attractive forces that make molecules.


-200

-150

-100

-50

0

50

100

150

1 2 3 4 5

Bo

ilin

g P

oin

t, °C

Period

Relationship Between H-Bonding and Intermolecular Attraction

BP, HX

BP, H2X

BP, H3X

BP, XH4

CH4

NH3

HF

H2O

SiH4

GeH4

SnH4

H2S H2Se

H2Te


Molar Mass

(g/mol)

Boiling Point,

°C

Solubility in water

(g/100 g H2O) CH3CH2OCH2CH3 74 34.6 7.5

CH3CH2CH2CH2CH3 72 36 Insoluble

CH3CH2CH2CH2OH 74 117 9


formaldehyde, CH2O 30.03 g/mol polar molecule dipole–dipole attractions present

▪ polar C=O bond & asymmetric fluoromethane, CH3F

34.03 g/mol polar molecule dipole–dipole attractions present

▪ polar C−F bond & asymmetric hydrogen peroxide, H2O2

34.02 g/mol polar molecule dipole–dipole attractions present

▪ polar H−O bonds & asymmetric H−O bonds Hydrogen bonding present

• formaldehyde, CH2O. 30.03 g/mol. polar molecule dipole–dipole attractions present.

Polar C=O bond and asymmetric.

• fluoromethane, CH3F. 34.03 g/mol. polar molecule dipole–dipole attractions present.

Polar C−F bond and asymmetric.

• hydrogen peroxide, H2O2 34.02 g/mol. polar molecule dipole–dipole attractions present.

Polar H−O bonds and asymmetric. H−O bonds hydrogen-bonding present.

C

F

HHH

HC

H

O

O O

HH


CH3CH2OCH2CH3 or CH3CH2CH2CH2CH3.

CH3CH2NHCH3 or CH3CH2CH2CH3.

CH3CH2OH or CH3CH2CH2CH2CH2OH.


CH3CH2OCH2CH3 or CH3CH2CH2CH2CH3 contains polar O.

CH3CH2NHCH3 or CH3CH2CH2CH3 contains polar N.

CH3CH2OH or CH3CH2CH2CH2CH2OH contains less nonpolar parts.

Type of force

Relative strength

Present in Example

Dispersionforce

Weak, but increases

with molar mass

All atoms and

moleculesH2

Dipole– Dipole force

ModerateOnly polar

moleculesHCl

Hydrogen Bond

Strong

Molecules having H bonded to F, O, or N

HF

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