Chapter 7 - Gases, Liquids, And Solids

2 Kinetic Molecular Theory of matter

Five statements of the kinetic molecular theory of matter:•1. matter is ultimately composed of tiny particles (atoms, molecules or ions) that have

definite and characteristic sizes that do not change

•

2. the particles are in constant random motion and therefore possesses kinetic energy.•3. the particles interact with one another through attractions and repulsions and

therefore possesses potential energy.

•

4. The kinetic energy of the particle increases as the temperature is increased•5. The particles in a system transfer energy to each other through elastic collisions.•In elastic collision total kinetic energy is conserved. In a inelastic collision some of the

energies are lost during collision. In real world every collision is inelastic.

•

Slide 7-3 Kinetic Molecular Theory of matter

Kinetic energy is energy that matter possess because the motion of the particle. It can be

considered as disruptive force that tends to make the particles of a system increasingly independent of one another.

•

Potential energy is stored energy that matter possesses as a result of its position,

condition or composition. Potential energy of attraction can be considered as cohesive force that tends to cause order and stability among the particles of the system.

•

Electrostatic attraction is an attraction or repulsion that occurs between charged particle.

It is a form of potential energy

•

Slide 7-4 Solids

When liquids are cooled, their molecules come so close together and attractive forces

between them become so strong that random motion stops and a solid is formed.

•

A solid is a physical state characterized by a dominance of potential energy (cohesive

forces) over kinetic energy (disruptive forces).

•

Characteristic properties of solids•1. Definite volume and definite shape: the strong, cohesive forces hold the particles in

essentially fixed positions, resulting in definite volume and definite shape

•

Slide 7-5 Solids

2. high Density: the constituent particles of solids are located as close together as

possible (touching each other). Therefore a given volume contains large number of particles , resulting in high density.

•

3. Small compressibility: Because there is very little space between particles, increased

pressure cannot push the particles any closer together; therefore it has little effect on solid’s volume.

•

4. Very small thermal expansion: An increased temperature increase the kinetic energy

(disruptive forces), thereby causing more vibrational motion of the particles. Each occupies a slightly larger volume, and the result is a slight expansion of the solid. The

strong cohesive forces prevent this effect from becoming very large.

•

CH 7 - Gases, Liquids, and SolidsWednesday, November 14, 20074:38 PM

CHEM 1151 - Organic & Biological Chemistry.one (On 1-6-2008).one (On 6-5-2008)

6 Liquids

As pressure increases in a real gas, its molecules come closer and closer with the result

that attractions between molecules become important.

•

When distances decrease so that almost all molecules touch or almost touch, a gas

condenses to a liquid.

•

A liquid is the physical state characterized by potential energy (cohesive forces) and

kinetic energy (disruptive forces) of about the same magnitude.

•

Characteristic properties of liquids:•1. Definite volume and indefinite shape: The attractive forces are strong enough to

restrict particles to movement within a definite volume. They are not strong enough, however to prevent the particles from moving over each other in a random manner that

is limited only by the container walls. Thus liquids have no definite shape except that

they maintain a horizontal upper surface in container that are not completely filled

•

Slide - 7-7 Liquids

2. High Density: The particles in a liquid are not widely separated; they are still touching

one another, therefore a large number of particles in a given volume- a high density

•

3. Small compressibility: Because the particles in a liquid are still touching each other,

there is a very little empty space. Therefore an increase in pressure cannot squeeze the particles much closer together.

•

4. Small thermal expansion: Most of the particle movement in a liquid is vibrational

because a particle can move only a short distance before they collide with a neighbor. The increased particle velocity that accompanies a temperature increase results only in

increased vibrational amplitudes. The net effect is an increase in the effective volume a

particle occupies, which causes a slight volume increase in the liquid.

•

Slide 7-8 Gases

A gas is the physical state characterized by a complete dominance of kinetic energy

(disruptive forces) over potential energy (cohesive forces).

•

Properties of gases:•1. Indefinite volume and indefinite shape: the attractive forces between particles have

been overcome by kinetic energy, and the particles are free to travel in all directions. Therefore , gas particles completely fill their container, and the shape of the gas is that of

the container.

•

2. Low density: The particles of a gas are widely separated. There are relatively few

particles in a given volume (compared with liquids and solids), which means little mass per volume.

•

Slide 7-9 Gases

3. Large Compressibility: particles in a gas are widely separated; essentially a gas is

mostly empty space. When pressure is applied the particles are easily pushed closer together, decreasing the amount of empty space and the volume of the gas.

•

4. Moderate thermal expansion: An increase in temperature means an increase in particle

velocity. The increased kinetic energy of the particles enables them to push back whatever barrier is confining them into a given volume, and the volume increases. Size of

the particle is not changed during expansion or compression, they move further apart or

closer. It is the distance between two particles that changes.

•


10 Gases

Gas law is the generalization that describes in mathematical terms the relationships

among the amount, pressure, temperature and volume of a gas

•

most commonly measured in millimeters of mercury (mm Hg), atmospheres (atm),

and torr.

•

1 atm = 760 mm Hg = 760 torr

= 101,325 pascals = 28.96 in. Hg

=14.7psi (lb/in2+)•pressure is measured using a barometer (next screen).•

Gas pressure: the pressure per unit area exerted against a surface.•

Slide 7-11 Gas Pressure

Figure 6.2 A mercury barometer.•

Slide 7-12 Gas Pressure


12 Gas Pressure

Figure 6.3 A manometer.•

Slide 7-13 Gas Laws

PV = constant or P1V1 = P2V 2

Boyle’s law: for a fixed mass of gas at a constant temperature, the volume is inversely

proportional to the pressure.

•

V

T

V1

T1

V2

T2

= a constant or =

Charles’s Law: the volume of a fixed mass of gas at a constant pressure is directly

proportional to the temperature in kelvins (K).

•

Slide 7-14 Gas Law

Gay-Lussac’s Law: for a fixed mass of gas at constant volume, the pressure is directly

proportional to the temperature in kelvins (K).

•

P

T

P1

T1

P2

T2

= a constant or =


T

V 2

T2

V 1

T1

P2

T2

P1

T1

P

V

Name Expression Constant

Boyle's law

Charles's Law

Gay-Lussac's law

P1V 1 = P2V2

=

=

in summary:•

Slide 7-15 Gas Laws

P1 V1

T1

P2 V2

T2

PV

T== a constant or

Boyle’s law, Charles’s law and Gay-Lussac’s law can be combined into one law called the

combined gas law.

•

Slide 7-16 Gas Laws

P1 = 2.00 atm V1 = 3.00 LInitial:

Final: P2 =10.15 atm V2 = ?

Problem: a gas occupies 3.00 L at 2.00 atm. Calculate its volume when the pressure is

10.15 atm at the same temperature.

•

because the temperature is constant T1 = T2•

P1 V1T2

T1 P2

V2 = =(2.00 atm)(3.00 L)

10.15 atm= 0.591 L

Slide 7-17 Gas Laws

The actual temperature and pressure at which we compare two or more gases does

not matter.

•

For convenience in making comparisons, chemists have selected one pressure as a

standard pressure, and one temperature as a standard pressure.

•

The standard temperature and pressure (STP) selected are 0°C (273 K) and 1 atm

pressure.

•

Avogadro’s law: equal volumes of gas at the same temperature and pressure contain the

same numbers of molecules.

•


18 Gas Laws

All gases at STP or any other combination of pressure and temperature contain the same

number of molecules in a given volume. But how many is that?

•

One mole contains 6.022 x 1023 formula units; what volume of gas at STP contains this

many molecules?

•

This volume has been measured and found to be 22.4 L.•Thus, one mole of any gas at STP occupies 22.4 L•

Slide 7-19 Ideal Gas Law

Avogadro’s law allows us to write a gas law that is valid not only for any P, V, and T but

also for any mass of gas.

•

PV = nRTP = pressure of the gas in atmospheres (atm)

V = volume of the gas in liters (L)

n = moles of the gas (mol)

T = temperature in kelvins (K)

R = ideal gas constant (a constant for all gases)

Ideal gas law:•

Slide 7-20 Ideal Gas Law

We find the value of R by using the fact that 1.00 mol of any gas at STP occupies 22.4 L•

PVR =

nT=

(1.00 atm)(22.4 L)

(1.00 mol)(273 K)= 0.0821

L• atmmol• K

Problem: 1.00 mol of CH4 gas occupies 20.0 L at 1.00 atm. What is the temperature

of the gas in kelvins?

•

Solution: solve the ideal gas law for T and plug in the given values:•

PVnR

T = = 244 K(1.00 atm)(20.0 L)

(1.00 mol)(0.0821 L• atm• mol-1• K

-1)

=

Slide 7-21 Gas Laws

Dalton’s law of partial pressures: the total pressure, PT, of a mixture of gases is the sum

of the partial pressures of each individual gas:

•

PT = P1 + P2 + P3 + . . .

Problem: to a tank containing N2 at 2.0 atm and O2 at 1.0 atm we add an unknown

quantity of CO2 until the total pressure in the tank is 4.6 atm. What is the partial pressure of CO2?

•

4.6 atm 2.0 atm 1.0 atm 1.6 atm

Totalpressure

Partial pressure

of N 2

Partial pressure

of O2

Partial pressureof CO2

+ +=

Solution:•

Slide 7-22 Gases


22 Gases

Ideal gas: the six assumptions of the KMT give us an idealized picture of the particles of a

gas and their interactions with one another.

•

their atoms or molecules do occupy some volume.•there are forces of attraction between their atoms or molecules.•

Real gases•

at pressures above 1 to 2 atm and temperatures well above their boiling points,

real gases behave in much the same way as predicted by the KMT.

•In reality, no gases are ideal•

Slide 7-23 Changes of States

A change of state is a process in which a substance is transformed from one physical

state to another physical state. They are normally accomplished by heating or cooling.

•

There are six possible changes of states: •Freezing: liquid to solid state•Melting: solid to liquid state•Evaporation: liquid to gaseous state•Condensation: gaseous to liquid state•Sublimation: solid to gaseous state•Deposition: gaseous to solid state•Endothermic Change: change of state in which heat energy is absorbed. Ex: melting,

evaporation, sublimation

•

Exothermic change: change of state in which heat energy is given off. Ex: freezing,

condensation, deposition

•

Slide 7-24 Intermolecular Forces

At or near STP, the forces of attraction between molecules of most gases are so

small that they can be ignored.

•

When T decreases or P increases or both, the forces of attraction become important

to the point that they cause condensation (gases to liquids) and ultimately solidification (liquids to solids).

•

In order to understand the properties of liquids and solids, we must look at the

nature of these intermolecular forces of attraction.

•

The strength of attractive forces between molecules determines whether any sample of

matter is a gas, liquid, or solid.

•


25 Intermolecular Forces

Their origin is electrostatic; that is, the attraction between positive and negative

dipoles.

•

The strengths of covalent bonds are shown for comparison.•

We discuss three types of intermolecular forces•

Slide 7-26 London Dispersion Forces

London dispersion forces are the attraction between temporary induced dipoles.•


27 London Dispersion Forces

London dispersion forces exist between all atoms and molecules.•

They are the only forces of attraction between atoms and nonpolar molecules.•

They range in strength from 0.01 to 2 kcal/mol depending on mass, size, and shape of

the interacting molecules.

•

In general, their strength increases as the mass and number of electrons in a molecule

increases.

•

Even though these forces are very weak, they contribute significantly to the attractive

forces between large molecules because they act over large surface areas.

•

Slide 7-28 Dipole-Dipole Interactions

CH3 CH2 CH2 CH3 CH3 -C- CH3

O-

+Butane

(bp 0.5°C)

Acetone

(bp 58°C)

consider butane and acetone, compounds of similar molecular weight•

Butane is a nonpolar molecule; the only interactions between butane molecules are

London forces.

•

Acetone is a polar molecule; its molecules are held together in the liquid state by

dipole-dipole interactions.

•

Dipole-dipole interactions; the electrostatic attraction between positive and negative

dipoles.

•

Slide 7-29 Hydrogen Bonds

Hydrogen bond: a noncovalent force of attraction between the partial positive charge on

a hydrogen bonded to an atom of high electronegativity, most commonly O or N, and the partial negative charge on a nearby O or N.

•

H O

H

H

O

H

- +

hydrogenbond

hydrogen

bond

- +

(a) (b) (c)


30 Hydrogen Bonds

The strength of hydrogen bonds ranges from 2 to 10 kcal/mol.•

that in liquid water is approximately 5.0 kcal/mol•

By comparison, the strength of an O-H covalent bond in a water molecule is 119

kcal/mol.

•

Nonetheless, hydrogen bonding in liquid water has an important effect on the physical

properties of water.

•

The relatively high boiling point of water is due to hydrogen bonding between water

molecules; extra energy is required to separate a water molecule from its neighbors.

•

Hydrogen bonds are not restricted to water; they form whenever there is are O-H or N-H

groups.

•

Slide 7-31 Evaporation

In a liquid there is a distribution of kinetic energies (KE) among its molecules.•

Some have high KE and move rapidly; others have low KE and move more slowly.•

If a molecule at the surface is moving slowly (has a low KE), it cannot escape from

the liquid because of the attractions of neighboring molecules.

•

If, however, it is moving more rapidly (has a higher KE) and moving upward, it can

escape the liquid and enter the gaseous space above it.

•

An important property of liquids is that they evaporate:•

Slide 7-32 Evaporation

If the container is open, this process continues until all molecules escape.•

If the container is closed, molecules remain in the air space above the liquid.•

At equilibrium, molecules continue to escape from the liquid while an equal number are

recaptured by it.

•

The partial pressure of the vapor in equilibrium with the liquid is called the vapor

pressure of the liquid.

•

Vapor pressure is a function of temperature. Vapor pressure increases with temperature

until it equals the atmospheric pressure.

•

Evaporation is the process by which molecules escape from the liquid phase to the gas

phase. Evaporation is surface phenomena.

•

A vapor is a gas that exists at a temperature and pressure at which it ordinarily would be

thought of as a liquid or solid

•

Slide 7-33 Vapor Pressure of Liquids

Vapor pressure is the pressure exerted by a vapor above a liquid when the liquid and the

vapor are in equilibrium with each other.

•

A volatile substance is a substance that readily evaporates at room temperature because

of a huh vapor pressure. Gasoline is a substance whose components are highly volatile.

•

Boiling is a form of evaporation where conversion from the liquid state to the vapor state

occurs within the body of the liquid through bubble formation.

•

Boiling point is the temperature at which the vapor pressure of a liquid becomes equal to

the external (atmospheric) pressure exerted on the liquid

•


34 Boiling Point

Normal boiling point: the temperature at which the vapor pressure of a liquid equals the

atmospheric pressure.

•

Slide 7-35 Conditions that affect Boiling Point

Boiling point of a liquid can be increased by increasing the external pressure. This

principle is used in pressure cooker cooking. Normally at high altitude cooking is slow because water boils at low temperature due to low pr. But in pressure cooker water boils

above 1000C at elevated pr. So the cooking is faster than normal.

•

Liquids that have high normal boiling points or undergo undesirable reaction at a higher

temperature can be made to boil at low temperatures by reducing the external pressure. This principle is used in numerous food products like juice preparation where water is

boiled off at reduced pr. Thus concentrating the juice without heating it at high

temperature.

•

Slide 7-36 Boiling Point

CHCl3

CH3 CH2 OH

H2 O

CH3 COOH

120

46

18

60

62

78

100

118

Chloroform

Ethanol

Water

Acetic acid

NameMolecular

Formula

Molecular

Weight

(amu)

Boiling

Point

(°C)

CH3 CH2 CH2 CH2 CH2 CH3 86 69Hexane

Table 6.3•

Boiling points of covalent compounds depend primarily on two factors: (1) the nature and

strength of intermolecular forces and (2) molecular size and shape.

•


37 Boiling Points

Consider CH4 (MW 16, bp -164°C) and H2O (MW 18, bp 100°C). The difference in

boiling points between them is due to the greater strength of hydrogen bonding in water compared with the much weaker London dispersion forces in methane.

•

Consider methane, CH4 (MW 16, bp -164°C), and hexane C6H14 (MW 86, bp 69°C).

Because of its larger surface area, London dispersion forces are stronger between hexane molecules than between methane molecules.

•

Intermolecular forces•

Slide 7-38 Boiling Points

when molecules are similar in every way except shape, the strength of London forces

determines boiling point

•

CH3 CH2CH2CH2 CH3CH3 -C-CH3

CH3

CH3

Pentane(bp 36.2°C)

2,2-Dimethylpropane(bp 9.5°C)

Both are C5H12 and have the same molecular weight.•

2,2-dimethylpropane is roughly spherical while pentane is a linear molecule.•

Pentane has the higher boiling point because it has the larger surface area and

stronger London dispersion forces between its molecules.

•

Molecular shape•


Chapter 7 - Gases, Liquids, And Solids

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