chapter seven gases, liquids, and solids. chapter 7 | slide 2 kinetic molecular theory. theory...

Chapter SevenChapter Seven

Gases, Liquids, and Solids

Chapter 7 | Slide 2

Kinetic Molecular Theory.

• Theory developed to explain physical behavior of the ______ states of matter (solid, liquid, and gas)

• Theory of _________ molecules.

Chapter 7 | Slide 3

Assumptions of KMT

• Matter is composed of _____ particles (atoms, molecules or ions) that have definite and characteristic sizes that do not change.

• The particles are in constant, random _________ and therefore possess kinetic energy (energy of _______).

• The particles interact with one another through attractions and repulsions and therefore possess _____________ energy.

• The _______ of the particles (and their kinetic energy) increases as the _____________ is increased.

• The particles in a system ___________ energy to each other through elastic collisions.

Chapter 7 | Slide 4

Figure 7.3 (a) In a solid, the particles (atoms, molecules, or ions) are close together and vibrate about fixed sites. (b) The particles in a liquid, though still close together, freely slide over one another. (c) In a gas, the particles are in constant random motion, each particle being independent of the others present.

Gases, Liquids, and Solids cont’d

Chapter 7 | Slide 5

Solid State

• Characterized by a dominance of __________ energy over ____________ energy There are strong forces that hold individual molecules together Low kinetic energy = little motion of molecules Particles in a solid are tightly packed together in a definite

arrangement

• Consequences: ___________ volume and ______________ shape High density Small compressibility

Chapter 7 | Slide 6

Liquid State

• Characterized by __________ energy and _________ energy that are approximately __________ in magnitude Particles in a liquid are randomly packed, touching each other, and

have enough kinetic energy that they constantly slide over each other.

• Consequences: ___________ volume and ____________ shape High density Small compressibility Small thermal expansion

Chapter 7 | Slide 7

Gaseous State

• Characterized by dominance of ________ energy over ______________ energy Gas particles have enough kinetic energy to move far

apart from each other (moving in straight lines) There is very little attraction between particles

• Consequences: ___________ volume and _____________ shape Low density Large compressibility Moderate Thermal Expansion

Chapter 7 | Slide 8

Gases move in straight lines

Atoms or molecules bounce off each other and the sides or walls of container.

Chapter 7 | Slide 9


Chapter 7 | Slide 10


→ Fig. 7.5

When a gas is compressed, the amount of empty space in the container is decreased.


Four Important Gas Variables

• 1. n = number of moles n = mass/MM

• 2. T = temperature; must be in Kelvin! K = oC + 273.15

• 3. V = Volume, usually measured in Liters 1000 mL = 1 L

• 4. P = Pressure = force/area 1 atm = 760 mm Hg = 760 torr = 14.68 lb/in2 = 101.325 Pa

• Pressure of a gas results from the number of collisions per unit time on the walls of container.


Two Important Relationships

• Directly proportional: if the value of one variable increases, the value of the other variable also increases

• Inversely proportional: if the value of one variable increases, the value of the other variable decreases

2

1

2

1

B

B

A

A

1

2

2

1

B

B

A

A


Relationship Between Pressure and Volume (Boyle’s Law)

• When the pressure is increased on a gas, its volume goes down – inversely proportional!

1

2

2

1

V

V

P

P



Fig. 7.9 When the volume of a gas at constant temperature decreases by half, the average number of times a molecule hits the container walls is doubled.


Volume/Pressure Calculation

• A balloon is inflated to a volume of 12.6L on a day when the atmospheric pressure is 675 mm Hg. The next day, a storm drops the pressure to 651 mm Hg. Assuming constant temperature, what is the new volume of the balloon?


Relationship Between Temperature and Volume (Charles’ Law)

• When the temperature is increased on a gas, its volume goes up – directly proportional!

2

1

2

1

V

V

T

T

KELVIN !!!


Volume/Temperature Calculation

• The volume of a sample of air at 30. oC is 3.50 L. Assume that the pressure is held constant. What would be the volume of air at 300. oC?


Combined Gas Laws

• Relates the changes in pressure, volume and temperature of a gas

2

22

1

11

T

VP

T

VP


Combined Gas Law Practice

• A fixed quantity of nitrogen gas in a 275 mL container at a pressure of 625 mm Hg is transferred to a container with a volume of 750. mL. What is the new pressure if the temperature changes from 25 oC to 17 oC?


The Ideal Gas Law

• Describes the relationships among the four variables for gaseous substances: P, T, V, n PV = nRT

• R is called the ideal gas constant 0.0821 L·atm/mol·K Always make sure you have all your units correct!


Ideal Gas Law Calculations

• Methane, CH4, can be used as fuel for an automobile; however, it is a gas at normal temperatures and pressures, which causes some problems with storage. One gallon of gasoline could be replaced by 655 g of CH4. What is the volume of this much methane at 25 oC and 745 mm Hg.


Ideal Gas Law Calculations, II

• What is the temperature (in oC) of a sample of 3.00 moles of He at 27.5 atm of pressure and in a 6.00 L container?


Dalton’s Law of Partial Pressures

• Since gas molecules are so far apart, we can assume they behave independently.

• Dalton’s Law: in a gas mixture the total pressure is given by the sum of partial pressures of each component:

• Partial Pressure The pressure that a gas in a mixture of gases would exert if it were

present alone under the same conditions

321total PPPP


Figure 7.14 A set of four containers can be used to illustrate Dalton's law of partial pressures. The pressure in the fourth container (the mixture of gases) is equal to the sum of the pressures in the first three containers

(the individual gases).


Dalton’s Law Calculation

• A mixture of H2, N2 and Ar gases is present in a steel cylinder. The total pressure within the cylinder is 675 mm Hg and the partial pressures of N2 and Ar are, respectively, 354 mm Hg and 235 mm Hg. If CO2 gas is added to the mixture, at constant temperature, until the total pressure reaches 842 mm Hg, what is the partial pressure, in mm Hg, of the following? A) CO2 B) N2 C) Ar D) H2


Gas Laws

Some common examples illustrating applications of gas laws.

Filling a syringe

Popping corn

Use of dynamite


← Fig. 7.10 Filling a syringe with a liquid is an application of Boyle’s law.




Phil Degginger/Color-Pic

CC 7.1


Rapid Expansion of Gases

4 C3H5O3(NO2)3(s) →

12 CO2(g) + 10 H2O(g) + 6 N2(g) + O2(g)

_________________ reaction takes place in a small confined volume of space and intense heat is produced. What happens to gases at very high temperatures?


Changes of StateChanges in state occur with changes in kinetic energy



(a) The beaker contains iodine crystals.

(b) Iodine has an appreciable vapor pressure even below its melting point.Fig. 7.15


Evaporation of Liquids

• Process by which molecules _____________ from the liquid phase to the _______ phase

• In order for molecules in the _________ phase to change into molecules in the __________ phase, the molecules must ______ __________ __________ is gained through collisions with other molecules Some molecules in a liquid have more energy than others It is easier for surface molecules to escape than molecules in the

middle of the liquid (because they are subjected to fewer attractive forces)

• Rate of evaporation ______________ with temperature The average ______________ energy of the molecules increases


Vapor Pressure in a Closed Container

• Liquid and vapor in a closed container eventually reach equilibrium A condition in which two opposite processes take place at the

same rate In this case, the two processes are evaporation and

condensation

• The vapor above the liquid in a closed container exerts vapor pressure The pressure exerted by a vapor above a liquid when the liquid

and vapor are in equilibrium with each other



Fig. 7.17

(a) the liquid level drops for a time, (b) then becomes constant. At that point a state of equilibrium has been reached in which (c) the rate of evaporation equals the rate of condensations.


Volatility

• Substances with _________ vapor pressures evaporate _________ They are the substances with _________ attractive

forces between molecules

• Volatile substance: A substance that readily evaporates at room

temperature because of a high vapor pressure


Boiling and Boiling Point

• Boiling 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 The temperature at which the vapor pressure of a

liquid becomes equal to the external (atmospheric) pressure exerted on the liquid

Normal boiling point: boiling point at 1 atm


Conditions Affecting Boiling Point

• Boiling Point is increased by increasing the external pressure Pressure cooker

• Boiling Point is decreased by decreasing the external pressure Boiling point at higher elevations is lower than at sea

level


Table 7.3 Boiling Point of Water at Various Locations That Differ in Elevation.



Same principle applies to a pressurized automobile cooling system.


Intramolecular v. Intermolecular Forces

• The covalent bond holding a molecule together is an intramolecular force.

• The attraction between molecules is an intermolecular force.

• Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl).

• When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).


Strength of Bonds


Intermolecular Forces

• There are different intermolecular forces _________ - ___________ Forces _____________ “Bonds” London ______________ Forces

• Most of these interactions are driven by one general idea: opposite charges attract!


Dipole-Dipole Interactions

Attraction between polar molecules


Hydrogen Bonds

Unusually strong dipole-dipole interactions that occur between molecules with H-N, H-O, or H-F covalent bonds and a lone pair of electrons on another small, electronegative atom (N, O, or F).


Figure 7.22 Diagrams of hydrogen bonding involving selected simple molecules. The solid lines represent covalent bonds; the

dotted lines represent hydrogen bonds.


Figure 7.23 If there were no hydrogen bonding between water molecules, the boiling point of water would be

approximately - 80C.


London Dispersion Forces

• Formed between adjacent neutral molecules The nucleus of one molecule (or atom) attracts the electrons of the

adjacent molecule (or atom). For an instant, the electron clouds become distorted. In that instant a dipole is formed (called an instantaneous dipole). One instantaneous dipole can induce another instantaneous dipole

in an adjacent molecule (or atom).


London Dispersion Forces, II

• Polarizability is the ease with which an electron cloud can be deformed.

• The larger the molecule (the greater the number of electrons) the more polarizable.

• London dispersion forces ____________ as molecular weight ________________.

• London dispersion forces exist between all molecules.• London dispersion forces depend on the shape of the

molecule.



→Fig. 7.24

Nonpolar molecules can develop instantaneous dipoles and induced dipoles.


Boiling Points of Nonpolar Substances


Relative Strengths of Interactions

• Intramolecular Bonds: Ionic Bonds Strongest Covalent Bonds Weakest

• Intermolecular Forces Hydrogen “Bonds” Strongest Dipole-Dipole Forces London Dispersion Forces Weakest

The stronger the forces/attractions, the more energy that must be added to break the attractions (the higher the melting point and the higher the boiling point).


Question!

• Which of the following molecules will not form hydrogen bonds?

H3C CH2 C

O

OH

H3C CH2 C

O

CH3 H3C CH2 N

H

CH3

HF(A) (B)

(C) (D)


Question!

• Which forces would you expect to find in samples of the following molecules? A. C3H8 (propane)

B. H2O

C. CH3F


Question!

• Which of the following compounds would you expect to have the lowest boiling point? A. C3H8 (propane)

B. H2O

C. CH3F


Question!

• Which of the following compounds would you expect to have the highest boiling point? A. C2H6

B. C3H8

C. C4H10

D. C5H12

• Why?

chapter seven gases, liquids, and solids. chapter 7 | slide 2 kinetic molecular theory. theory...

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