Transcript Chapter 6 - DePaul University Department of Chemistry

Chapter 6:
The States of Matter
1
PHYSICAL PROPERTIES OF MATTER
• All three states of matter have certain properties that help
distinguish between the states. Four of these properties
are density, shape, compressibility, and thermal
expansion.
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DENSITY
• Density is equal to the mass of a sample divided by the
volume of the same sample.
d e n sity 
m a ss
v o lu m e
3
SHAPE
• The shape matter takes depends upon the physical state
of the matter.
COMPRESSIBILITY
• Compressibility is the change in volume of a sample of
matter that results from a pressure change acting on the
sample.
THERMAL EXPANSION
• Thermal expansion is the change in volume of a sample of
matter resulting from a change in the temperature of the
sample.
4
CHARACTERISTIC PROPERTIES OF THE THREE
STATES OF MATTER
5
THE KINETIC MOLECULAR THEORY OF MATTER
• The kinetic molecular theory of matter is a useful tool for
explaining the observed properties of matter in the three
different states of solid, liquid and gas.
POSTULATES OF THE THEORY
• Postulate 1: Matter is made up of tiny particles called
molecules.
• Postulate 2: The particles of matter are in constant motion
and therefore possess kinetic energy.
• Postulate 3: The particles possess potential energy as a
result of repelling or attracting each other.
• Postulate 4: The average particle speed increases as the
temperature increases.
• Postulate 5: The particles transfer energy from one to
another during collisions in which no net energy is lost
from the system.
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KINETIC ENERGY
• Kinetic energy is the energy a particle has as a result of
being in motion.
• Kinetic energy (KE) is calculated using the equation:
In this equation, m is the mass of a particle and v is its
velocity.
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POTENTIAL ENERGY
• Potential energy is the energy a particle has as a result of
being attracted to or repelled by other particles.
COHESIVE FORCE
• A cohesive force is an attractive force between particles. It
is associated with potential energy.
DISRUPTIVE FORCE
• A disruptive force results from particle motion. It is
associated with kinetic energy.
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THE SOLID STATE
• The solid state is characterized by a high density, a definite
shape that is independent of its container, a small
compressibility, and a very small thermal expansion.
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THE LIQUID STATE
• The liquid state is characterized by a high density, an
indefinite shape that depends on the shape of its container,
a small compressibility, and a small thermal expansion.
10
THE GASEOUS STATE
• The gaseous state is characterized by a low density, an
indefinite shape that depends on the shape of its container,
a large compressibility, and a moderate thermal expansion.
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A KINETIC MOLECULAR VIEW OF SOLIDS, LIQUIDS,
AND GASES
12
THE GAS LAWS
• The gas laws are mathematical equations that describe the
behavior of gases as they are mixed, subjected to
pressure or temperature changes, or allowed to diffuse.
• The pressure exerted on or by a gas sample and the
temperature of the sample are important quantities in gas
law calculations.
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PRESSURE
• Pressure is defined as a force pushing on a unit area of
surface on which the force acts.
• In gas law calculations, pressure is often expressed in
units related to the measurement of atmospheric pressure.
STANDARD ATMOSPHERE OF PRESSURE
• A pressure of one standard atmosphere is the pressure
needed to support a 760-mm (76.0-cm) column of mercury
in a barometer.
ONE TORR OF PRESSURE
• One torr of pressure is the pressure needed to support a
1-mm column of mercury in a barometer. A pressure of
760 torr is equal to one standard atmosphere.
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OFTEN-USED UNITS OF PRESSURE
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TEMPERATURE
• The temperature of a gas sample is a measurement of the
average kinetic energy of the gas molecules in the sample.
• The Kelvin temperature scale is used in all gas law
calculations.
ABSOLUTE ZERO
• A temperature of 0 K
is called absolute zero.
It is the temperature at
which gas molecules
have no kinetic energy
because all motion
stops. On the Celsius
scale, absolute zero is
equal to -273°C.
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PRESSURE, TEMPERATURE AND VOLUME
RELATIONSHIPS FOR GASES
• Mathematical equations relating the pressure, temperature
and volume of gases are called gas laws.
• All of the gas laws are named after the scientists who first
discovered them.
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BOYLE'S LAW
• Boyle's law is a gas law that describes the pressure and
volume behavior of a gas sample that is maintained at
constant temperature.
• Mathematically, Boyle's law is written as follows:
or
In these equations, P is the pressure, V is the volume, and k
is an experimentally determined constant.
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CHARLES'S LAW
• Charles's law is a gas law that describes the temperature
and volume behavior of a gas sample that is maintained at
constant pressure.
• Mathematically, Charles's law is written as follows:
or
In these equations, V is the volume, T is the temperature in
Kelvin, and k' is an experimentally determined constant.
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19
THE COMBINED GAS LAW
• Boyle's law and Charles's law can be combined to give the
combined gas law that is written mathematically as follows:
In this equation, P, V and T have the same meaning as
before and k'' is another experimentally determined constant.
• The combined gas law can be
expressed in another useful form
where the subscript i refers to an
initial set of conditions and the
subscript f refers to a final set of
conditions for the same gas sample.
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• Example: A gas sample has a volume of 2.50 liters when it
is at a temperature of 30.0°C and a pressure of 1.80 atm.
What volume in liters will the sample have if the pressure
is increased to 3.00 atm, and the temperature is increased
to 100°C?
• Solution: The problem can be solved:
•
•
•
•
using the combined gas law.
by identifying the initial and final conditions.
making sure all like quantities are in the same units.
expressing the temperatures in Kelvin.
• Thus, we see that the combined gas law must be solved for
Vf .
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•The result is:
•Substitution of appropriate values gives:
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AVOGADRO’S LAW
• Equal volumes of gases measured at the same temperature
and pressure contain equal numbers of molecules.
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STANDARD CONDITIONS
• STP = standard temperature and pressure
• 0°C (273 K)
• 1.00 atm
MOLAR VOLUME AT STP
• 1 mole of any gas molecules
has a volume of 22.4 L at STP.
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THE IDEAL GAS LAW
• The ideal gas law allows calculations to be done in which
the amount of gas varies as well as the temperature,
pressure, and volume.
• Mathematically, the ideal gas law is written as follows:
PV= nRT
In this equation, P is the pressure of a gas sample, V is the
sample volume, T is the sample temperature in Kelvin, n is
the number of moles of gas in the sample, and R is a
constant called the universal gas constant. A commonlyused value for R is:
• In calculations, the quantities V, P, and T must be expressed
in units that match the units of R, liters (L), atm, and Kelvin,
respectively.
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EXAMPLES OF IDEAL GAS LAW CALCULATIONS
• Example 1: A 2.50 mole sample of gas is confined in a
6.17 liter tank at a temperature of 28.0°C. What is the
pressure of the gas in atm?
• Solution: The ideal gas equation is first solved for P:
• The known quantities are then substituted into the equation,
making sure the units cancel properly to give units of atm in
the answer:
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• Example 2: A 4.00 g sample of gas is found to exert a
pressure of 1.71 atm when confined in a 3.60 L container
at a temperature of 27°C. What is the molecular weight of
the gas in grams per mole?
• Solution:
• The molecular weight is equal to the sample mass in
grams divided by the number of moles in the sample.
• Because the sample mass is known, the molecular
weight can be determined by calculating the number of
moles in the sample.
• The ideal gas equation
is first solved for n:
• The known quantities are then substituted into the equation,
making sure units cancel properly to give the units of mol for
the answer.
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• The units are seen to cancel properly to give the number of
moles as the answer. The molecular weight is calculated by
dividing the number of grams in the sample by the number of
moles in the sample:
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IDEAL GASES vs. REAL GASES
• No ideal gases actually exist.
• If they did exist, they would behave exactly as predicted by
the gas laws at all temperatures and pressures.
• Real gases deviate from the behavior predicted by the gas
laws, but under normally encountered temperatures and
pressures, the deviations are small.
• Consequently, the gas laws can be used for real gases.
• Interparticle attractions make gases behave less ideally.
• The gases laws work best for gases made up of single
atoms or nonpolar molecules.
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DALTON'S LAW OF PARTIAL PRESSURES
• According to Dalton's law, the total pressure exerted by a
mixture of gases is equal to the sum of the partial
pressures of the gases in the mixture.
Pto ta l 
P
in d iv id u a l g a s e s
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PARTIAL PRESSURE
• The partial pressure of an individual gas of a mixture is the
pressure the gas would exert if it were alone in the
container at the same temperature as the mixture as shown
in the following illustration:
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GRAHAM'S LAW
• Graham's law is a mathematical expression that relates
the rates of effusion or diffusion of two gases to the
masses of the molecules of the two gases.
EFFUSION
• Effusion is a process in which a gas escapes from a
container through a small hole in the container.
DIFFUSION
• Diffusion is a process that causes gases to spontaneously
mix when they are brought together.
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MATHEMATICAL EXPRESSION OF GRAHAM'S LAW
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EXAMPLE OF GRAHAM'S LAW CALCULATIONS
• Compare the rates of effusion or diffusion for neon and
krypton gases.
• Solution: The molecular masses of neon and krypton are
20.18 u and 83.80 u respectively. These are the molecular
weights of the gases from the periodic table. Substitution
into the Graham's law equation gives the following:
• Thus, the rate of Ne = (2.038) rate of Kr. Stated another way,
neon gas effuses or diffuses about twice as fast as krypton
gas.
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CHANGES IN STATE
• Changes in state are often accomplished by adding or
removing heat from a substance.
• Changes in state caused by adding heat to a substance
are classified as endothermic (heat in) processes.
• Changes in state caused by removing heat are classified
as exothermic (heat out) processes.
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ENDOTHERMIC PROCESSES:
EVAPORATION OR VAPORIZATION
• Evaporation or vaporization is an endothermic process in
which a liquid is changed to a gas.
SUBLIMATION
• Sublimation is an endothermic process in which a solid is
changed to a gas without first melting to a liquid.
MELTING OR FUSION
• Melting or fusion is an endothermic process in which a
solid is changed to a liquid.
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EXOTHERMIC PROCESSES:
LIQUEFACTION OR CONDENSATION
• Liquefaction or condensation is an exothermic process in
which a gas is changed to a liquid.
DEPOSITION OR CONDENSATION
• Deposition or condensation is an exothermic process in
which a gas is changed into a solid.
FREEZING OR CRYSTALLIZATION
• Freezing or crystallization is an exothermic process in
which a liquid is changed into a solid.
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VAPOR PRESSURE
• Vapor pressure is the pressure exerted by a vapor that
is in equilibrium with its liquid.
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BOILING POINT
• The boiling point of a liquid is the temperature at which
the vapor pressure of the liquid is equal to the prevailing
atmospheric pressure.
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VARIATION OF WATER BOILING POINT WITH
ELEVATION
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SUBLIMATION AND MELTING
• Sublimation is the endothermic process in which a solid
is changed directly to a gas without first becoming a
liquid.
Solid CO2 (left) and solid H2O (right) both
sublime.
These freeze-dried coffee crystals
were prepared by subliming water
from frozen coffee.
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ENERGY AND THE STATES OF MATTER
• At 760 torr, constant heat is applied until a 1 g sample of
ice at -20°C is converted to steam at 120°C.
• This is a five step process: (AB) heating ice to melting
point, (BC) melting ice, (CD) heating liquid to boiling point,
(DE) boiling water, and (EF) heating steam.
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41
SPECIFIC HEAT
• The specific heat of a substance is the amount of heat
required to raise the temperature of exactly 1 g of a
substance exactly 1°C.
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42
HEAT OF FUSION
• The heat of fusion of a substance is the amount of heat
required to melt exactly 1g of a solid substance at constant
temperature.
HEAT OF VAPORIZATION
• The heat of vaporization of a substance is the amount of
heat required to vaporize exactly 1g of a liquid substance
at constant temperature.
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