Chapter 4. THE PROPERTIES OF GASES. THE NATURE OF GASES. 4.1 Observing Gases 4.2 Pressure 4.3 Alternative Units of Pressure. THE GAS LAWS. 4.4 The Experimental Observations 4.5 Applications of the Ideal Gas Law 4.6 Gas Density 4.7 The Stoichiometry of Reacting Gases
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Chapter 4.
THE PROPERTIES OF GASES
THE NATURE OF GASES
4.1 Observing Gases
4.2 Pressure
4.3 Alternative Units of Pressure
THE GAS LAWS
4.4 The Experimental Observations
4.5 Applications of the Ideal Gas Law
4.6 Gas Density
4.7 The Stoichiometry of Reacting Gases
4.8 Mixtures of Gases
2012 General Chemistry I
4.1 Observing Gases
Many of physical properties of gases are very similar, regardless of the identity of the gas. Therefore, they can all be described simultaneously.
Samples of gases large enough to study are examples of bulk matter – forms of matter that consist of large numbers of molecules
Two major properties of gases:
Compressibility – the act of reducing the volume of a sample of a gas
Expansivity  the ability of a gas to fill the space available to it rapidly
4.2 Pressure
 Pressure arises from the collisions of gas molecules on the walls of the container.

 SI unit of pressure, the pascal (Pa)
Measurement of Pressure
where h = the height of a column, d = density of liquid, and g = acceleration of gravity (9.80665 ms2)
This is a Ushaped tube filled with liquid and connected to an experimental system, whose pressure is being monitored.
 1 bar = 105 Pa = 100 kPa
 1 atm = 760 Torr = 1.01325×105 Pa (101.325 kPa)
 1 Torr ~ 1 mmHg
Weather map
mbar
4.4 The Experimental Observations
This applies to an isothermal system (constant T) with a fixed amount of gas (constant n).
 For isothermal changes between two states (1 and 2),
This applies to an isobaric system (constant P) with a fixed amount of gas (constant n).
The Kelvin Scale of Temperature
If a Charles’ plot of V versus T (at constant P and n) is extrapolated to V = 0, the intercept on the T axis is ~273 oC.
 Kelvin temperature scale
T = 0 K = 273.15 oC,
when V → 0.
 Celsius temperature scale
t (oC) = T (K)  273.15
0 oC = 273.15 K
Another aspect of gas behavior (GayLussac’s Law)
This applies to an isochoric system (constant V) with a fixed amount of gas (constant n).
Avogadro’s Principle
 This defines molar volume
The Ideal Gas Law
This is formed by combining the laws of Boyle,
Charles, GayLussac and Avogadro.
Gas constant, R = PV/nT.
It is sometimes called a “universal constant” and
has the value 8.314 J K1 mol1 in SI units, although
other units are often used (Table 4.2).
Table 4.2. The Gas Constant, R
 For conditions 1 and 2,
 Molar volume
 Standard ambient temperature and pressure (SATP)
298.15 K and 1 bar, molar volume at SATP = 24.79 L·mol1
 Standard temperature and pressure (STP)
0 oC and 1 atm (273.15 K and 1.01325 bar)
 Molar volume at STP
EXAMPLE 4.4
In an investigation of the properties of the coolant gas used in an
airconditioning system, a sample of volume 500 mL at 28.0 oC was
found to exert a pressure of 92.0 kPa. What pressure will the sample
exert when it is compressed to 30 mL and cooled to 5.0 oC?
Calculating the pressure of a given sample
Using the combined gas law when one variable is changed
Using the combined gas law when two variables are changed
Molar concentration of a gas is the number moles divided by the volume
occupied by the gas.
Molar concentration of a gas at STP (where molar volume is 22.4141 L):
This value is the same for all gases, assuming ideal behavior.
Density, however, does depend on the identity of the gas.
Gas Density Relationships
Density at STP
4.7 The Stoichiometry of Reacting Gases
e.g. sodium azide (NaN3) for air bags
EXAMPLE 4.6
The carbon dioxide generated by the personnel in
the artificial atmosphere of submarines and
spacecraft must be removed form the air and the
oxygen recovered. Submarine design teams have
investigated the use of potassium superoxide, KO2,
as an air purifier because this compound reacts with
carbon dioxide and releases oxygen:
4 KO2 (s) + 2 CO2(g) → 2 K2CO3(s) + 3 O2(g)
Calculate the mass of KO2 needed to react with 50 L
of CO2 at 25 oC and 1.0 atm.
Vm = 24.47 Lmol1; 1 mol CO2 > 2 mol KO2; MKO2 = 71.10 gmol1
 A mixture of gases that do not react with one another behaves like a
single pure gas.
P = PA + PB + … for the mixture containing A, B, …
 Humid gas: P = Pdry air + Pwater vapor (Pwater vapor = 47 Torr at 37 oC)
EXAMPLE 4.7
Air is a source of reactants for many chemical processes. To determine
how much air is needed for these reactions, it is useful to know the
partial pressures of the components. A certain sample of dry air of
total mass 1.00 g consists almost entirely of 0.76 g of nitrogen and
0.24 g of oxygen. Calculate the partial pressures of these gases when
the total pressure is 0.87 atm.
Chapter 4.
THE PROPERTIES OF GASES
MOLECULAR MOTION
4.9 Diffusion and Effusion
4.10 The Kinetic Model of Gases
4.11 The Maxwell Distribution of Speeds
REAL GASES
4.12 Deviations from Ideality
4.13 The Liquefaction of Gases
4.14 Equations of State of Real Gases
2012 General Chemistry I
4.9 Diffusion and Effusion
through a small hole into a
vacuum
Strictly,Graham’s law relates to effusion, but it can also be used for diffusion.
 For two gases A and B with molar masses MA and MB,
 Rate of effusion and average speed increase as the square root of the temperature:
1. A gas consists of a collection of molecules in
continuous random motion.
2. Gas molecules are infinitesimally small points.
3. The molecules move in straight lines until
they collide.
4. The molecules do not influence one another
except during collisions.
 Collision with walls: consider molecules
traveling only in one dimensional x with a
velocity of vx.
The change in momentum (final – initial)
of one molecule: 2mvx
All the molecules within a distance vxDt of the wall
and traveling toward it will strike the wall during the
Interval Dt.
If the wall has area A, all the particles in a volume
AvxDt will reach the wall if they are traveling toward it.
The number of molecules in the volume AvxDt is that
fraction of the total volume V, multiplied by the total
number of molecules:
The average number of collisions with the wall during
the interval Dt is half the number in the volume AvxDt:
The total momentum change = number of collisions × individual molecule
momentum change
Force = rate of change of momentum =
(total momentum change)/Dt
for the average value of <vx2>
Mean square speed:
Pressure on wall:
where vrms is the root mean square speed,
or
 The temperature is proportional to the mean square speed of the
molecules in a gas.
 This was the first acceptable physical interpretation of temperature:
a measure of molecular motion.
EXAMPLE 4.7
What is the root mean square speed of nitrogen
Molecules in air at 20 oC?
Maxwell derived equation 22, for calculating the fraction of gas molecules
having the speed v at any instant, from the kinetic model.
v = a particle’s speed
DN = the number of molecules with speeds in the range
between v + Dv
N = total number of molecules; M = molar mass
f(v) = Maxwell distribution of speeds
For an infinitesimal range,
average speed
 Molar mass (M) dependence:
as M increases, the fraction of molecules with
speeds greater than a specific speed decreases.
 Temperature dependence:
as T increases, the fraction of molecules with speeds greater than a specific speed increases.
 Deviations from the ideal gas law are significant at high pressures and low temperatures (where significant intermolecular interactions exist).
4.12 Deviations from Ideality
 Gases condense to liquids when cooled or compressed (attraction).
 Liquids are difficult to compress (repulsion).
Deviation from ideal gases
For an ideal gas, Z = 1
Long range attractions; smaller Z,
condensation of gases
Short range repulsions; larger Z, low
compressibility of liquids and solids,
finite molecular volume
 For many gases, attractions dominate at low pressure (Z < 1), while repulsive interactions dominate at high pressure (Z > 1).
 Linde refrigerator for the liquefaction
of gases
i.e. Adiabatic cooling; temperature
decrease under isentropic expansion
of any gas (w 0)
or
–nb volume excluded since molecules cannot overlap
b volume excluded by 1 mol ~ molar volume in the liquid state
pressure reduced due to attractions between pairs of molecules
Virial expansion of the van der Waals equation
At low particle densities
Table 4.5
Van der Waals Parameters for some Common Gases
1. A large number of gas molecules in ceaseless, random, and straight motion.
2. The average speed and the spread of speeds increase with T and decrease with m.
3. Molecules travel in straight lines until they collide with other molecules or the container wall.
4. Widely separated. Intermolecular forces have only a weak effect on the properties.
5. Repulsions increase the molar volume, whereas attractions decrease the molar volume.
Refrigerant gas (a = 16.2 L2 atm mol–2, b = 0.084 L/mol), 1.50 mol in 5.00 L at 0 oC; Estimate the pressure.
EXAMPLE 4.9