2. 2
Objectives
• Develop rules for determining nonreacting gas mixture
properties from knowledge of mixture composition and
the properties of the individual components.
• Define the quantities used to describe the composition
of a mixture, such as mass fraction, mole fraction, and
volume fraction.
• Apply the rules for determining mixture properties to
ideal-gas mixtures and real-gas mixtures.
• Predict the P-v-T behavior of gas mixtures based on
Dalton’s law of additive pressures and Amagat’s law of
additive volumes.
3. 3
COMPOSITION OF A GAS MIXTURE: MASS AND
MOLE FRACTIONS
The mass of a mixture is equal to the
sum of the masses of its components.
The number of moles of a nonreacting
mixture is equal to the sum of the number
of moles of its components.
To determine the properties of a mixture, we need to know the composition of the
mixture as well as the properties of the individual components. There are two ways to
describe the composition of a mixture:
Mass
fraction
Mole
fraction
Molar analysis: specifying the
number of moles of each
component
Gravimetric analysis:
specifying the mass of each
component
4. 4
The sum of the mole
fractions of a mixture is
equal to 1.
Apparent (or average) molar mass M
Gas constant
The molar mass of a mixture
Mass and mole fractions of a
mixture are related by
The sum of the mass and mole
fractions of a mixture is equal
to 1.
7. 7
P-v-T BEHAVIOR OF GAS MIXTURES: IDEAL AND
REAL GASES
An ideal gas is defined as a gas whose molecules are spaced far
apart so that the behavior of a molecule is not influenced by the presence
of other molecules.
The real gases approximate this behavior closely when they are
at a low pressure or high temperature relative to their critical-point
values.
The P-v-T behavior of an ideal gas is expressed by the simple
relation Pv=RT, which is called the ideal-gas equation of state.
The P-v-T behavior of real gases is expressed by more complex
equations of state or by Pv=ZRT, where Z is the compressibility factor.
8. 8
P-v-T BEHAVIOR OF GAS MIXTURES: IDEAL AND
REAL GASES
The prediction of the P-v-T
behavior of gas mixtures is usually
based on two models:
Dalton’s law of additive pressures: The
pressure of a gas mixture is equal to the
sum of the pressures each gas would
exert if it existed alone at the mixture
temperature and volume.
Amagat’s law of additive volumes: The
volume of a gas mixture is equal to the
sum of the volumes each gas would
occupy if it existed alone at the mixture
temperature and pressure.
Dalton’s law of additive pressures for a
mixture of two ideal gases.
Amagat’s law of additive volumes
for a mixture of two ideal gases.
9. 9
The volume a component would occupy if it existed alone at the
mixture T and P is called the component volume (for ideal gases,
it is equal to the partial volume yiVm).
For ideal gases, Dalton’s and Amagat’s laws are
identical and give identical results.
Pi component pressure Vi component volume
Pi /Pm pressure fraction Vi /Vm volume fraction
10. 10
Ideal-Gas Mixtures
This equation is only valid for ideal-gas mixtures as it is derived by
assuming ideal-gas behavior for the gas mixture and each of its components.
The quantity yiPm is called the partial pressure (identical to the
component pressure for ideal gases), and the quantity yiVm is called the
partial volume (identical to the component volume for ideal gases).
Note that for an ideal-gas mixture, the mole fraction, the pressure
fraction, and the volume fraction of a component are identical.
11. 11
Real-Gas Mixtures
One way of predicting the P-v-T
behavior of a real-gas mixture is to
use compressibility factor.
Zi is determined either at Tm and Vm
Dalton’s law) or at Tm and Pm (Amagat’s law)
for each individual gas.
Compressibility factor
Amagat’s law involves the use
of mixture pressure Pm, which accounts
for the influence of intermolecular
forces between the molecules of
different gases.
Dalton’s law disregards the influence of dissimilar molecules in a
mixture on each other. As a result, it tends to underpredict the pressure of a
gas mixture for a given Vm and Tm.
Dalton’s law is more appropriate for gas mixtures at low pressures.
Amagat’s law is more appropriate at high pressures.
12. 12
Real-Gas Mixtures
Zm is determined by using these
pseudocritical properties.
The result by Kay’s rule is
accurate to within about 10% over a wide
range of temperatures and pressures.
Kay’s rule
17. 17
PROPERTIES OF GAS MIXTURES:
IDEAL AND REAL GASES
The extensive
properties of a
mixture are
determined by
simply adding the
properties of the
components.
Extensive properties of a gas mixture
Changes in properties of a gas mixture
18. 18
The intensive
properties of a
mixture are
determined by
weighted
averaging.
Extensive properties of a gas mixture
Properties per unit mass involve mass fractions (mfi) and properties
per unit mole involve mole fractions (yi).
The relations are exact for ideal-gas mixtures, and approximate for
real-gas mixtures.
19. 19
Ideal-Gas Mixtures
Partial pressures (not
the mixture pressure)
are used in the
evaluation of entropy
changes of ideal-gas
mixtures.
Gibbs–Dalton law: Under the ideal-gas
approximation, the properties of a gas are
not influenced by the presence of other
gases, and each gas component in the
mixture behaves as if it exists alone at the
mixture temperature Tm and mixture volume
Vm.
Also, the h, u, cv, and cp of an ideal gas
depend on temperature only and are
independent of the pressure or the volume
of the ideal-gas mixture.
26. 26
Real-Gas Mixtures
It is difficult to predict the
behavior of nonideal-gas
mixtures because of the
influence of dissimilar
molecules on each other.
This equation suggests that the generalized property
relations and charts for real gases developed in Chap. 12
can also be used for the components of real-gas mixtures.
But TR and PR for each component should be evaluated
using Tm and Pm.
If the Vm and Tm are specified instead of Pm and Tm,
evaluate Pm using Dalton’s law of additive pressures.
Another way is to treat the mixture as a pseudopure
substance having pseudocritical properties, determined in
terms of the critical properties of the component gases by
using Kay’s rule.
T ds relation for a gas mixture
