Chapter 7 States of Matter and Changes in State
MIXTURES OF GASES If molecules are to react, they must come into contact with one another. That is, reactants must be mixed. Consequently, an understanding of mixtures is essential to an understanding of how reactions occur. In this section, we discuss mixtures of gases.
Each component of a gaseous mixture exerts its own pressure, called its
partial
pressure
. The partial pressure of component A, which is written P
, can be obtained from A
the ideal gas law
PA
=
nA
RTV
=
⎛nA⎜V⎝
⎞RT⎟⎠
Eq
7.7
The composition of a mixture of gases is th
e same throughout, so a mixture is said to
be
homogeneous
. Homogeneous mixtures are also called
solutions
. Solutions are
characterized by the
concentrations
(the number of particles per unit volume) of their
components. The molarity of a solution is th
e most common type of concentration in the
laboratory. The
molar concentration
of A, which is given the symbol [A], is defined as
the number of moles of A per liter of the solution:
[A] =
nA V
Eq 7.8
nA
is the number of moles of A in V liters of solution. [A] = 1 M is read as “the
concentration of A is 1 molar.” Substitution
of Equation 7.8 into Equation 7.7 yields the
relationship of the partial pressure of a gas to its molar concentration.
PA
= [A]RT
Eq 7.9
The total pressure of a mixture is related to the partial pressures by
Dalton’s Law of
partial pressures
:
The total pressure in a mixture of gases is the sum of the partial pressures of all gases in the mixture;
i.e.
, P
TOT
=
Σi
P, where Pi
is the partial pressure of the ii
th gas.
Thus, the total pressure in a mixture of A and B is P
total
= P
- PA
, which can be related to B
the number of moles or concentrations of the
components with the use of the ideal gas law
Ptotal
= P
+ PA
= B
nA
RTV
+
nB
RTV
= RT
(n
+ nA
)B
V
= RT ([A] + [B])
The total pressure of a mixture of non-interacting gases depends only upon the partial pressures or the temperature and concentrations of its components; it is independent of their identities.
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