Chemistry - A Molecular Science

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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State

University