Page 19 - Basic physical chemistry for the atmospheric sciences

Chemical equilibrium 5

[N2(g)] = 0 . 18 M
At 25°C the value of Kc for Reaction (l.7) i s only l x 1 0 -3o! This
implies that the equilibrium concentration of NO(g) is very low at
normal temperatures and that the equilibrium "lies to the left" of
Reaction (l .7), favoring the reactants. Hence, in the troposphere,
(
negligible quantities of NO(g) are produced by Reaction 1 .7).
In the case of chemical reactions involving only gases, it is often
more convenient to express the equilibrium constant for the reaction
in terms of the partial pressures of the reactants and products instead
of their molarities. However, before doing this we must review the
ideal gas equation.
Laboratory experiments show that for a wide range of conditions
the pressure (p), volume (V) and temperature (D of all gases follow
closely the same relationship, which is called the ideal gas equation.
I
In SI units (see Appendix ) , the ideal gas equation can be written in
the following forms. For mass m (in kilograms) of a gas
l
p V = mRT ( . 8a)
where p is in pascal s , V in cubic meters, T in K (K =
°C + 273. 1 5 = °C + 273), and R is the gas constant for I kg of a gas. The
value of R depends on the number of molecules in I kg of the ga , and
s
therefore varies from one gas to another. Since m/V = p, where p is the
density of the gas,
p=RpT ( I .Sb)

For I kg of gas (m = I), Eq. ( I .Sa) becomes

pa = R T (l 8c)
.
where a is the specific volume of the gas (i. e . , the volume occupied by
1 kg of the gas). One mole of any gas contains the same number of
molecules (NA)· Therefore, the gas constant for I mole is the same for
all gases and is called the universal gas constant R*(8. 3 1 4 3 J deg-1
1
mol- ) . Therefore,
p V = nR*T (l . 8d)
where n is the number of moles of the gas, which is given by

I O OOm
--
n=
M