Page 29 - Basic physical chemistry for the atmospheric sciences

Chemical equilibrium l .'i

sion for the equilibrium constant K P for the reaction in terms
off and the total pressure p of the system at equilibrium.

1 . 22. Solid ammonium mercaptan, N H 4HS(s), dissociates rapidly
at room temperature to form ammonia and hydrogen sulfide
NH4HS(s) � N H 3(g) + H2S(g)

where KP = . 08 x 1 0 -1 at 25°C. If some NH4HS(s) is placed
l
in a closed 2.00-L flask that already contains 0.300 g of
NHJ(g), what will be the total pressure in the flask after
chemical equilibrium is established?
s
1 . 23. In automobile engine , why is some NO(g) produced by the
following endothermic reaction?

With reference to Exercise l . 23, why does not most of the
l.24 .
NO(g) that is produced quickly revert to N2(g) and 0 2 (g)
when the emissions attain normal atmospheric tempera­
tures?

Notes

The phase of a substance is indicated in the parenthesis following the chemical
symbol, where "g" stands for gas, "'!'' for liquid, "s" for solid , and "aq" for aqueous
(i .e. , water solution) phase .
2 If this is not intuitively obvious , it can be proved as follows. Let the masses of the
molecules of two compounds I and 2 be m1 and m2 and their molecular weights M1
and M2, respectively. Then, by definition, I mole or compound I is M1 g of
compound I , and I mole of compound 2 is Mi g of compound 2. Let the numbers of
molecules in M1 g of compound I and M2 g of compound 2 be 11 1 and 11 2 • respectively.
Then. M1 = n1m and Mi = n2m2• Therefore, M1/M2 = n 1 m 1/n2m,. However , the ratio
1
of the molecular weights of any compounds is equal to the ratio of the masses of their
molecules, that is, Mif M2 = m / m2• It follows that 11 1 = ni. that is, the number of
molecules in I mole of compound I is equal to the number of molecules in I mole of
compound 2.
3 Prior to 1 9 82, the standard pressure was one atmosphere (I atm) , and this is still in
common use. The difference in the two definitions is not great since I atm = 1 . 0 1 3
bar. The unit f pressure i n the International System of Units (i.e., the SI system) i s
o
i
the pascal (Pa). The basic and derived units for the SI system are given n Appendix
I; for the most part, we will adhere to the SI system in this book.
4 A solution is a homogeneous mixture. For example, air is a gaseous solution of
several gases, seawater is a liquid solution of sodium chloride and other materials.
The component of a solution that is present in the greatest amount, and therefore
determines the state of matter (solid, liquid or gas) of the solution , is called the
.wli·ent; the other components are called solutes. A solution in which water is the
solvent (e.g. , seawater) is called an aqueous solution . An ideal solution is one for
which both solvent and solutes obey Raoult 's law (see Section 4.4) at all
concentrations.
� hir reactions between gases at high pressures , or for reactions in nonideal solutions,
/{.. defined hy Eq . ( 1 .6) is not strictly constant. In these cases, a thermodynamic