Page 16 - Basic physical chemistry for the atmospheric sciences

2 Basic physical chemistry

02(g). Some chemical reactions proceed very quickly, others very
slowly; and some never reach completion. However, what Reaction
(I. a ) does tell us is that at any given instant in time the ratio of the
l
numbers of molecules of C02(g), H20(1), C6H1206(s), and 0 2 (g) that
have reacted is 6:6: I :6.
The relative masses of the various atoms are represented by their
atomic weights (dimensionless) referenced to carbon-12 (i. e . , a carbon
atom containing six protons and six neutrons), where carbon-12 is
arbitrarily assigned an exact atomic weight of 12. Atomic weights are
listed in Appendix III. Similarly, the relative masses of molecules are
represented by their molecular weights (dimensionless), where the
molecular weight is obtained by adding together the atomic weights of
all the atoms in the molecule. For example, since the atomic weights of
hydrogen and oxygen are 1 . 008 and 1 5 . 9 99, respectively, the molecular
weight of water (H20) is (2 x 1 . 008) + 15.999= 18. 0 1 5 .
One gram-molecular weight (abbreviation mole or mo/) of any com­
pound is a mass of that compound equal to its molecular weight in
m
gra s . Thus, I mole of water is 18.015 g of water. One mole of any
compound contains the same number of molecules as one mole of any
2
other compound. The number of molecules in I mole of any com­
pound is 6.022 x 1 0 23 • which is called Avogadro' s number (NA). Since
the volume occupied by a gas depends on its temperature, pressure,
and the number of molecules in the gas, at the same temperature and
pressure I mole of the gas of any compound occupies the same volume
as I mole of the gas of any other compound. At standard temperature
and pressure (STP), which are defined as 0°C and I bar ( = HP Pa), 3
the volume occupied by I mole of any gas is about 22.4 L.
l
If we now multiply every term in the Relation (I. b ) by NA we get
6N A molecules of C0 2 (g) + 6N A molecules of H 2 0(1)�
I NA molecule of C6H 1206(s) + 6N A molecules of 02(g)
or,
6 moles of COi(g) + 6 moles of H20(1)�
I mole of C6H 1206(s) + 6 moles of 0 2(g) (I.le)

Relations ( I . I b) and ( I . le) demonstrate how we can move direct! y
from a balanced chemical equation, such as Reaction (I. l a ), to a
statement about the relative numbers of molecules (I. I b) or the rela­
(
tive numbers of moles I . le) involved in the reaction.
Exe'rcise 1 .1. An important chemical reaction in atmospheric, earth,