Page 150 - Basic physical chemistry for the atmospheric sciences

1 3 6 Basic physical chemistry

6 . 2 9. What is the normality of 1 . 0 L of a solution that contains
35.6 g of HN03 if the only chemical reaction is ionization?

Notes
The reader might wonder why a reaction such as (6.3), which does not involve
oxygen, is called an oxidation-reduction reaction. The reason is as follows.
Originally the term "oxidation" was applied to reactions in which a substance
combines with oxygen , for example, the oxidation of copper in air
2Cu(s) + 02(g) _,. 2Cu0(s)
This reaction involves each copper atom losing two electrons, and each oxygen atom
gaining two electrons
+
2Cu(s)_,. 2Cu2 ( aq) + 4e -
02(g) + 4e - __,. 202 - (aq)
2
Net: 2Cu(s) + Oi(g) _,. 2Cu 2 + (aq) + 20 - (aq)_,. 2Cu0(s)
Thus, by analogy , all chemical reactions involving simultaneous electron loss and
electron gain came to be called oxidation-reduction reactions.
2 From the viewpoint of atomic structure, every atom consists of a positive nucleus
surrounded by negative electrons. In forming chemical bonds, atoms donate , receive,
or share electrons. The number of electrons of an atom that is involved in this way in
forming bonds with other atoms is the oxidation number of the atom. A positive
oxidation number indicates that the atom has donated one or more electrons (e.g. ,
+ I for the hydrogen atom in a molecule), and a negative oxidation number indicates
that the atom has received one or more electrons (e.g., - 2 for the oxygen atom in a
molecule).
3 In redox reactions it is better to indicate the aqueous proton by H ( aq) rather than
+
by H,O + (aq), since it decreases the number of water molecules that must be written .
4 For example, in the half-cell on the right side of Figure 6. 1 , Cu 2 + (aq) ions are
released from the copper electrode into the solution. These attract SOi - (aq) ions
with which they combine: Cu2 + (aq) + S o� - (aq)_,. CuS04(aq). This would leave the
solution with a net positive charge if it were not for negative ions drifting into this
.
half-cell from the salt bridge. In the half-cell on the left side of Figure 6 1 , Ag + (aq)
ions from the solution are deposited onto the silver electrode . This leaves N 0 3 ( aq)
ions in the vicinity of this electrode ; these attract Ag + (aq) ions with which they
combine: NO ) (aq) + A g + (aq)-> A gN03(aq). Again, the cell is maintained electrically
neutral by positive ions drifting into it from the salt bridge.
5 A I M solution is in its standard state only if it behaves ideally (see Section 4.4), but
this complication need not concern us here.
23
6 The number of electrons in I mole of electrons is Avogadro's number (6.0229 x 10 ) ,
o
and the charge o n one electron is 1 . 602 1 x 1 0 - 19 coulombs. Therefore, the charge n
o
I mole f electrons is (6.0229 x 1023) x 1 .602 1 x 1 0 - 19) r %,489 coulombs; this is the
o
(
Faraday constant (F).
7 The following is an alternative explanation for the fact that type (a) combinations are
significant (i.e . , spontaneous) and type (b) combinations are not . In type (a)
combinations the better oxidant serves as the oxidant, and the better reductant
serves as the reductant. However, in type (b) combinations the better oxidant is
forced into the role of a reductant and the better reductant into the role of the
oxidant.
8 Of course, c1 - ( aq) will react with Fe3 + (aq) until a small amount of Cl2(g) is formed
to bring the reaction to equilibrium. Therefore, the phrase "Cl ( aq) will not react to
-
2
3
a significant extent with Fe + (aq)" really means "the ratio of [Cl(aq)] to [Fe + (aq)) is
very large at equilibrium."