Page 82 - [B._MURPHY,_C._MURPHY,_B._HATHAWAY]_A_working_meth

66 Chapter 6
Table 6.1 (contd)

4. Hydrogen, ls', usually has an oxidation number of I (sl --+ so). In metal
hydrides, such as NaH, hydrogen has an oxidation number of -I.
5. The sum of the oxidation numbers of all the atoms is equal to the charge
on the anion, cation, compound or complex. For example in the
tetrafluoroborate anion, BF4-, the sum of the oxidation numbers must
be equal to -1, whereas in the complex [Fe(OH&]C13 the sum of
the oxidation numbers must be equal to zero, i.e. [Bir1F~']- and
[Fe'"(O-'1H~)6] Cly'
~ ~~ ~~
Examples of the calculation of oxidation numbers: What is the
electron configuration of the free element marked with an asterisk (*),
and of the element in the appropriate oxidation state in the given
anion, cation, compound or complex in:
H2S*04; (b) KAl*C14; (c) Cr;O?-; (a) N*&+;
[co* (NH3)61Cl3
S: [Ne]3s23p4; 2(I) + x + 4( - 11) = 0; x = 6; Sv' = [N.]s:p:.
Al: [Ne]3s23p'; (I) + x + 4( - I) = 0; x = 3; Al"' = pels p .
Cr: [Ar]4s23d4, but is more correctly written as [Ar]4s*3d5 due to
the extra stability of the half-filled 3d sublevel;
2x + 7( - 11) = -2; x = 6; CrV' = [Ar].
N: [He]2s22p3;x + 4(I) = 1;x = -3;N-'I' = [Ne]sopo.
[co*(NH3)6]c13: first separate into cation and anion, i.e.
[Co*(NH3)6]3+ and 3C1- respectively. NH3 is a neutral ligand
and therefore has an oxidation number of 0; Co: [Ar]4s23d7;
x + 6(0) = 3; x = 3; but electrons are removed from the 4s level
before the 3d, therefore Co"' = [Ar]3d6.
Oxidation is the loss of electrons and reduction is the gain of
electrons ('OILRIG'). A more useful definition of reduction is (as its
name suggests) a decrease in the oxidation number of a species, and
consequently, oxidation is an increase in the oxidation number.

e.g. MnV"04- + 5e --+ Mn"
VII: [Ar]3do 11: [Ar]3d
(7 --+ 2 . . . decrease, therefore reduction!)
Fe" -+ Fe"' + e
11: [Ar]3& 111: [Ar]38
(2 + 3 . . . increase, therefore oxidation)

An oxidising agent is a species which causes some other species to be
oxidised, while itself being reduced. A reducing agent is a species which
causes some other species to be reduced, the reducing agent being