mass
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
P4. OXIDATION & REDUCTION: REDOX PROCESSES
P4.3 Oxidation Number, Redox Reactions & Redox Equations
P4.31 Limitations of electron transfer in assigning redox
Appearing first in 1928, "REDOX" is a portmanteau of the terms "reduction" and "oxidation".
As we have seen in the previous section, identifying separately reduction and oxidation processes is straightforward where simple monatomic ions are involved,
However, there are plenty of redox processes where identifying all of the four important chemical forms is less obvious. Take the reaction of concentrated nitric acid with copper metal.
Cu(s) - 2e =
2+
Cu
(aq)
It is not difficult in assigning oxidation to the loss of electrons from copper atoms forming copper(II) ions.
,
Far less evident is that the production of brown fumes of nitrogen dioxide,
NO
2
from not obviously
colourless nitrate ions,
,
NO
3
1-
turns out to be the reduction process.
And not all redox processes are easily explained in terms of electron transfer ideas propagated since the early 1900s, however. Take the reaction of carbon dioxide with hydrogen:
There is no change in the number of valence electrons on any of the atoms as is evident from inspection of their Lewis structures:
C
_
_
_
O
Consequently, the oxidation number concept was developed as a logical extension of the existing ideas, that would cope not only with redox reactions where, in reality, electrons are not actually gained or lost, but also help to deal easily with the arithmetic of electron transfer occurring in the vast majority of redox reactions.
Where oxy-anions are involved in redox processes, for example, the chemical equations that accurately describe the reactions are often too cumbersome to be memorized. Consider again the reaction of copper metal with nitric acid: at least two distinct chemical equations are required depending on whether the concentrated acid (about 70 % w/w) or a rather more dilute solution is used.
Finally, consider the oxidation of aqueous hydrogen peroxide by an acidified solution containing permanganate [aka mangante(VII)] ions. Below are just a few of the equations which can be written that account for the four redox forms participating.
Trial and error hardly provides an adequate method for constructing equations such as these.