Auto-redox
Some particles can be found in both columns of the redox tables.
This means that they can be oxidator as well as reductor, that they can capture as well as donate electrons.
Wether they will go one way or the other depends on the circumstances and of the presence of certain helping substances.
In general we know that a substance wil react as an oxydator when it meets a strong reductor.
and vice versa: react as e reuctor in contact with a strong oxydator.
Examples:
Tin en tin-ions can be met in three ways: Sn Sn2+ Sn4+
There you have one: Sn2+ can capture as well as donate.
The same for Manganese: Mn2+ MnO2 MnO4-
In the middle you find MnO2 with the oxidation number of Mn: +4 (Mn4+); this one can capture and donate.
Even odder it becomes when one and the same substance simultaneously is reacting as oxydator and as reductor, or:
this substance reacts with itself.
electrons are transferred internally.
Sn2+-ions can react with one another, where one ion will donate electrons (becoming Sn4+) and the other ion will capture and becomes neutral Sn).
Such particular situations we call "autoredox reactions".
Taken from a redox table:
| Oxidator |
|
Reductor |
| H2O2+ 2H+ + 2e- |
 |
2H2O |
| O2 + 2H+ + 2e- |
|
H2O2 |
The substance Hydrogen(hy)per oxyde can act as an oxydator and as a reductor.
Suppose that this (H2O2) is in an aqueous and acid solution.
It can be an oxydator: H2O2 + 2H+ + 2e- 2H2O
but also:
it can be a reductor: H2O2 O2 +2H+ + 2e-
In the table you find this oxydator above the reductor, or: the substance reacts spontaneously, is strong enough.
This means that it wan't be easy to keep hydrogen peroxyde in the bottle for a long time.
It has the tendency to be transformed (via this auto redox reaction) into water and oxygen.
Study well the following halfreactions:
| oxidators |
|
reductors |
| Cu+ + e- |
|
Cu |
| Cu2+ +2e- |
|
Cu |
| Cu2+ + e- |
|
Cu+ |
The ion Cu+ is in both columns and can participate in an autoredoxreaction.
The following reactions are applied in Lead batteries.
I. When a lead battery is delivering:
Pb(s) + SO42-
PbSO4(s) + 2e-
and
PbO2(s) + SO42-+ 4H+ + 2e-
PbSO4(s) + 2 H2O
II. When a lead battery is being charged:
PbSO4(s) + 2e-
Pb(s) + SO42-
and
PbSO4(s) + 2 H2O
PbO2(s) + SO42-+ 4H+ + 2e-