-
“💡 Don’t forget to save this blog for quick revision before your exams!”
-
“Can a single substance oxidize and reduce itself? Let’s find out!”
What is Auto-Redox / Self-Redox reaction
-
Examples of Disproportionation Reactions
-
Real-life applications in Chemistry
-
Step-by-step solved problems
Auto-Redox reaction/Self-Redox reactions/ Disproportionation Reaction/Disputation Reaction
Definition
It is an important and
special type of redox reaction in which a single substance (specie) undergoes simultaneous oxidation and reduction
i.e. it occurs when a same element is both oxidized and reduced simultaneously
(i.e. in the meantime). A specie undergoing auto-redox reaction is said to be
disproportionate.
disproportionation, also
called disputation reaction,
is basically a redox reaction involving simultaneous reduction and
oxidation of atoms of the same element of a substance of
intermediate oxidation state from one oxidation state to two different
oxidation states forming two compounds, one with higher and one with lower
oxidation states. So a species is simultaneously reduced and oxidized to form
two different products.
Example
Reason
The requirement for disproportionation reaction to
occur is, the element undergoing disproportionation should exhibit minimum
three different oxidation states and the element must be less stable in a
particular oxidation state from which it can be both oxidized as well as
reduced to relatively more stable oxidation states.
Examples of Auto-Redox
Reactions
1. Decomposition or Disproportionation of potassium chlorate to potassium perchlorate and potassium chloride
2. Decomposition of
nitrogen (III) oxide into nitric oxide and nitrogen dioxide
3. Decomposition of
hydrogen peroxide into water and oxygen
Decomposition reaction of hydrogen peroxide into water and oxygen involves disproportionation of oxygen. In this auto-redox reaction, the relatively less stable oxygen of peroxide in the -1 oxidation state disproportionates into relatively more stable compounds i.e. water and dioxygen changing its oxidation state to the -2 oxidation state in water and zero oxidation state in oxygen gas at the same time.
4. Dissolution of
chlorine gas in water (Reaction of chlorine gas with water)
5. Photolysis of
Mercurous chloride into mercuric chloride and mercury
Upon UV-irradiation, Mercurous chloride or mercury(I)
chloride undergoes disproportionation. under UV light to give mercury and
mercuric chloride. The Hg22+ ion is oxidized to Hg2+ and
reduced to Hg.
6. Dissolution of
nitrogen dioxide in water
When nitrogen dioxide in which oxidation state of
nitrogen is +5 reacts with water (Ostwald process), it undergoes
disproportionation reaction resulting in the formation of both nitric acid and
nitrous acid (or nitric oxide; O.S of N = +2) wherein nitrogen has oxidation
states +5 and +3 respectively. In this reaction, nitrogen of NO2
with +4 oxidation state is simultaneously oxidized to nitric acid (+5 oxidation
state) and reduced to nitrous acid or NO (with oxidation state +3 or +2). Thus,
it is a disproportionation reaction.
7. Decomposition of Cuprous
chloride into cupric chloride and copper
Decomposition of Cuprous chloride into cupric chloride
and copper involves disproportionation of copper. When cuprous chloride in
which oxidation state of copper is +1 is heated it is decomposed and
simultaneously oxidized to copper chloride changing the oxidation state of
copper from +1 to +2 and reduced to elemental copper changing the oxidation state
of copper from +1 to 0. Thus, this is a disproportionation reaction
8. Dissolution of
metal superoxides with water
This reaction can serve as a
convenient source of oxygen in masks of self-contained breathing apparatus worn
by fire fighters. The source of oxygen is the reaction between KO2
and exhaled water vapours. The KOH so formed serves to remove CO2
from the exhaled breath.
9.
disproportionation of
Phosphorus to phosphine and hypophosphite in alkaline medium.
Phosphorus disproportionates to phosphine and
hypophosphite in alkaline medium. In this case, one P atom is reduced to -3
oxidation number (in PH3) and three P atoms get oxidized to +1 (in
NaH2PO2).
10. Auto-redox
Reactions of chlorine gas with dilute or conc Alkalis (sodium hydroxide &
lime water)
Chlorine
undergoes auto-redox reaction with water, sodium hydroxide (cold and hot) and
lime water (cold, hot and dry) in which it reduces to chloride (Cl-)
ion (in HCl or NaCl or CaCl2) as well as oxidizes itself to Cl+1
(in hypochlorite; ClO1-) or Cl+5 (in chlorate; ClO31-).
11. Cannizaro’s reaction/Auto-redox
Reactions of formaldehyde with conc. Alkalis
The self-addition oxidation
reduction and disproportionation Reaction in which two molecules of aldehyde
lacking a-hydrogen are disproportinated into
carboxylic acid (which form salt with alkali) and alcohol is known as
Cannizaro’s Reaction.
Aldehydes lacking a-hydrogen like formaldehyde and
benzaldehyde (do not show aldol condensation) undergo self-redox reaction in
presence of aqueous alkali, two molecules of such aldehydes disproportionate
and simultaneously oxidize and reduce one another into acid and alcohol
respectively.
For example
formaldehyde on heating with conc.
Solution of strong alkali like NaOH undergoes self-oxidation reduction reaction
in one molecule of formaldehyde is reduced to methanol and the other is
oxidized to formic acid that forms salt with alkali.
