XI Physics Textbook Federal Board

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XI MCQs on Stoichiometry (Fundamental Concepts of Chemistry)

 

 XI Balancing of Redox Reactions by Ion Electron Method

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 BIOLOGY MDCAT KIPPS NOTES

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XII Chemistry Model Test Questions Chapter # 5 Hydrocarbons (Alkanes, alkenes, Alkynes, benzene and phenols)

 XII Chemistry Model Test Questions Chapter # 5 Hydrocarbons (Alkanes, alkenes, Alkynes, benzene and phenols) 

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XII Alkyl Halides and Amines

 XII Alkyl Halides and Amines

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Auto-Redox reaction/Self-Redox reactions/ Disproportionation Reaction/Disputation Reaction

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; ClO^1-) or Cl⁺⁵ (in chlorate; ClO₃^1-).

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.

                                 

XI Chemistry Chapter # 9 Chemical Kinetics

 

XI Chemistry Chapter # 9 Chemical Kinetics

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