Question
A solution of KMnO4 on reduction yields either a colourless solution or a brown precipitate or a green solution depending on pH of the solution. What different stages of the reduction do these represent and how are they carried out?
✓
Answer
Oxidising behaviour of KMnO4 depends on pH of the solution.
In acidic medium $\text{(pH<7)}$
$\text{MnO}_4^-+8\text{H}^++5\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{Mn}^{2+}+4\text{H}_2\text{O}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Colourless)}$
In alkaline medium $\text{pH<7}$
$\text{MnO}_4^-+\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{MnO}^{2-}_4\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Green)}$
In neutral medium $\text{(pH=7)}$
$\text{MnO}_4^-+2\text{H}_2\text{O}+3\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{MnO}_{2}+4\text{OH}^-\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Brown precipitate)}$
In acidic medium $\text{(pH<7)}$
$\text{MnO}_4^-+8\text{H}^++5\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{Mn}^{2+}+4\text{H}_2\text{O}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Colourless)}$
In alkaline medium $\text{pH<7}$
$\text{MnO}_4^-+\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{MnO}^{2-}_4\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Green)}$
In neutral medium $\text{(pH=7)}$
$\text{MnO}_4^-+2\text{H}_2\text{O}+3\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{MnO}_{2}+4\text{OH}^-\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Brown precipitate)}$
Need a full question paper?
Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.
Explore more
Similar questions
How shall you convert the following:
(i) Aniline to bromobenzene,
(ii) Chloroethane to butane.
(i) Aniline to bromobenzene,
(ii) Chloroethane to butane.
What do you understand by the glucosidic linkage?
Write structures of compounds A, B and C in each of the following reactions:
→- $ \text{C}_6\text{H}_5\text{Br}\xrightarrow{\text{Mg/dry ether}}\text{A}\xrightarrow[(b )\text{ }\text{H}_3\text{O}^+]{(a)\text{ }\text{Co}_{2\text{(g)}}}\text{B}\xrightarrow{\text{PC}l_5}\text{C}$
- $ \text{C}\text{H}_3\text{CN}\xrightarrow[(b )\text{ }\text{H}_3\text{O}^+]{(a)\text{ }\text{SnC}l_{2}/\text{HC}l}\text{A}\xrightarrow{\text{dil.NaOH}}\text{B}\xrightarrow{\text{ }\text{ }\text{ }\text{ }\text{ }\triangle\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }}\text{C}$
Compare the chemistry of actinoids with that of the lanthanoids with special reference to:
Chemical reactivity
Chemical reactivity
Explain the nucleophilic substitution reaction of aldehydes and ketones with Grignard reagent (R'-Mg-X) with chemical equations ### Write only chemical reactions to obtain 1º, 2º and 3º alcohols from aldehyde andketone compounds.
Predict the major product formed when HCl is added to isobutylene. Explain the mechanism involved.
- What is the basis of formation of the spectro-chemical series?
- Draw the structures of geometrical isomers of the following coordination complexes:
[Co(NH3)3Cl3] and [CoCl2(en)2]+.
(en = ethylenediamine and atomic number of Co is 27).
Calculate the boiling point of a 1M aqueous solution (density 1.04g mL-1) of potassium chloride (Kb for water = 0.52K kg mol-1, Atomic masses: K = 39u, Cl = 35.5u) Assume, potassium chloride is completely dissociated in solution.
What is the difference between Galvanic cell (Electrochemical cell) and Electrolytic cell.