Oxidation Number — Rules, calculating
Redox Reactions
5
JEE Qs
8%
Hard
60
min
Master the hierarchical rules for assigning oxidation numbers, especially the exceptions for oxygen and hydrogen, and always verify the sum equals the net charge for accuracy.
🧮 Key Formulas
✅ Key Points for JEE
- 1Oxidation number of an element in its free or uncombined state is always 0 (e.g., O₂, N₂, Na).
- 2Group 1 metals always have +1 oxidation number, Group 2 metals always +2, and Fluorine always -1 in compounds.
- 3Oxygen typically has -2, but is -1 in peroxides (e.g., H₂O₂, Na₂O₂), -1/2 in superoxides (e.g., KO₂), and +2 in OF₂.
- 4Hydrogen typically has +1, but is -1 in metal hydrides (e.g., NaH, CaH₂).
- 5The algebraic sum of oxidation numbers of all atoms in any neutral compound is zero, and in a polyatomic ion, it is equal to the net charge on the ion.
- 6Fractional oxidation numbers indicate an average; the actual oxidation states of individual atoms might be integers and vary within complex structures, requiring structural analysis.
⚠️ Common Mistakes
- ✕Incorrectly assigning oxidation numbers to oxygen in peroxides/superoxides or to hydrogen in metal hydrides.
- ✕Forgetting to equate the sum of oxidation numbers to the net charge for polyatomic ions.
- ✕Confusing the average oxidation number with the actual oxidation state of an atom in a compound where multiple atoms of the same element exist in different environments (e.g., calculating for S in Na₂S₂O₃ without considering its structure).
- ✕Incorrectly identifying the maximum or minimum possible oxidation states for elements based on their group number.
📝 Practice Questions
See allQ63.Given below are two statements: Statement I: In the oxalic acid vs KMnO4 (in the presence of dil H2SO4 ) titration the solution needs to be heated initially to 60∘C, but no heating is required in Ferrous ammonium sulphate (FAS) vs KMnO4 titration (in the presence of dil H2SO4 ) Statement II: In oxalic acid vs KMnO4 titration, the initial formation of MnSO4 takes place at high temperature, which then acts as catalyst for further reaction. In the case of FAS vs KMnO4 , heating oxidizes Fe2+ into Fe3+ by oxygen of air and error may be introduced in the experiment. In the light of the above statements, choose the correct answer from the options given below (1) Both Statement I and Statement II are false (2) Both Statement I and Statement II are true (3) Statement I is false but Statement II is true (4) Statement I is true but Statement II is false 2025 (28 Jan Shift 1) JEE Main Previous Year Paper
Q69.Given below are two statements : Statement (I): In partition chromatography, stationary phase is thin film of liquid present in the inert support. Statement (II) : In paper chromatography, the material of paper acts as a stationary phase. In the light of the above statements, choose the correct answer from the options given below : (1) Statement I is true but Statement II is false (2) Statement I is false but Statement II is true (3) Both Statement I and Statement II are false (4) Both Statement I and Statement II are true
Q57.The species which does not undergo disproportionation reaction is : (1) ClO−3 (2) ClO− (3) ClO−2 (4) ClO−4
Q53.Which of the following oxidation reactions are carried out by both K2Cr2O7 and KMnO4 in acidic medium? A. Γ−→I2 B. S 2−→S C. Fe2+ →Fe3+ D. Γ−→IO3− E. S2O32−→SO42− Choose the correct answer from the options given below: (1) C, D and E Only (2) B, C and D Only (3) A, D and E Only (4) A, B and C Only
Q54.Only 2 mL of KMnO4 solution of unknown molarity is required to reach the end point of a titration of 20 mL of oxalic acid (2M) in acidic medium. The molarity of KMnO4 solution should be M.
Q58.The difference in the 'spin-only' magnetic moment values of KMnO4 and the manganese product formed during titration of KMnO4 against oxalic acid in acidic medium is _______ BM. (nearest integer)
NCERT Chapters
- Class 11 Chemistry Ch 8: Redox Reactions