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MHT-CET Chemistry · Redox Reactions

Oxidation Number: The Rules, the Structural Exceptions, and the Change Across a Reaction

The oxidation number of an atom is the charge it would carry if every bond were fully ionic; it is found from a few fixed values and the rule that the numbers in a species add up to its charge.

All Redox Reactions notesMHT-CET Chemistry strategy

Why this matters

26 PYQs, the heart of the chapter, and only one HARD — the tetrathionate structure. Fifteen ask for one atom's number in a compound or ion, five need the structure (a peroxide oxygen, an S–S chain, an average value), and six ask how the number changes across a reaction. Three cards.

Concept 1 of 3: Finding One Atom's Oxidation Number

Write down the atoms whose numbers never change, call the unknown x, and make the total equal the charge on the species — zero for a compound, the ion's charge for an ion. Every one of the fifteen questions of this kind is that one line of algebra.

Definition

  • Fixed values: F −1 always; O −2 (except peroxides −1, OF₂ +2); H +1 with non-metals (−1 in metal hydrides); group 1 metals +1, group 2 metals +2; an element on its own 0.
  • Sum rule: the numbers add up to 0 in a neutral compound and to the charge in an ion.
  • A covalent compound works the same way: in methanal (CH₂O) carbon is 0; in oxalate (C₂O₄²⁻) each carbon is +3.

Sum rule

∑(oxidation numbers)=charge on the species\sum (\text{oxidation numbers}) = \text{charge on the species}

Worked example

Find the oxidation number of P in Ca₃(PO₄)₂ and of V in V₂O₇⁴⁻.
Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 10th May Shift 1 · Q68Moderate

Example 1 · Redox Reactions · Oxidation Number Calculation and Determination

What is the oxidation number of carbon in K2C2O4\text{K}_2\text{C}_2\text{O}_4?

Treating an ion as neutral

In MnO₄⁻ the total is −1, not 0: x−8=−1x - 8 = -1 gives +7. Setting the sum to zero gives +8, which is not an option for Mn.

Giving oxygen −2 next to fluorine

Fluorine is always −1, so in OF₂ oxygen is +2 — the one compound where oxygen is positive.
Drill 14 more on finding one atom's oxidation number

Concept 2 of 3: When the Formula Is Not Enough: Peroxides, Chains and Averages

The sum rule gives the AVERAGE over atoms of one element. When those atoms sit in different places — a peroxide oxygen next to ordinary ones, sulphurs bonded to oxygen and sulphurs bonded only to sulphur — the average hides the real values, and you need the structure to split it.

Definition

  • Peroxide linkage (–O–O–): each of those oxygens is −1. H₂SO₅ has one peroxide pair and three ordinary O, so S is +6, not +8.
  • S–S chain: a sulphur bonded only to sulphur is 0. In tetrathionate S₄O₆²⁻ the end sulphurs are +5 and the two middle ones 0 — average 2.5.
  • Average (fractional) values: Fe₃O₄ (Fe 8/3), Mn₃O₄, Pb₃O₄ are mixed-state oxides.
  • Highest in a series: count the oxygens — HClO +1, HClO₂ +3, HClO₃ +5, HClO₄ +7; KIO₄ (+7) beats KIO₃ and IF₅ (+5).

Peroxide acid

H2SO5: 2(+1)+x+2(−1)+3(−2)=0⇒x=+6\mathrm{H_2SO_5}:\ 2(+1) + x + 2(-1) + 3(-2) = 0 \Rightarrow x = +6

Worked example

Sulphur in S₄O₆²⁻ averages 2.5. Why do the question's four numbered sulphurs read +5, 0, 0, +5?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 11th May Shift 2 · Q51Moderate

Example 2 · Redox Reactions · Oxidation Number Calculation and Determination

What is the oxidation number of sulfur in H2SO5\text{H}_2\text{SO}_5?

Reading +8 for sulphur

No element in group 16 exceeds +6. If the sum rule gives +8 for S in H₂SO₅, a peroxide oxygen has been counted as −2.
Drill 4 more on when the formula is not enough: peroxides, chains and averages

Concept 3 of 3: The Change in Oxidation Number Across a Reaction

Find the atom's number on the left and on the right; the difference is the change, and its direction tells you oxidation (up) or reduction (down). If the number is the same on both sides, that atom is neither oxidised nor reduced — even when its formula changes.

Definition

  • Up = oxidation, down = reduction; the size of the change is the electrons per atom.
  • Common pairs: Cr₂O₇²⁻ → Cr³⁺ +6 → +3; NO₃⁻ → NH₄⁺ +5 → −3; H₂S → S −2 → 0; KMnO₄ → MnO₂ +7 → +4; ClO₃⁻ → Cl⁻ (in ICl) +5 → −1.
  • No change: CrO₄²⁻ ⇌ Cr₂O₇²⁻ keeps Cr at +6 — an acid–base equilibrium, not redox.

Change in oxidation number

Δ=ONproduct−ONreactant(Δ>0: oxidation)\Delta = \text{ON}_{\text{product}} - \text{ON}_{\text{reactant}}\quad(\Delta > 0:\ \text{oxidation})

Worked example

In I₂ + KClO₃ → ICl + KIO₃, by how much does chlorine's oxidation number change?
Practice this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 12th May Shift 2 · Q52Easy

Example 3 · Redox Reactions · Oxidation Number Calculation and Determination

What is the change in oxidation number of Cr in the following redox reaction? 3H2O2(aq)+Cr2O72−(aq)+8H+(aq)→3O2(g)+2Cr3++7H2O3H_2O_2(aq) + Cr_2O_7^{2-}(aq) + 8H^+(aq) \rightarrow 3O_2(g) + 2Cr^{3+} + 7H_2O

Calling chromate ⇌ dichromate a redox reaction

The formula changes and so does the colour, but Cr stays +6. It is set every few years precisely because it looks like redox.
Drill 5 more on the change in oxidation number across a reaction

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (3)

Watch out for (4)

Test yourself on Redox Reactions

15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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