PYQ Vault

MHT-CET Chemistry · Formula sheet

Redox Reactions formulas

5 formulas, 1 reference table and 8 common traps for MHT-CET Chemistry Redox Reactions, grouped by subtopic.

Full notes with worked examples

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

Learn this subtopic in the notes

Finding One Atom's Oxidation Number

Sum rule

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

When the Formula Is Not Enough: Peroxides, Chains and Averages

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

The Change in Oxidation Number Across a Reaction

Change in oxidation number

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

Common traps

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.

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.

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.

Redox Reactions: Spotting What Is Oxidised, Counting Electrons, and Balancing

Learn this subtopic in the notes

Electrons Transferred and the Balancing Coefficients

Electron balance

nlost×(coefficient)=ngained×(coefficient)n_{\text{lost}} \times (\text{coefficient}) = n_{\text{gained}} \times (\text{coefficient})

Common traps

Picking hydrogen or oxygen because they appear on both sides

H (+1) and O (−2) are spectators in almost every aqueous redox reaction. Look for the element whose number actually changes.

Counting electrons per ion instead of per atom

Cr₂O₇²⁻ → 2Cr³⁺ is 3 electrons per Cr but 6 per dichromate ion. Read which the question asks for.

Forgetting that N₂ has two atoms

In CuO + NH₃ → N₂, one N₂ needs 2 N at 3 electrons each — 6 electrons, so 3 Cu (2 each) balance it, not 6.

Oxidising and Reducing Agents, and Acidic, Basic and Neutral Oxides

Learn this subtopic in the notes

Oxidising and Reducing Agents and Their Strength

Can A reduce B?

A reduces B if EB∘>EAn+/A∘\text{A reduces B if } E^\circ_{\text{B}} > E^\circ_{\text{A}^{n+}/\text{A}}

Acidic, Basic, Amphoteric and Neutral Oxides

OxideCharacterWhy
CaOBasicMetal oxide; CaO + 2HCl → CaCl₂ + H₂OQ
N₂O₅AcidicAnhydride of HNO₃Q
SO₃AcidicAnhydride of H₂SO₄
Al₂O₃AmphotericDissolves in HCl and in NaOH
CO, NO, N₂ONeutralNo acid or base forms with water
The trap: N₂O₅ is acidic, but N₂O and NO are neutral — the oxidation state decides.
Higher oxidation state of a non-metal → more acidic oxide.

Common traps

Confusing the strongest oxidising agent with the weakest reducing agent

F₂ is the strongest oxidising agent; its partner F⁻ is the weakest reducing agent. The question may offer both — read which one it asks for.

More MHT-CET Chemistry formula sheets