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CBSE Class 11 · Chemistry

Redox Reactions

Official NCERT chapter from Chemistry Part I–II (book code kech1). ExamMaster notes are original teaching at CBSE Class 11 depth.

This lesson follows the official chapter “Redox Reactions” in Chemistry Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 11
  • Medium level
  • 8 concepts

1Classical Idea of Redox Reactions-Oxidation and Reduction Reactions

Classical: oxidation is oxygen-gain or hydrogen-loss; reduction is the reverse — as the older school names. A redox pair runs both. Rusting-iron as oxygen-gain is the classical look. Classical is a first language, not the only one.

Calling every colour-change “oxidation” without a gain/loss is a miss.

Figure. Classically, adding oxygen oxidises magnesium to MgO. Taking oxygen off copper(II) oxide reduces it to copper. The two changes are a pair, not two separate stories.

How it works

  1. Name O-gain / H-loss or the reverseClassical.
  2. Keep both sides — one oxidised, one reducedRedox.
  3. Land on electron-talk in the next headingThe newer language.

2Redox Reactions in Terms of Electron Transfer Reactions

Electron-transfer: oxidation is loss of electrons; reduction is gain. OIL RIG as a memory if taught. 2Na + Cl2 → 2NaCl: Na loses, Cl gains. Transfer-talk matches the same reaction the classical names already held.

A lone “Na loses” with no who-gains is half a redox.

Figure. Oxidation is electron loss and reduction is electron gain. Zinc frees two electrons; Cu²⁺ takes those two electrons. One transfer, two names.

How it works

  1. Name who loses e− (oxidised)The donor.
  2. Name who gains e− (reduced)The acceptor.
  3. Keep the pairTransfer.

3Oxidation Number

Oxidation number is a bookkeeping charge as taught: H is +1 (usually), O is −2 (usually), an element is 0. In H2O, H is +1, O is −2. A number-change tells oxidised versus reduced. ON is a rule-set, not a measured ion-charge always.

Giving O as −1 in H2O because “minus looks small” is a miss.

Figure. In H₂SO₄ the house rules give H as +1 and O as −2. Two hydrogens contribute +2 and four oxygens contribute −8, so sulphur must be +6 for the molecule to sum to zero.

How it works

  1. Assign ON by the taught rulesThe bookkeeping.
  2. See which ON rose (oxidised) and which fell (reduced)The redox read.
  3. Keep element-free as 0The zero.

ON in H2O

Oxidation numbers in H2O (H=+1, O=−2 as usual).

  • H (two of them)each +1
  • O−2
  • Check sum2(+1)+(−2)=0

Pro tip. Usual H and O rules; molecule sums to 0.

4Redox Reactions and Electrode Processes

Electrode processes: oxidation at the anode, reduction at the cathode as taught. A cell is two half-writes plus a circuit. Electrode is a where-the-e−-leave/enter story, not a battery-ad.

Calling both ends “cathode” because they look metallic is a miss.

Figure. Electrode processes split the same Zn/Cu transfer across two half-cells. Electrons travel the top wire from zinc to copper. The dashed salt bridge is an ion path, not an electron path. Signs are for spontaneous discharge.

How it works

  1. Name anode = oxidation, cathode = reductionThe map.
  2. Write a half-reaction as taughtThe process.
  3. Keep a circuit if a cell was the caseElectrons have a path.

5A definition is a test you can run

Redox-word is a test: O-gain, e−-loss, an ON-rise, or an anode. If you only say “change”, you have a heading.

A rust-sticker is not the test.

Figure. The definition is a test you can run on the oxidation number: if it rises the species is oxidised; if it falls the species is reduced. The picture is the test, not a decoration.

How it works

  1. Name classical, e−, ON, or electrodeThe object.
  2. Give the school sentenceThe test.
  3. Then the word has contentThe definition ran.

6Name the given before the unknown

The given is a formula or a half-write. The unknown is who is oxidised or the ON. Copy H2O before you treat O as −1.

Using a peroxide-rule on ordinary water is a silent swap.

Figure. Name the given rules before the unknown atom. Oxygen is −2 and hydrogen is +1 in H₂SO₄; only then does sulphur come out +6 so the sum is zero.

How it works

  1. Copy the formula or the cell-endThe given.
  2. Name ON, e−, or the electrode-wordThe unknown.
  3. Then assignGiven first.

7One worked case is enough at this class

One worked case is enough: 2Mg + O2 → 2MgO. Mg from 0 to +2 (oxidised); O from 0 to −2 (reduced). Do not stack four more combustions. The case teaches the ON-read.

A poster of ten rusts is not more science.

Figure. One school case is enough: magnesium loses two electrons as its number goes 0 to +2, and oxygen gains two as its number goes 0 to −2. The electron counts already match.

How it works

  1. Take one balanced redoxThe case.
  2. Assign ON before and afterThe read.
  3. Stop — one case taught the moveThis class.

8A miss: swapping the school name for the picture

A miss: swapping the school name for a picture. “Oxidation” is e−-loss or ON-rise as taught — a flame-drawing is a setting. Keep the name that the test can run.

A pretty rust-photo without ON or e− is a mute picture.

Figure. The miss is using the classroom word oxidation as if it always meant add oxygen. Sodium becoming Na⁺ is oxidation by electron loss with no oxygen in the picture.

How it works

  1. Keep the taught name (ON, e−, or classical)The science.
  2. Use a picture only as a settingHonest.
  3. Refuse a swap of name for glowThe miss named.
Oxidation number of O in H2O is usually
  1. −2
  2. +2
  3. 0

Usual oxygen rule.

Notes

  • Mapped to the official NCERT chapter “Redox Reactions”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • ON(H) = +1 usual; ON(O) = −2 usual
  • element ON = 0
  • anode = ox, cathode = red

Recap

Hold these pegs from the official chapter “Redox Reactions”. The wording is ExamMaster’s teaching, not a textbook recap.

Classical Idea of Redox Reactions-Oxidation and Reduction Reactions
Classical: oxidation is oxygen-gain or hydrogen-loss; reduction is the reverse — as the older school names.
Redox Reactions in Terms of Electron Transfer Reactions
Electron-transfer: oxidation is loss of electrons; reduction is gain.
Oxidation Number
Oxidation number is a bookkeeping charge as taught: H is +1 (usually), O is −2 (usually), an element is 0.
Redox Reactions and Electrode Processes
Electrode processes: oxidation at the anode, reduction at the cathode as taught.
A definition is a test you can run
Redox-word is a test: O-gain, e−-loss, an ON-rise, or an anode.
Name the given before the unknown
The given is a formula or a half-write. The unknown is who is oxidised or the ON. Copy H2O before you treat O as −1.

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