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NDA Chemistry · Chemical Reactions

Redox — Oxidation, Reduction and Reducing Agents

Oxidation is loss of electrons (oxidation number goes up); reduction is gain of electrons (oxidation number goes down); the two always happen together in a redox reaction.

All Chemical Reactions notesNDA Chemistry strategy

Why this matters

The biggest and HARDEST subtopic in the chapter — 10 PYQs and the source of almost every HARD question. The bank tests whether you can ASSIGN oxidation numbers, name the oxidising and reducing agent, order reducing power from the activity series, and spot the one reaction that is NOT redox. Master the oxidation-number rules and these marks are reliable.

Concept 1 of 4

Assigning oxidation numbers

Intuition

An oxidation number is the charge an atom WOULD have if every bond were ionic. Fix the easy atoms (H is +1, O is −2, free elements are 0), then make the numbers add to the overall charge to find the unknown. Tracking how this number changes tells you instantly what is oxidised and what is reduced.

Definition

The standard rules, applied in order:

  • A free element (Fe, O₂, H₂, Cl₂) has oxidation number 0.
  • Hydrogen is +1 (except −1 in metal hydrides like NaH); oxygen is −2 (except −1 in peroxides).
  • A monatomic ion's oxidation number equals its charge (Fe³⁺ is +3).
  • The oxidation numbers in a neutral molecule sum to 0; in an ion they sum to the ion charge.
  • Oxidation = increase in oxidation number (loss of electrons); reduction = decrease (gain of electrons).
  • A compound can hold the same element in two oxidation states — Pb₃O₄ has Pb²⁺ and Pb⁴⁺; Fe₃O₄ has Fe²⁺ and Fe³⁺; Mn₃O₄ has Mn²⁺ and Mn³⁺. But Fe₂O₃ has only Fe³⁺ (uniform).

Oxidation-number bookkeeping

∑(oxidation numbers)=net charge;H=+1, O=−2, element=0\sum (\text{oxidation numbers}) = \text{net charge}; \qquad \text{H} = +1,\ \text{O} = -2,\ \text{element} = 0

Worked example

In the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, find the oxidation number of carbon before and after, and state what happens to carbon and hydrogen.
  1. In CH₄: H is +1 (four of them = +4 total), so C must be −4 for the molecule to be neutral.
  2. In CO₂: O is −2 (two of them = −4), so C must be +4.
  3. Carbon goes −4 → +4: its oxidation number INCREASES, so carbon is oxidised.
  4. Hydrogen is +1 in CH₄ and +1 in H₂O — unchanged, so hydrogen is neither oxidised nor reduced.
Answer:Carbon goes from −4 to +4 (oxidised); hydrogen stays +1 (unchanged). Only 'carbon is oxidised' is correct.
Practice this conceptself-check · 4 quick reps

Try it yourself

What is the oxidation number of sulphur in sulphuric acid, H₂SO₄?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Oxidation number of a free element such as O₂ or Fe?
  2. 2.
    Oxidation number of carbon in CO₂?
  3. 3.
    Which compound has iron in only ONE oxidation state: Fe₂O₃ or Fe₃O₄?
  4. 4.
    An increase in oxidation number means oxidation or reduction?

From the bank · past-year question

Example 1Chemical ReactionsHARD
Consider the following reaction: CH4+2O2⟶CO2+2H2O\text{CH}_4 + 2\text{O}_2 \longrightarrow \text{CO}_2 + 2\text{H}_2\text{O} Which of the following about the reaction given above is/are correct? 1. Carbon is oxidized. 2. Hydrogen is oxidized. 3. Hydrogen is reduced. 4. Carbon is reduced. Select the correct answer using the code given below:

[Q140 · Sep · 2017]

Fe₂O₃ is uniform; Fe₃O₄ is mixed

Fe₂O₃ contains only Fe³⁺ (one oxidation state), but Fe₃O₄ contains BOTH Fe²⁺ and Fe³⁺. The same trap applies to Pb₃O₄ (Pb²⁺ + Pb⁴⁺) and Mn₃O₄ (Mn²⁺ + Mn³⁺).

Unchanged oxidation number = neither oxidised nor reduced

In CH₄ combustion, hydrogen stays +1 throughout, so it is NEITHER oxidised nor reduced. Only carbon (−4 → +4) changes. Do not assume every atom in a redox reaction is itself oxidised or reduced.
Drill 2 more on assigning oxidation numbers

Concept 2 of 4

Defining oxidation and reduction

Intuition

There are four equivalent ways to spot oxidation: loss of electrons, increase in oxidation number, gain of oxygen, or loss of hydrogen. Reduction is the exact opposite of each. Memorise the four pairs and the definition questions are automatic.

Definition

Oxidation and reduction, four equivalent definitions each:

  • Oxidation = loss of electrons = increase in oxidation number = gain of oxygen = loss of hydrogen.
  • Reduction = gain of electrons = decrease in oxidation number = loss of oxygen = gain of hydrogen.
  • A memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
  • So 'loses hydrogen → reduced' is wrong — losing hydrogen is OXIDATION.
Change to a substanceOxidation or reduction?
Loses electronsOxidation
Gains electronsReduction
Gains oxygenOxidation
Loses oxygenReduction
Loses hydrogenOxidation
Losing hydrogen is OXIDATION, not reduction — this is the bank's favourite false statement.
Gains hydrogenReduction
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Loss of electrons is oxidation or reduction?
  2. 2.
    Gain of oxygen is oxidation or reduction?
  3. 3.
    A substance that LOSES hydrogen has been oxidised or reduced?
  4. 4.
    Gain of hydrogen corresponds to which process?

From the bank · past-year question

Example 2Chemical ReactionsMODERATE
Which one of the following statements regarding oxidation and reduction reactions is NOT correct?

[Q83 · Apr · 2025]

'Loses hydrogen → reduced' is false

Loss of hydrogen is OXIDATION, not reduction. The four markers of reduction are: gains electrons, loses oxygen, gains hydrogen, oxidation number falls. Any statement pairing 'loses hydrogen' with 'reduced' is incorrect.

Concept 3 of 4

Oxidising and reducing agents

Intuition

The reducing agent is the one that GETS oxidised (it gives electrons away, reducing the other species). The oxidising agent is the one that GETS reduced (it takes electrons). It feels backwards but it's consistent: the agent does the opposite to itself than its name suggests.

Definition

How to name the agents:

  • Reducing agent — the species that is itself oxidised (loses electrons / oxidation number rises). It REDUCES the other reactant.
  • Oxidising agent — the species that is itself reduced (gains electrons / oxidation number falls). It OXIDISES the other reactant.
  • In Zn + CuSO₄ → ZnSO₄ + Cu: Zn (0 → +2) is oxidised, so Zn is the reducing agent; Cu²⁺ (+2 → 0) is reduced, so CuSO₄ is the oxidising agent.
  • In N₂H₄ + 2H₂O₂ → N₂ + 4H₂O: nitrogen goes −2 → 0 (oxidised), so N₂H₄ is the reducing agent.
  • In a halogen displacement Br₂ + 2I⁻ → 2Br⁻ + I₂, Br₂ gains electrons (reduced), so Br₂ is the oxidising agent — calling it the reductant is wrong.

The agent does the opposite to itself

Reducing agent⇒itself oxidised;Oxidising agent⇒itself reduced\text{Reducing agent} \Rightarrow \text{itself oxidised}; \qquad \text{Oxidising agent} \Rightarrow \text{itself reduced}

Worked example

In Zn + CuSO₄ → ZnSO₄ + Cu, identify the oxidising agent and the reducing agent.
  1. Zinc goes from 0 (free metal) to +2 in ZnSO₄ — its oxidation number rises, so Zn is oxidised.
  2. Copper goes from +2 in CuSO₄ to 0 as Cu metal — its oxidation number falls, so Cu²⁺ is reduced.
  3. The species oxidised (Zn) is the reducing agent; the species reduced (CuSO₄) is the oxidising agent.
Answer:Reducing agent = Zn; oxidising agent = CuSO₄ (the Cu²⁺ ion).
Practice this conceptself-check · 4 quick reps

Try it yourself

In Br₂ + 2I⁻ → 2Br⁻ + I₂, which species is the oxidising agent?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    The reducing agent is the species that is itself ...?
  2. 2.
    In Zn + CuSO₄ → ZnSO₄ + Cu, name the reducing agent.
  3. 3.
    In N₂H₄ + 2H₂O₂ → N₂ + 4H₂O, which is the reducing agent?
  4. 4.
    Is Br₂ in Br₂ + 2I⁻ → 2Br⁻ + I₂ an oxidising or reducing agent?

From the bank · past-year question

Example 3Chemical ReactionsMODERATE
In the following redox reaction, which compound is acting as a reducing agent ? N2H4+2H2O2→N2+4H2O\text{N}_2\text{H}_4 + 2\text{H}_2\text{O}_2 \to \text{N}_2 + 4\text{H}_2\text{O}

[Q104 · Sep · 2025]

The reducing agent is the one OXIDISED

The naming feels reversed: the REDUCING agent is the species that itself gets OXIDISED (it donates electrons). Mixing this up flips every answer. Remember: the agent does to itself the opposite of its name.

A halogen in a displacement is the OXIDISING agent

In Br₂ + 2I⁻ → 2Br⁻ + I₂, Br₂ gains electrons (0 → −1), so it is REDUCED and is the oxidising agent — not the reductant. Calling Br₂ the reducing agent is a classic false statement.

Concept 4 of 4

Spotting a redox reaction and oxidation in daily life

Intuition

A reaction is redox only if some oxidation number actually CHANGES. If atoms only swap partners with no change in oxidation state (hydrolysis, precipitation, neutralisation), it is NOT redox. Several everyday processes — rusting, burning, food spoiling — are slow oxidations.

Definition

Identifying redox and the daily-life examples:

  • A reaction is redox only if at least one element changes oxidation number. Combination of elements (2Mg + O₂ → 2MgO) and metal displacements (Cu + Zn salt) ARE redox.
  • Not redox: double displacement / hydrolysis / precipitation where ions just swap with no oxidation-state change — e.g. AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl is hydrolysis, not redox.
  • Electron-releasing tendency (reducing power) follows the activity series: Zn > Cu > Ag — a more reactive metal releases electrons more readily.
  • Oxidation in daily life: rusting of iron, burning of fuel, rancidity of oils and fats, and browning of cut fruit are all oxidation reactions.
Reaction or processRedox? / Note
2Mg + O₂ → 2MgORedox — Mg oxidised (0 → +2), O reduced
Cu + Zn-salt displacementRedox — electron transfer between metals
AlCl₃ + 3H₂O → Al(OH)₃ + 3HClNOT redox — hydrolysis, no oxidation-state change
Hydrolysis/double displacement with no oxidation-number change is NOT a redox reaction.
Rusting of ironOxidation — Fe → hydrated Fe³⁺ oxide
Burning of fuelOxidation — carbon/hydrogen oxidised
Rancidity of oils and fatsOxidation — fatty acids oxidise
Browning of cut fruitOxidation — chemical/enzymatic
Practice this concept5 quick reps

Practice — Level 1 (5 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Is 2Mg + O₂ → 2MgO a redox reaction?
  2. 2.
    Is AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl redox?
  3. 3.
    Order the electron-releasing tendency of Zn, Cu, Ag.
  4. 4.
    Rusting of iron is an example of which process?
  5. 5.
    Is the browning of cut fruit oxidation or reduction?

From the bank · past-year question

Example 4Chemical ReactionsMODERATE
Which of the following are oxidation reactions? I. Rusting of iron II. Burning of fuel III. Rancidity of oils and fats IV. Browning of fruits

[Q80 · Apr · 2026]

No oxidation-state change → not redox

AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl LOOKS like a reaction, but every element keeps its oxidation number (Al stays +3, Cl stays −1). With no electron transfer it is hydrolysis, NOT a redox reaction.

Reducing power follows reactivity

Electron-releasing tendency (reducing power) is Zn > Cu > Ag, mirroring the activity series — not the reverse. A more reactive metal gives up electrons more readily.
Drill 4 more on spotting a redox reaction and oxidation in daily life

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 (2)

  • Assigning oxidation numbers

    Oxidation-number bookkeeping

    ∑(oxidation numbers)=net charge;H=+1, O=−2, element=0\sum (\text{oxidation numbers}) = \text{net charge}; \qquad \text{H} = +1,\ \text{O} = -2,\ \text{element} = 0
  • Oxidising and reducing agents

    The agent does the opposite to itself

    Reducing agent⇒itself oxidised;Oxidising agent⇒itself reduced\text{Reducing agent} \Rightarrow \text{itself oxidised}; \qquad \text{Oxidising agent} \Rightarrow \text{itself reduced}

Reference tables (2)

Defining oxidation and reduction6 rows
Change to a substanceOxidation or reduction?
Loses electronsOxidation
Gains electronsReduction
Gains oxygenOxidation
Loses oxygenReduction
Loses hydrogenOxidation
Losing hydrogen is OXIDATION, not reduction — this is the bank's favourite false statement.
Gains hydrogenReduction
Spotting a redox reaction and oxidation in daily life7 rows
Reaction or processRedox? / Note
2Mg + O₂ → 2MgORedox — Mg oxidised (0 → +2), O reduced
Cu + Zn-salt displacementRedox — electron transfer between metals
AlCl₃ + 3H₂O → Al(OH)₃ + 3HClNOT redox — hydrolysis, no oxidation-state change
Hydrolysis/double displacement with no oxidation-number change is NOT a redox reaction.
Rusting of ironOxidation — Fe → hydrated Fe³⁺ oxide
Burning of fuelOxidation — carbon/hydrogen oxidised
Rancidity of oils and fatsOxidation — fatty acids oxidise
Browning of cut fruitOxidation — chemical/enzymatic

Watch out for (7)

Mastery check — 5 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Chemical ReactionsHARD
Consider the following reaction-property pairs: I. Fe+2HCl→FeCl2+H2\text{Fe} + 2\text{HCl} \to \text{FeCl}_2 + \text{H}_2 | Fe is reductant II. Zn+CuSO4→ZnSO4+Cu\text{Zn} + \text{CuSO}_4 \to \text{ZnSO}_4 + \text{Cu} | Cu is being reduced III. Br2+2I−→2Br−+I2\text{Br}_2 + 2\text{I}^- \to 2\text{Br}^- + \text{I}_2 | Br2\text{Br}_2 is reductant IV. CH4+2O2→CO2+2H2O\text{CH}_4 + 2\text{O}_2 \to \text{CO}_2 + 2\text{H}_2\text{O} | C atom in CH4\text{CH}_4 is oxidized Which pairs are correct?

[Q90 · Apr · 2026]

Example 2Chemical ReactionsMODERATE
Which one of the following represents the correct order of electron releasing tendency of metals?

[Q90 · Sep · 2019]

Example 3Chemical ReactionsMODERATE
Which one of the following compounds does not\textbf{\text{not}} exhibit a different oxidation number of the same element?

[Q56 · Sep · 2019]

Example 4Chemical ReactionsMODERATE
Which one of the following is not\textbf{\text{not}} an example of a redox reaction?

[Q85 · Apr · 2022]

Example 5Chemical ReactionsEASY
Which one among the following is an example of oxidation reaction ?

[Q116 · Sep · 2024]

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