ChemistryClass 11CBSE

Why an Iron Nail Turns Copper-Coloured in Blue Copper Sulphate

Identify oxidation and reduction by oxygen and hydrogen transfer and by electron transfer, name the oxidant and reductant, assign oxidation numbers including fractional averages, and classify redox reactions while predicting displacement from the activity series.

What do rusting, bleaching and burning fuel have in common?

Iron rusts, a cut apple turns brown, LPG burns in a stove, and bleach whitens clothes. In each case one substance loses electrons while another gains them. These are redox reactions — reduction and oxidation happening together.

Keeping track of electrons with oxidation numbers makes it possible to spot and classify any redox change.

This part covers the definitions of oxidation and reduction, oxidation numbers, and the main types of redox reactions.

How do you identify oxidation, reduction, the oxidant and the reductant?

Classically, oxidation is gaining oxygen or losing hydrogen and reduction is the reverse; in terms of electrons, oxidation is loss of electrons and reduction is gain; the oxidant (oxidising agent) is the species that is reduced, and the reductant (reducing agent) is the species that is oxidised.

Worked example 1 — oxygen and hydrogen transfer.

- 2Mg + O

_{2}

\to

2MgO: magnesium gains oxygen — oxidised
- CuO + H

_{2}

\to

Cu + H

_{2}

O: CuO loses oxygen — reduced; H

_{2}

is oxidised
- H

_{2}

S + Cl

_{2}

\to

2HCl + S: H

_{2}

S loses hydrogen — oxidised

Worked example 2 — electron transfer. In Zn + Cu

^{2 +}

\to

^{2 +}

+ Cu:

- Zn

\to

^{2 +}

+ 2e

^{-}

— zinc loses electrons, so it is oxidised and is the reductant
- Cu

^{2 +}

+ 2e

^{-}

\to

Cu — copper ions gain electrons, so they are reduced and are the oxidant

Worked example 3. In 2Na + Cl

_{2}

\to

2NaCl, each sodium atom gives one electron to chlorine: sodium is the reductant, chlorine the oxidant.

An everyday example. A freshly cut apple turns brown as substances in it are oxidised by oxygen from the air.

The substance. Oxidation and reduction always occur together — electrons lost by one species must be gained by another.

How do you assign oxidation numbers, including fractional average values?

An oxidation number is the charge an atom would carry if all its bonds were ionic; it is found from fixed rules, with all oxidation numbers in a species adding up to its overall charge.

Rules:

- Free elements:

0

— Na, O

_{2}

, P

_{4}

- Monatomic ions: equal to the charge
- Oxygen: usually

- 2

;

- 1

in peroxides such as H

_{2}

_{2}

;

+ 2

in OF

_{2}

- Hydrogen:

+ 1

with non-metals;

- 1

in metal hydrides such as NaH
- Fluorine: always

- 1

- Sum equals the charge on the molecule or ion

**Worked example 1 — chromium in K

_{2}

_{2}

_{7}

.**

2 (+ 1) + 2 x + 7 (- 2) = 0 \Rightarrow x = + 6

**Worked example 2 — manganese in MnO

_{4}^{-}

.**

x + 4 (- 2) = - 1 \Rightarrow x = + 7

**Worked example 3 — nitrogen in NH

_{4}^{+}

.**

x + 4 (+ 1) = + 1

, so

x = - 3

.

Worked example 4 — fractional averages.

- **Fe

_{3}

_{4}

**:

3 x = + 8

, so the average is

+ \frac{8}{3}

— really one Fe

^{2 +}

and two Fe

^{3 +}

- **S

_{4}

_{6}^{2 -}

**:

4 x - 12 = - 2

, so the average is

+ 2.5

— really two sulphur atoms at

+ 5

and two at

0

An everyday example. Potassium permanganate, the purple "lal dawa" used to wash vegetables, contains manganese at

+ 7

, which makes it a strong oxidising agent.

The substance. A fractional oxidation number is only an average — individual atoms always have whole-number values.

How do you classify redox reactions, and how does the activity series predict displacement?

Redox reactions are combination (two species join), decomposition (one breaks up), displacement (one element replaces another) or disproportionation (one element is both oxidised and reduced); a metal displaces another from its salt only if it lies higher in the activity series.

Worked examples of each type.

- Combination: C + O

_{2}

\to

_{2}

— carbon goes from

0

+ 4

- Decomposition: 2KClO

_{3}

\to

2KCl + 3O

_{2}

— chlorine

+ 5 \to - 1

, oxygen

- 2 \to 0

- Metal displacement: Zn + CuSO

_{4}

\to

ZnSO

_{4}

+ Cu
- Non-metal displacement: Cl

_{2}

+ 2KBr

\to

2KCl + Br

_{2}

- Disproportionation: 2H

_{2}

_{2}

\to

_{2}

O + O

_{2}

— oxygen at

- 1

goes to both

- 2

and

0

; also Cl

_{2}

+ 2OH

^{-}

\to

^{-}

+ ClO

^{-}

+ H

_{2}

O, with chlorine going from

0

- 1

and

+ 1

Activity series (most to least reactive): K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au.

Predicting displacement.

- Fe + CuSO

_{4}

— yes, iron is above copper
- Cu + ZnSO

_{4}

— no, copper is below zinc
- Zn + 2HCl

\to

ZnCl

_{2}

+ H

_{2}

— yes; Cu + HCl — no

An everyday example. An iron nail dipped in blue copper sulphate solution gets a reddish copper coating while the blue colour fades, because iron displaces copper.

The substance. Not every decomposition is a redox reaction — in CaCO

_{3}

\to

CaO + CO

_{2}

, no oxidation number changes.

Exam tip

What earns full marks on oxidation numbers and redox types?

Write the oxidation number above every atom on both sides, then look for which values rise and which fall.

- Oxidation: electron loss, oxidation number rises; reduction: gain, number falls
- Oxidant is reduced; reductant is oxidised
- Rules: elements

0

, F

- 1

, O usually

- 2

, H usually

+ 1

, total

=

charge
- Disproportionation: one element both rises and falls
- Activity series: higher metal displaces lower

The trap. Using

- 2

for oxygen in peroxides. **In H

_{2}

_{2}

and Na

_{2}

_{2}

, oxygen is

- 1

.**

Did you know

Why does silver jewellery slowly turn black?

Silver ornaments and old coins gradually darken because of tiny amounts of hydrogen sulphide in the air, which comes from sources such as eggs, some foods and pollution. The silver reacts to form a thin black layer of silver sulphide:

4 Ag + 2 H_{2} S + O_{2} \to 2 Ag_{2} S + 2 H_{2} O

Silver goes from

0

+ 1

— oxidised — while oxygen goes from

0

- 2

— reduced. Polishing removes the sulphide layer, and a quick redox reaction with aluminium foil in warm baking soda water can even turn it back into silver.

Exam relevance

How are redox reactions tested in JEE Main and NEET?

Redox Reactions is a chapter in both JEE Main and NEET Chemistry, and oxidation numbers are used throughout JEE Advanced inorganic chemistry.

What gets asked. Oxidation numbers in tricky species such as K

_{2}

_{2}

_{7}

, S

_{4}

_{6}^{2 -}

and Fe

_{3}

_{4}

, identifying disproportionation reactions, naming the oxidant and reductant, and predicting displacement from the activity series. These skills are needed for balancing redox equations, electrochemistry and the d- and f-block elements.

Question types. Short multiple-choice questions, match-the-column lists and statement-based questions.

The trap that costs marks. Treating a fractional average oxidation number as the real value of every atom.

Key takeaways

What must you be able to do from this part?

- Definitions: in Zn + Cu

^{2 +}

, zinc loses electrons and is the reductant; Cu

^{2 +}

is the oxidant
- Oxidation numbers: Cr

+ 6

in K

_{2}

_{2}

_{7}

; Mn

+ 7

in MnO

_{4}^{-}

; N

- 3

in NH

_{4}^{+}

; average

+ \frac{8}{3}

in Fe

_{3}

_{4}

and

+ 2.5

in S

_{4}

_{6}^{2 -}

- Types: combination, decomposition, displacement, and disproportionation as in 2H

_{2}

_{2}

\to

_{2}

O + O

_{2}

- Activity series: Fe displaces Cu; Cu cannot displace Zn or H

_{2}

Find the oxidation number of sulphur in H

_{2}

_{4}

, Na

_{2}

_{2}

_{3}

and SO

_{2}

, and decide whether 3Cl

_{2}

+ 6OH

^{-}

\to

5Cl

^{-}

+ ClO

_{3}^{-}

+ 3H

_{2}

O is a disproportionation.