Redox Reactions — Part 2: Types of redox reactions, balancing and redox titrations: visual lesson, CBSE Class 11 Chemistry

Lesson

Types and balancing

Part 2 — four patterns that cover most redox reactions, and two systematic ways to balance any of them.

In this lesson you will

Classify redox reactions as combination, decomposition, displacement and disproportionation
Describe a Daniell cell as redox split between two electrodes
Balance redox equations by the oxidation-number and half-reaction methods in acidic and basic medium
Use mole ratios from balanced redox equations in permanganate titrations

Table of four types of redox reaction with general pattern and an example
Type	General pattern	Example
Combination	A + B gives AB	$\ce{N2 + 3H2 -> 2NH3}$
Decomposition	AB gives A + B	$\ce{2KClO3 -> 2KCl + 3O2}$
Displacement	A + BC gives AC + B	$\ce{Zn + CuSO4 -> ZnSO4 + Cu}$
Disproportionation	One species gives two products at higher and lower ON	$\ce{2H2O2 -> 2H2O + O2}$

Concept

Combination and decomposition

A combination reaction is redox if an element takes part; a decomposition is redox if an element is formed. Reactions between compounds with no ON change, such as $\ce{CaO + CO2 -> CaCO3}$ , are not redox.

Table of combination and decomposition reactions showing which are redox
Reaction	Type	ON changes	Redox?
$\ce{N2 + 3H2 -> 2NH3}$	Combination	N: 0 to −3; H: 0 to +1	Yes
$\ce{CaO + CO2 -> CaCO3}$	Combination	None	No
$\ce{2KClO3 -> 2KCl + 3O2}$	Decomposition	Cl: +5 to −1; O: −2 to 0	Yes
$\ce{CaCO3 -> CaO + CO2}$	Decomposition	None	No

Concept

Displacement reactions

In a displacement reaction an atom or ion is replaced by another that is a stronger reducing agent (metals) or stronger oxidising agent (non-metals). Activity series (decreasing): K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au.

Table of four kinds of displacement reaction with examples
Kind	Example	Replaced by
Metal by metal	$\ce{Zn + CuSO4 -> ZnSO4 + Cu}$	A more reactive metal
Hydrogen by metal	$\ce{Zn + H2SO4 -> ZnSO4 + H2}$	A metal above hydrogen
Halogen by halogen	$\ce{Cl2 + 2KBr -> 2KCl + Br2}$	Order: F₂ > Cl₂ > Br₂ > I₂
Oxygen by fluorine	$\ce{2F2 + 2H2O -> 4HF + O2}$	Fluorine is the strongest oxidant

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