Revision: Redox Reactions Chemistry Science (English Medium) Class 11 CBSE

"Oxidation" is defined as the addition of oxygen/electronegative element to a substance or the removal of hydrogen/ Electron/ electropositive element from a substance.

or

A process involving an increase in oxidation number by the loss of electrons.

"Reduction" is defined as the removal of oxygen/ electronegative element from a substance or the addition of hydrogen/Electron/ electropositive element to a substance.

or

A process involving decrease in oxidation number by gain of electrons.

The species which gets itself reduced and oxidise another species is called oxidising agent.

or

A substance which involves a decrease in the oxidation number of one or more of its elements. An oxidising agent helps oxidise the other substance by being reduced itself.

\[\begin{aligned} & \mathrm{S}+6\mathrm{HNO}_{3}\longrightarrow\mathrm{H}_{2}\mathrm{SO}_{4}+2\mathrm{H}_{2}\mathrm{O}+6\mathrm{NO}_{2} \\ & \mathrm{Oxidising} \\ & \mathrm{agent} \end{aligned}\]

The species which gets itself oxidised and reduce another species is called reducing agent.

or

A substance which involves an increase in the oxidation number of one or more of its elements. A reducing agent helps reduce the other substance by being oxidised.

\[\begin{aligned} & \mathrm{ZnO}+\mathrm{C}\longrightarrow\mathrm{Zn}+\mathrm{CO} \\ & \mathrm{Reducing} \\ & \mathrm{agent} \end{aligned}\]

Oxidation number (also called oxidation state) is the charge that an atom of an element appears to have when present in a combined state with other atoms. It is a hypothetical charge assigned by assuming all bonds are ionic — atoms in real molecules like H₂O do not actually carry these charges.

Rules for Assigning Oxidation Numbers:

• Oxidation number of N can be −3 (bonded to less electronegative atoms) or +3 (bonded to more electronegative atoms)
• Oxidation number of halogens is always −1 in metal halides
• In interhalogen compounds, the more electronegative halogen gets the oxidation number of −1
• Oxidation number of metals in amalgams and carbonyls is zero (e.g., Fe in [Fe(CO)₅] = 0)
• In complex ions, the algebraic sum of oxidation numbers of all atoms = net charge on the ion
• Oxidation number can be positive, negative, zero, a whole number, or a fraction
• Oxidation number greater than +6 or less than −4 is unusual — double-check for errors

Stock Notation

Variable oxidation states are indicated using Roman numerals in parentheses after the element symbol:

Two methods are used to balance redox reactions:

Method 1: Oxidation Number Method

The change in oxidation number is used to balance electron gain and loss.

Steps (Acidic Medium):

1. Write the skeleton equation; balance all atoms except O and H first
2. Identify which atoms change oxidation number; calculate the net increase and decrease
3. Multiply coefficients to make total increase in oxidation number = total decrease
4. Balance O atoms by adding H₂O to the side with fewer O atoms
5. Balance H atoms by adding H⁺ ions
6. Check that all atoms and charges are balanced

Method 2: Ion Electron Method (Half-Reaction Method)

The reaction is split into two half-reactions (oxidation and reduction) which are balanced separately and then combined.

Steps:

1. Write the redox reaction in ionic form
2. Split into oxidation half-reaction and reduction half-reaction
3. Balance atoms in each half-reaction (except O and H first)
4. Balance O by adding H₂O; balance H by adding H⁺ (acidic) or OH⁻ (basic)
5. Balance charge by adding electrons to the appropriate side
6. Equalise electrons transferred — multiply one or both half-reactions by suitable factors so electrons cancel
7. Add both half-reactions; cancel identical species on both sides
8. Check that all atoms and charges are balanced

For Basic Medium (Ion Electron Method):
After balancing in acidic conditions:

• Add OH⁻ ions to both sides equal to the number of H⁺ ions
• H⁺ + OH⁻ → H₂O (combine)
• Eliminate H₂O molecules appearing on both sides
• Final check: all elements and charges must balance