Rules for Identifying Redox Reactions
Oxidation, Reduction & Oxidation Numbers | Class 11
1. What is a Redox Reaction?
Definition: A chemical reaction in which Reduction and Oxidation occur simultaneously.
2. Ways to Identify Redox Reactions
A. Classical Concept (Old Definition)
| Oxidation | Reduction |
|---|---|
| Addition of Oxygen | Removal of Oxygen |
| Removal of Hydrogen | Addition of Hydrogen |
| Addition of Electronegative element | Removal of Electronegative element |
| Removal of Electropositive element | Addition of Electropositive element |
B. Electronic Concept (Modern Definition)
Remember: OIL RIG
Oxidation Is Loss of electrons.
Reduction Is Gain of electrons.
Oxidation Is Loss of electrons.
Reduction Is Gain of electrons.
- $Na \rightarrow Na^+ + e^-$ (Loss = Oxidation)
- $Cl_2 + 2e^- \rightarrow 2Cl^-$ (Gain = Reduction)
C. Oxidation Number Concept (Most Reliable)
- Oxidation: Increase in Oxidation Number.
- Reduction: Decrease in Oxidation Number.
3. Rules for Assigning Oxidation Number (ON)
- Free State: Elements in their natural, uncombined state have ON = 0 (e.g., $H_2, O_2, Fe, Na, P_4, S_8$).
- Monoatomic Ions: ON = Charge on the ion (e.g., $Na^+ = +1$, $Mg^{2+} = +2$, $Cl^- = -1$).
- Hydrogen: Generally +1 (with non-metals). -1 in metal hydrides (e.g., $NaH, CaH_2$).
- Oxygen: Generally -2.
- Peroxides ($H_2O_2$): -1
- Superoxides ($KO_2$): -0.5
- With Fluorine ($OF_2$): +2
- Fluorine: Always -1 (Most electronegative).
- Alkali Metals (Group 1): Always +1.
- Alkaline Earth Metals (Group 2): Always +2.
- Sum Rule: The algebraic sum of ONs of all atoms in a molecule is 0, and in an ion is equal to the charge.
4. Oxidizing and Reducing Agents
- Oxidizing Agent (Oxidant): The substance that gets reduced (accepts electrons) and oxidizes others.
- Reducing Agent (Reductant): The substance that gets oxidized (donates electrons) and reduces others.
If an element is in its Maximum Oxidation State (e.g., $Mn$ in $KMnO_4$ is +7), it can only be an Oxidizing Agent.
If an element is in its Minimum Oxidation State (e.g., $S$ in $H_2S$ is -2), it can only be a Reducing Agent.
If an element is in its Minimum Oxidation State (e.g., $S$ in $H_2S$ is -2), it can only be a Reducing Agent.
5. Types of Redox Reactions
- Combination: $C + O_2 \rightarrow CO_2$
- Decomposition: $2H_2O \rightarrow 2H_2 + O_2$
- Displacement: Metal ($Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$) or Non-metal.
- Disproportionation: A reaction in which the same element is simultaneously oxidized and reduced.
Ex: $2H_2O_2 \rightarrow 2H_2O + O_2$ (Oxygen goes from -1 to -2 and 0).
Practice Quiz
Test your ability to identify Redox changes.
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