Oxidation Number Method — Revision Notes

⚡ 30-Second Revision

Oxidation Number (ON) — Hypothetical charge. Increase = Oxidation, Decrease = Reduction.
Rules — Elements (ON=0), Monatomic ions (ON=charge), Group 1 (+1), Group 2 (+2), F (-1).
H — +1 (except metal hydrides -1).
O — 2 (except peroxides -1, superoxides -1/2, $ext{OF}_2$ +2).
Sum of ONs — 0 for neutral, charge for ion.
Method Steps

1. Assign ONs. 2. Identify changes (increase/decrease). 3. Equalize total increase/decrease using coefficients. 4. Balance other atoms. 5. Acidic: Balance O with $ext{H}_2\text{O}$, H with $ext{H}^+$. 6. Basic: Balance O with $ext{H}_2\text{O}$, H with $ext{H}^+$, then add $ext{OH}^-$ to both sides to neutralize $ext{H}^+$, simplify $ext{H}_2\text{O}$. 7. Verify charge.

2-Minute Revision

The Oxidation Number Method is a systematic way to balance redox reactions. Start by assigning oxidation numbers to all atoms, using standard rules (e.g., elements are 0, Group 1 metals are +1, oxygen is usually -2 but -1 in peroxides).

Identify the atoms whose oxidation numbers change; an increase signifies oxidation, and a decrease signifies reduction. Calculate the total change in oxidation number for both the oxidized and reduced species, remembering to multiply by the number of atoms involved.

Then, use stoichiometric coefficients to make the total increase in oxidation number equal to the total decrease. This balances the electron transfer. Next, balance all other atoms (not O or H) by inspection.

Finally, balance oxygen atoms by adding $ext{H}_2\text{O}$ molecules and hydrogen atoms by adding $ext{H}^+$ ions for acidic medium, or $ext{OH}^-$ ions for basic medium (after initially balancing with $ext{H}^+$ and then neutralizing).

Always perform a final check of the total charge on both sides to ensure it is balanced.

5-Minute Revision

The Oxidation Number Method is a powerful tool for balancing redox reactions, ensuring both mass and charge conservation. The process begins with accurately assigning oxidation numbers to every atom in the unbalanced equation.

Remember the key rules: elements are 0, Group 1 metals are +1, Group 2 are +2, fluorine is -1. Hydrogen is +1 (except in metal hydrides, -1), and oxygen is -2 (except in peroxides, -1; superoxides, -1/2; and with fluorine, +2).

The sum of oxidation numbers in a neutral compound is zero, and in an ion, it equals the ion's charge. Once assigned, identify the atoms undergoing a change in oxidation number. An increase indicates oxidation (electron loss), and a decrease indicates reduction (electron gain).

Calculate the total change for each species, multiplying the change per atom by the number of atoms involved in the formula unit. For example, if $ext{Cr}_2\text{O}_7^{2-}$ (Cr at +6) reduces to $2\text{Cr}^{3+}$ (Cr at +3), the decrease is $2 \times (6-3) = 6$.

Now, use appropriate stoichiometric coefficients to make the total increase in oxidation number equal to the total decrease. This balances the electron transfer. Next, balance all other atoms (not O or H) by simple inspection.

The final steps depend on the medium: in acidic medium, balance oxygen by adding $ext{H}_2\text{O}$ molecules to the oxygen-deficient side, then balance hydrogen by adding $ext{H}^+$ ions to the hydrogen-deficient side.

In basic medium, follow the acidic steps first, then add an equal number of $ext{OH}^-$ ions to both sides as there are $ext{H}^+$ ions. Combine $ext{H}^+$ and $ext{OH}^-$ to form $ext{H}_2\text{O}$, and simplify any common $ext{H}_2\text{O}$ molecules.

A final check of the total charge on both sides is essential to confirm a correctly balanced equation. Practice with diverse examples, especially those with polyatomic ions and different media, is key to mastering this method for NEET.

Prelims Revision Notes

Oxidation Number Method: NEET Quick Recall

1. Oxidation Number (ON) Rules - The Foundation:

* Elemental State: ON = 0 (e.g., $ext{O}_2$, $ext{Na}$, $ext{Cl}_2$). * Monatomic Ions: ON = ion's charge (e.g., $ext{Na}^+$ is +1, $ext{Cl}^-$ is -1). * Group 1 Metals: Always +1 in compounds (e.

g., $ext{KMnO}_4$). * Group 2 Metals: Always +2 in compounds (e.g., $ext{CaCO}_3$). * Fluorine (F): Always -1 in compounds (most electronegative). * Hydrogen (H): +1 (most compounds, e.

g., $ext{H}_2\text{O}$); -1 (metal hydrides, e.g., $ext{NaH}$). * Oxygen (O): -2 (most compounds, e.g., $ext{CO}_2$); -1 (peroxides, e.g., $ext{H}_2\text{O}_2$); -1/2 (superoxides, e.g., $ext{KO}_2$); +2 (with F, e.

g., $ext{OF}_2$). * Sum of ONs: 0 for neutral compound; equals ion's charge for polyatomic ion.

2. Core Principle:

* Total increase in ON (oxidation) = Total decrease in ON (reduction).

3. Balancing Steps (Acidic Medium):

1. Assign ONs: To all atoms, identify changing ones. 2. Calculate Change: Determine total increase/decrease in ON for redox species (multiply change per atom by number of atoms). 3. Equalize Changes: Use coefficients to make total increase = total decrease.

4. Balance Other Atoms: By inspection (not O or H). 5. Balance Oxygen: Add $ext{H}_2\text{O}$ to O-deficient side. 6. Balance Hydrogen: Add $ext{H}^+$ to H-deficient side. 7. Verify Charge: Sum charges on both sides; must be equal.

4. Balancing Steps (Basic Medium):

1. Follow steps 1-4 for acidic medium. 2. Balance Oxygen: Add $ext{H}_2\text{O}$ to O-deficient side. 3. Balance Hydrogen: Add $ext{H}^+$ to H-deficient side (as if acidic). 4. **Neutralize $ext{H}^+$**: Add $ext{OH}^-$ to *both* sides, equal to the number of $ext{H}^+$. 5. **Form $ext{H}_2\text{O}$**: $ext{H}^+ + \text{OH}^- \to \text{H}_2\text{O}$. Simplify any common $ext{H}_2\text{O}$ molecules. 6. Verify Charge: Sum charges on both sides; must be equal.

5. Key Traps for NEET:

* Forgetting exceptions for O and H oxidation numbers. * Incorrectly calculating total change for species with multiple changing atoms (e.g., $ext{C}_2\text{O}_4^{2-}$). * Errors in balancing $ext{H}_2\text{O}$ and $ext{H}^+$/$ext{OH}^-$. * Not performing a final charge check.

Vyyuha Quick Recall

Only Really Easy Balancing Always Brings Confidence!

Oxidation numbers assigned.
Redox species identified.
Equalize electron changes (total increase = total decrease).
Balance other atoms.
Acidic: $ext{H}_2\text{O}$ for O, $ext{H}^+$ for H.
Basic: $ext{H}_2\text{O}$ for O, $ext{H}^+$ for H, then add $ext{OH}^-$ to both sides.
Charge check (final verification).