Oxidation Number — Prelims Strategy
Prelims Strategy
To excel in NEET questions on oxidation numbers, a systematic approach is key. First and foremost, memorize all the fundamental rules and their exceptions without fail. This includes the rules for elements, monatomic ions, Group 1/2 metals, fluorine, hydrogen (especially metal hydrides), and critically, oxygen (peroxides, superoxides, ). Many trap questions revolve around these exceptions.
For numerical problems involving calculation of oxidation numbers in complex compounds or ions:
- Identify the unknown element: — Clearly mark the element whose ON needs to be found.
- Assign known ONs: — Apply the rules for all other elements in the compound/ion (e.g., H=+1, O=-2, K=+1, etc.).
- Check for exceptions: — Before assuming standard ONs for H and O, quickly scan for structural clues or compound types (e.g., peroxides, hydrides) that might indicate an exception. If a calculated ON exceeds the maximum possible for that element (e.g., +8 for sulfur), immediately suspect a peroxide linkage.
- Set up the equation: — For a neutral compound, the sum of all ONs is zero. For a polyatomic ion, the sum equals the ion's charge. Formulate the algebraic equation.
- Solve carefully: — Perform the arithmetic accurately to avoid silly mistakes.
For conceptual questions (e.g., identifying oxidizing/reducing agents, comparing ONs):
- Assign ONs to all relevant atoms: — For a reaction, assign ONs to the same element in both reactants and products.
- Track changes: — An increase in ON means oxidation (reducing agent). A decrease in ON means reduction (oxidizing agent).
- Beware of distractors: — Options might include spectator ions or substances that do not undergo a change in ON (like acting as a medium). Eliminate these carefully.
Practice a wide variety of problems, especially those involving common exceptions and complex structures like peroxoacids. Time management is crucial; these questions can be solved quickly with practice, saving time for more complex problems.