Physical Chemistry is where most Class 12 Chemistry marks are lost and where they can most reliably be recovered. The numericals follow fixed formulas and fixed patterns. A student who knows the formulas and practises the application consistently will score full marks in this section.
Chapter-Wise Formula Priority
| Chapter | Most Tested Formulas | Marks |
|---|---|---|
| Electrochemistry | Nernst equation, cell EMF, conductivity, Faraday's laws | 4–6 |
| Chemical Kinetics | Rate law, order of reaction, Arrhenius equation, half-life | 4–5 |
| Solutions | Raoult's law, molality, elevation in boiling point, depression in freezing point | 4–5 |
| Solid State | Packing efficiency, density of unit cell, defects | 3 |
| Surface Chemistry | Freundlich adsorption isotherm (qualitative mostly) | 2 |
The Standard 4-Step Numerical Method
- 1.Identify the chapter — which concept is this numerical testing? Immediately recall the formula.
- 2.Write 'Given' — list all quantities with symbols and SI units.
- 3.Write 'Formula' — write the complete formula you will apply. Even if your answer is wrong, a correct formula earns partial marks.
- 4.Substitute and solve — show every algebraic step. Do not skip. Final answer must include units.
Electrochemistry — The Highest-Value Chapter
- Cell EMF = E°cathode − E°anode — learn this formula and the sign convention.
- Nernst equation: E = E° − (RT/nF)lnQ — know how to apply at standard and non-standard conditions.
- Faraday's First Law: m = ZIt — mass deposited in electrolysis. Simple substitution but high marks.
- Kohlrausch's Law — molar conductivity at infinite dilution is additive. Practise 4–5 standard problems.
Chemical Kinetics — The Most Predictable Chapter
Chemical Kinetics questions in CBSE follow a rotation of about 6 question types: writing rate expressions, finding order of reaction from data, applying the integrated rate law, calculating half-life, applying Arrhenius equation, and effect of temperature on rate. Solve 3 previous year numericals from each type and you will recognise every kinetics question on the actual paper.
Tip
Create a 'formula card' for each Physical Chemistry chapter — a small index card with all formulas, SI units, and one example. Review these cards every morning in the last 3 weeks.
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marks available from Physical Chemistry numericals alone — all formulaic and fully practisable