General and Physical Chemistry
General and Physical Chemistry
This domain tests the quantitative core of the NEB Class 12 course: volumetric analysis, ionic equilibrium, chemical kinetics, thermodynamics and electrochemistry. Expect numericals that combine a formula with a unit check, plus short conceptual questions on why a reaction is spontaneous or why an indicator changes colour.
Core rules
- Volumetric analysis uses normality (N = M × n-factor) and the equivalence relation N1V1 = N2V2. KMnO4 in acidic medium is a self-indicator; oxalic acid and Na2CO3 are primary standards.
- Ionic equilibrium: Kw = [H+][OH−] = 1×10−14 at 25 °C; pH + pOH = 14. Buffers follow Henderson–Hasselbalch, pH = pKa + log([salt]/[acid]).
- Kinetics: order is experimental, molecularity is theoretical. For first order, k = (2.303/t)log(a/(a−x)) and t1/2 = 0.693/k.
- Thermodynamics: ΔG = ΔH − TΔS; ΔG < 0 means spontaneous at that temperature. Hess's law allows indirect ΔH.
- Electrochemistry: Nernst equation E = E° − (0.059/n)log Q at 298 K; F = 96500 C mol−1.
Worked example
A first-order reaction is 75% complete in 60 min. Find k and t1/2.
Reasoning: a = 100, a−x = 25, so k = (2.303/60)log(100/25) = (2.303/60)(0.602) = 0.0231 min−1. Then t1/2 = 0.693/0.0231 = 30 min. The 75% figure is two half-lives, which is a useful check.
Traps
Using molarity instead of normality in redox titrations; mixing °C and K in ΔG; forgetting to divide by n in the Nernst equation; reporting pH without checking acid/base strength; and assuming t1/2 is independent of concentration for every order.