JEE Chemistry — Important Questions & Concepts
Physical Chemistry
Q: Molarity vs molality — when to use each?
Molarity (M) = moles of solute / litres of solution
Changes with temperature (volume expands/contracts)
Used for: dilutions, titrations, stoichiometry
Molality (m) = moles of solute / kg of solvent
Temperature independent (mass doesn't change)
Used for: colligative properties (ΔTf, ΔTb, osmotic pressure)Q: Rate of reaction: 2NO + O₂ → 2NO₂. Write rate expression.
Rate = k[NO]²[O₂]
NO is second order (exponent = 2 from balanced equation for elementary)
O₂ is first order
Overall order = 3
Note: Order from equation only valid for elementary reactions
For complex reactions, order determined experimentallyQ: Calculate ΔG° for reaction with Kp = 10⁵ at 300K.
ΔG° = -RT ln Kp = -8.314 × 300 × ln(10⁵)
= -2494.2 × 5 × 2.303
= -28,722 J/mol ≈ -28.7 kJ/mol
Negative ΔG° → spontaneous reaction (Kp > 1 → products favoured)Organic Chemistry
Q: Explain SN1 vs SN2 mechanisms.
SN1 (Substitution Nucleophilic Unimolecular):
2 steps: ionisation → carbocation intermediate
Rate = k[substrate] (first order — only substrate in slow step)
Favoured: tertiary > secondary substrates
Polar protic solvents (water, alcohol)
Weak nucleophiles
Stereochemistry: racemisation (carbocation attacked from both sides)
SN2 (Bimolecular):
1 step: nucleophile attacks as leaving group leaves (backside attack)
Rate = k[substrate][nucleophile] (second order)
Favoured: primary > secondary substrates (less steric hindrance)
Polar aprotic solvents (DMSO, acetone)
Strong nucleophiles
Stereochemistry: inversion (Walden inversion)Inorganic Chemistry
Q: Why is the second ionisation energy of Na very high?
Na (Z=11): 1s² 2s² 2p⁶ 3s¹
First IE: remove 3s¹ electron (outermost, easiest) → Na⁺ = noble gas config
Second IE: remove electron from 2p⁶ (inner shell, full shell, much harder)
IE₂ >> IE₁ because noble gas configuration is highly stableRevision Notes
MOLE CONCEPT: n = mass/molar mass | PV = nRT | 1 mol = 6.022×10²³
SOLUTIONS:
Molarity (M): moles/litres | Molality (m): moles/kg solvent
Normality (N) = n × M (n = equivalents)
Henry's law: p = KH·x | Raoult's law: P_total = x_A·P_A° + x_B·P_B°
KINETICS: rate = k[A]^m[B]^n | t½ for 1st order = 0.693/k
THERMOCHEMISTRY: ΔG = ΔH - TΔS | ΔG° = -RT lnK
ΔG < 0: spontaneous | ΔG > 0: non-spontaneous
ORGANIC MECHANISMS:
SN1: tertiary, racemisation, protic solvent
SN2: primary, inversion, aprotic solvent, strong nucleophile
E1: tertiary, carbocation, Zaitsev product
E2: secondary/tertiary, strong base, anti-periplanar