Chemistry: Moles, Trends, and Mechanism
Chemistry in NEET UG spans physical, organic and inorganic chemistry. Physical questions test numerical relationships, inorganic questions test periodic and bonding patterns, and organic questions test structure, stability and mechanism. The common thread is that a small number of principles explain many reactions.
Core rules
- Mole concept: n = m/M, N = nN_A. At STP, one mole of an ideal gas occupies 22.4 L. Use molar mass, not atomic number, to convert mass.
- Periodic trends: atomic radius decreases across a period and increases down a group; ionisation enthalpy generally increases across a period, with exceptions at Be-B and N-O; electron gain enthalpy of Cl is more negative than that of F.
- Bonding: VSEPR predicts shape from electron-pair repulsion; hybridisation explains geometry; molecular orbital theory explains bond order and magnetism.
- Organic reactivity: identify nucleophile/electrophile, then consider inductive, resonance and hyperconjugation effects. SN1 proceeds via carbocation, SN2 via concerted backside attack.
Worked example
Calculate the number of formula units in 5.85 g of NaCl. M(NaCl) = 23 + 35.5 = 58.5 g/mol. n = 5.85/58.5 = 0.100 mol. Formula units = 0.100 × 6.022 × 10²³ = 6.022 × 10²². If NaCl dissociates completely, ions = 2 × 6.022 × 10²² = 1.2044 × 10²³.
Traps
Using 23 as the molar mass of NaCl; applying 22.4 L to solids or non-STP gases; forgetting periodic-trend exceptions; confusing moles of molecules with moles of atoms; ignoring carbocation stability in substitution.