Chemistry: bonding, reactions, organic and inorganic
The chemistry section covers electronic structure and bonding, quantitative reaction chemistry (stoichiometry, equilibrium, acid–base and redox), organic functional groups and reaction types, and inorganic periodic trends and identification tests.
Core rules
- Bonding: VSEPR predicts shapes (linear, trigonal planar, tetrahedral, trigonal pyramidal, bent); polar bonds plus asymmetry give a polar molecule; hydrogen bonding requires H attached to N, O or F.
- Quantitative: n=m/M; c=n/V; balance equations before using mole ratios; the limiting reagent is the one giving the least product.
- Equilibrium: Le Chatelier shifts the position on concentration, pressure or temperature change; a catalyst changes rate only; pH=−log[H⁺].
- Organic: alkanes undergo substitution, alkenes addition, alcohols oxidation; a chiral carbon carries four different groups.
Worked example
4.0 g of H₂ (M=2.0, n=2.0 mol) reacts with 16 g of O₂ (M=32, n=0.50 mol) via 2H₂+O₂→2H₂O. Oxygen is limiting: 0.50 mol O₂ gives 1.0 mol H₂O, i.e. 18 g. Hydrogen used = 1.0 mol, leaving 1.0 mol = 2.0 g unreacted. Compare mole ratios, never mass ratios.
Traps that cost marks
Using an unbalanced equation; forgetting that H₂, O₂, N₂ and Cl₂ are diatomic; treating a catalyst as an equilibrium shift; confusing pH with pOH; writing structural isomers without checking for chirality; assigning oxygen −2 in peroxides.
Key skills to drill
Convert every quantity to moles first, identify the limiting reagent, then convert back to the requested unit.