PSEB Senior Secondary Chemistry - Theory
What the theory paper tests
The PSEB Senior Secondary Chemistry theory domain covers Class XI and XII core chemistry: mole concept and stoichiometry, atomic structure and periodicity, chemical bonding, thermodynamics, equilibrium, redox and electrochemistry, kinetics, coordination compounds, and organic functional groups and mechanisms. Questions mix short definitions, reasoning about trends, equation balancing, and multi-step numericals, so you must show units and states.
Core rules to keep ready
- Balance atoms and charge; in redox use half-reactions and match electrons before adding.
- Periodic trends: atomic radius falls across a period and rises down a group; ionisation enthalpy and electronegativity follow the opposite pattern, with exceptions at group 13/16.
- VSEPR: count bond pairs and lone pairs; lone-pair repulsion > bond-pair repulsion, which explains bent H2O and pyramidal NH3.
- Thermodynamics: ΔG = ΔH − TΔS; spontaneous only when ΔG < 0. Sign conventions matter.
- Organic: identify the functional group, then the reagent; SN1 favours tertiary halides and polar protic solvents, SN2 favours primary and aprotic solvents.
Worked example
25.0 mL of 0.20 M FeSO4 is titrated with 0.10 M KMnO4 in acidic medium. What volume is needed? MnO4− + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O. Moles Fe2+ = 0.0250 × 0.20 = 0.00500 mol. From the equation, 1 mol MnO4− oxidises 5 mol Fe2+, so MnO4− needed = 0.00500/5 = 0.00100 mol. Volume = moles/molarity = 0.00100/0.10 = 0.0100 L = 10.0 mL. Reasoning check: the 5:1 electron ratio comes from Mn changing +7 to +2 (5 e−) and Fe2+ to Fe3+ (1 e−).
Traps that cost marks
- Omitting the acid medium in permanganate equations; H+ and H2O must be balanced.
- Using concentration without converting mL to L.
- Reversing ΔH sign or forgetting ΔG = 0 at equilibrium.
- Naming coordination compounds with ligands in the wrong alphabetical order or missing the oxidation state.
- Drawing organic products without showing stereochemistry when asked.