Chemistry: Structure, Equilibrium, and Reaction Logic
Core concepts
Focus on atomic structure and electronic configuration, periodic trends, chemical bonding and intermolecular forces, stoichiometry, gases, solutions and colligative properties, chemical equilibrium, acid-base behavior, reaction kinetics, electrochemistry, and organic functional groups. Explain trends with effective nuclear charge and shielding: atomic radius decreases across a period, ionization energy generally increases, and electron affinity becomes more negative, with exceptions at half-filled and fully filled subshells. In organic chemistry, classify reactions as substitution, addition, elimination, or oxidation-reduction, and track the functional group that changes.
Exam traps
Do not confuse bond polarity with molecular polarity: a molecule with polar bonds can be nonpolar if the dipoles cancel, as in CO2. In equilibrium, a catalyst changes the rate but not the position. In pH calculations, distinguish strong from weak acids and check whether the acid is monoprotic. In electrochemistry, assign oxidation numbers before balancing half-reactions, and remember that oxidation occurs at the anode in both galvanic and electrolytic cells.
Worked example
Find the oxidation state of chromium in K2Cr2O7. Potassium is +1 (two atoms: +2), and oxygen is -2 (seven atoms: -14). Let chromium be x: 2(+1) + 2x + 7(-2) = 0, so 2 + 2x - 14 = 0, 2x = 12, and x = +6.
Rule of thumb
Balance charge and mass, then check periodic trends against noble-gas configuration.