Chemistry (Grades 9–12)
What the test targets
Chemistry (Grades 9–12) tests atomic structure, periodic trends, bonding, stoichiometry, thermodynamics, kinetics, equilibrium, acids/bases, redox, and laboratory safety/data analysis. Expect quantitative items and explanations of models.
Core rules
- Periodic trends follow effective nuclear charge and shielding: atomic radius decreases across a period and increases down a group; ionization energy trends opposite radius.
- Bonding lies on a continuum; use electronegativity differences and molecular geometry (VSEPR) to explain properties.
- Stoichiometry requires a balanced equation and mole ratios; identify the limiting reactant before calculating yield.
- Equilibrium is dynamic; changing concentration, pressure, or temperature shifts position, not the equilibrium constant (except temperature).
- Safety: goggles, no food, fume hood for volatiles, waste segregation, and documented procedures.
Worked example
Given 4.0 mol N2 and 9.0 mol H2 for N2 + 3H2 → 2NH3, which is limiting? H2 needs 3 mol H2 per 1 mol N2; 4.0 mol N2 would need 12.0 mol H2, but only 9.0 mol is present. H2 is limiting. Moles NH3 = 9.0 mol H2 × (2 NH3/3 H2) = 6.0 mol NH3. Reasoning: compare required vs available; do not simply pick the smaller number.
Traps
- Using grams directly in mole ratios.
- Forgetting to balance equations.
- Confusing reaction rate with equilibrium position.
- Reporting impossible significant figures or skipping unit labels.