Chemistry: Structure, Reactions, and Equilibrium
What to master
ECAT chemistry spans atomic structure and electronic configuration, periodic trends (atomic radius, ionisation energy, electronegativity), chemical bonding (ionic, covalent, coordinate, hydrogen bonding), stoichiometry and the mole concept, gas laws, thermochemistry, chemical equilibrium and Le Chatelier's principle, acids, bases, and pH, redox and electrochemistry, and organic functional groups and isomerism.
Traps that catch candidates
- Forgetting that ionisation energy increases across a period but decreases down a group.
- Confusing empirical and molecular formulas when only percentages are given.
- Miscalculating moles from mass without converting to grams, or ignoring molar mass units.
- Applying Le Chatelier incorrectly to changes in pressure for reactions with equal moles of gas (no shift).
- Mixing up oxidation (loss of electrons) and reduction (gain).
- Ignoring significant figures and units in pH and concentration problems.
Rule of thumb
For any numerical problem, convert everything to moles first, then use the balanced equation as a ratio map. For equilibrium, ask: which side has more gas moles? That side is favoured by lower pressure.
Worked example
What is the pH of 0.01 M HCl? HCl is a strong acid, so [H⁺] = 0.01 = 10⁻². pH = −log(10⁻²) = 2. For a weak acid, you would need Ka and an ICE table - never assume full dissociation.