Chemistry for Medical Entrance: Bonding, Equilibria, Organic Reactions and Biomolecules
What you must understand
Chemistry questions test layered reasoning: atomic structure and periodicity, chemical bonding and hybridisation, states of matter and gas laws, thermodynamics (enthalpy, entropy, Gibbs energy), chemical and ionic equilibria (Ka, Kb, pH, buffers, solubility product), redox and electrochemistry, chemical kinetics, coordination compounds, and organic chemistry - nomenclature, isomerism, inductive and resonance effects, and the characteristic reactions of alkanes, alkenes, aromatics, alcohols, carbonyls, carboxylic acids, amines, and biomolecules (carbohydrates, proteins, enzymes, nucleic acids, vitamins).
Traps examiners plant
- Stability vs reactivity: more stable carbocation or conjugate base means stronger acid or favoured pathway; students often invert this.
- Le Chatelier misapplication: catalysts do not shift equilibrium position, only the rate of attainment.
- pH and dilution: diluting a weak acid changes pH less than expected because dissociation increases.
- Hybridisation: count sigma bonds plus lone pairs on the central atom; do not count pi bonds.
- Optical isomerism: look for a chiral carbon with four different groups; internal symmetry can make a molecule achiral (meso).
- Electrochemistry signs: oxidation at anode always, but anode polarity differs between galvanic and electrolytic cells.
- Named reactions applied to the wrong substrate class.
Worked example (rule of thumb)
Find the pH of 0.01 M HCl. Strong acid fully dissociates: [H⁺] = 10⁻², so pH = 2. For 0.01 M acetic acid (Ka ≈ 10⁻⁵), [H⁺] ≈ √(Ka·C) = √(10⁻⁷) ≈ 3.2 × 10⁻⁴, pH ≈ 3.5. Rule of thumb: strong acid - use concentration directly; weak acid - take the square root of Ka times concentration, then sanity-check that the answer lies between the strong-acid value and 7.