Chemistry
Chemistry rewards precise bookkeeping: atomic structure, periodicity, bonding, mole concept and stoichiometry, thermodynamics, equilibrium, redox, electrochemistry, organic nomenclature and functional groups, biomolecules and polymers.
Core rules
- Periodic trends: atomic radius falls across a period and rises down a group; ionisation energy and electronegativity do the opposite.
- Bonding: large electronegativity difference gives ionic character; hydrogen bonding explains water's high boiling point.
- One mole = 6.022 × 10²³ particles = molar mass in grams = 22.4 L of any gas at STP.
- Le Chatelier: raising pressure shifts the equilibrium towards the side with fewer gas moles; raising temperature favours the endothermic direction.
- Group tests: Tollens'/Fehling's for aldehydes, iodoform for methyl ketones, bromine water for unsaturation.
Worked example
How many molecules are in 11.2 L of oxygen at STP? Moles = 11.2 ÷ 22.4 = 0.5 mol; molecules = 0.5 × 6.022 × 10²³ = 3.011 × 10²³; mass = 0.5 × 32 = 16 g. If the same 11.2 L were measured at 2 atm and 273 K, Boyle's law reduces it to 5.6 L at STP, giving 0.25 mol - the 'STP only' condition attached to 22.4 L is the deciding step.
Traps
Using 22.4 L outside STP; confusing molality (per kg solvent) with molarity (per litre solution); assigning oxygen −2 in peroxides; believing a catalyst shifts equilibrium; treating aldehydes and ketones as one functional group; substituting masses into stoichiometry without balancing the equation.