Chemistry - Chemical Reactions, Equilibrium, and Inorganic/Organic Chemistry
This domain tests quantitative reaction chemistry and the logic of equilibrium. Expect balancing equations, mole and limiting-reagent calculations, acid–base and pH, redox, Le Chatelier shifts, and functional-group recognition in organic chemistry.
Core rules
- Balance atoms first, then charge. Coefficients give mole ratios, not mass ratios.
- Limiting reagent: calculate moles of each reactant, divide by the coefficient, and choose the smaller quotient.
- Acid–base: pH = −log[H+]; each pH unit is a tenfold change. Strong acids dissociate completely; weak acids do not.
- Equilibrium: at equilibrium rates are equal, not concentrations. Le Chatelier: adding reactant shifts right; increasing pressure favors fewer gas moles; increasing temperature favors the endothermic direction.
- Organic: alkanes undergo substitution, alkenes addition; alcohols, aldehydes, carboxylic acids, and esters have characteristic tests and oxidation behavior.
Worked example
4.0 g of H2 reacts with 32.0 g of O2 to form water. Which is limiting, and what mass of water forms? Moles: H2 = 4.0/2.0 = 2.0 mol; O2 = 32.0/32.0 = 1.0 mol. The equation 2H2 + O2 → 2H2O needs H2:O2 = 2:1, so the reactants are in exact stoichiometric ratio. Water formed = 2.0 mol × 18.0 g/mol = 36.0 g. If one reactant had excess, the other would still set the product amount.
Traps
- Using grams directly in mole ratios.
- Forgetting that a catalyst speeds equilibrium but does not shift it.
- Treating pH 3 as three times more acidic than pH 6 instead of 1000 times.
- Confusing oxidation and reduction: oxidation is loss of electrons.
- Writing organic products without checking carbon valency.