Chemical Reactions and Stoichiometry
Core concepts
Master equation balancing, the mole concept, molar mass, molar volume of a gas at standard conditions, and the stoichiometric ratio as a conversion factor. Know the main reaction types - combination, decomposition, single and double displacement, combustion, neutralisation, redox - and be able to assign oxidation states and identify the oxidising and reducing agents. For solution reactions, use the ionic equation and remove spectator ions.
Traps that appear in questions
- Limiting reagent: never assume the first-listed reactant is limiting; divide moles by coefficients and compare.
- Percentage yield and purity: distinguish theoretical mass, actual mass and the mass of pure substance inside an impure sample.
- Gas volume conditions: molar volume differs at standard conditions from room conditions; check which is stated.
- Hydrated salts: include water of crystallisation in the molar mass when computing concentration or yield.
- Oxidation-state rules: oxygen is usually −2 but −1 in peroxides; hydrogen is +1 except in hydrides.
Worked example
How many grams of water form when 4.0 g of hydrogen reacts with excess oxygen? Moles H₂ = 4.0 / 2.0 = 2.0 mol. From 2H₂ + O₂ → 2H₂O, water is 2.0 mol. Mass = 2.0 × 18.0 = 36 g.
Rule of thumb
Convert everything to moles, apply the coefficient ratio, convert back to the requested unit. Balance the equation before touching the numbers.