Chemistry: Stoichiometry, Equilibrium, Periodicity and Organic Groups
Chemistry on the ETEA test draws on atomic structure and periodicity, stoichiometry, chemical equilibrium, acids and bases, electrochemistry, and organic functional groups. Most items are solved in two or three steps, so the marks come from choosing the right relationship rather than from long calculation.
Core rules to hold
- The mole is the bridge: n = m/M, and one mole contains 6.02 × 10²³ particles.
- A balanced equation gives mole ratios, never mass ratios; the limiting reagent is the one that runs out first.
- Across a period, atomic radius falls and ionization energy rises; down a group the trend reverses.
- Le Chatelier: adding reactant or removing product shifts forward; raising temperature favours the endothermic direction.
- pH = −log[H⁺] for dilute aqueous solutions, and oxidation is loss of electrons (OIL RIG).
Worked example
4 g of H₂ reacts with 16 g of O₂. What mass of water forms? Moles: H₂ = 4/2 = 2 mol, O₂ = 16/32 = 0.5 mol. From 2H₂ + O₂ → 2H₂O, 0.5 mol O₂ consumes 1 mol H₂, so H₂ is in excess and O₂ is limiting. Water formed = 1 mol = 18 g, leaving 1 mol (2 g) of H₂ unreacted.
Traps that cost marks
- Using an unbalanced equation, which corrupts every mole ratio after it.
- Comparing masses instead of moles when hunting the limiting reagent.
- Assuming a shift in equilibrium also changes the equilibrium constant; only temperature does that.
- Mixing up oxidation and reduction, or forgetting that oxidation numbers are assigned, not measured.