Chemistry - Chemical Reactions, Equations, Ions, Acids and Bases, Oxidation and Reduction
This domain tests balanced equations, mole calculations, ionic reactions, neutralisation, and electron transfer. Aichi items often combine an equation with a mass or gas-volume calculation, so show the mole ratio clearly.
Equations and moles
A balanced equation conserves atoms. Coefficients give mole ratios. Molar mass is the mass of one mole; one mole of any gas occupies about 22.4 L at standard temperature and pressure. For solutions, moles = concentration × volume in litres. A limiting reactant determines the maximum product. In ionic equations, cancel spectator ions that appear unchanged on both sides.
Acids and bases
Acids produce H⁺ in water; bases produce OH⁻. pH below 7 is acidic, above 7 basic. Neutralisation: acid + base → salt + water. Indicators such as litmus and phenolphthalein show the endpoint. Strength (degree of ionisation) differs from concentration (amount per volume).
Oxidation and reduction
Oxidation is loss of electrons or an increase in oxidation number; reduction is gain. The oxidising agent is itself reduced. The reactivity series predicts displacement: a more reactive metal displaces a less reactive metal from its salt solution.
Worked example
2.4 g of magnesium reacts with excess hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. Moles of Mg = 2.4 ÷ 24 = 0.10 mol. The ratio Mg : H₂ is 1 : 1, so 0.10 mol H₂ forms. Volume at STP = 0.10 × 22.4 = 2.24 L; mass = 0.10 × 2.0 = 0.20 g.
Traps
Do not forget diatomic H₂ and O₂ when balancing. Use the mole ratio, not the mass ratio. “Strong acid” does not mean “concentrated acid.” pH is logarithmic, not linear. Do not reverse oxidation and reduction. Gas-volume calculations require the same temperature and pressure.