Chemistry: Reactions, Acids–Bases, Metals and Carbon
Chemistry in the HSC paper covers chemical reactions and equations, acids–bases–salts, metals and non-metals, carbon compounds and periodic classification. Expect balanced equations with state symbols, ion identification from tests, and predictions based on the reactivity series.
Acids, bases and salts
A pH below 7 is acidic, 7 is neutral and above 7 is basic; pH falls as the hydrogen-ion concentration rises. Acid + base gives salt + water (neutralisation); acid + carbonate or hydrogencarbonate gives salt + water + carbon dioxide, and the gas turns limewater milky. Common salts: baking soda NaHCO₃, washing soda Na₂CO₃·10H₂O, plaster of Paris CaSO₄·½H₂O, bleaching powder Ca(OCl)Cl. Electrolysis of brine in the chlor-alkali process yields NaOH, Cl₂ and H₂.
Metals and carbon
Reactivity order: K > Na > Ca > Mg > Al > Zn > Fe > Pb > Cu > Ag > Au. A more reactive metal displaces a less reactive one from its salt solution, and metals above hydrogen displace H₂ from dilute acids. Al₂O₃ and ZnO are amphoteric oxides. Carbon shows catenation and tetravalency; members of a homologous series differ by CH₂, and functional groups decide chemical properties. Ethanol with ethanoic acid and an acid catalyst gives an ester with a sweet smell.
Worked example
A salt gives brisk effervescence with dilute hydrochloric acid, and the gas turns limewater milky. Identify the ion present and write the reaction for calcium carbonate. The gas is CO₂, and the milkiness is a calcium carbonate precipitate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. The salt therefore contains carbonate (CO₃²⁻) or hydrogencarbonate (HCO₃⁻). With acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Traps that cost marks
Unbalanced equations or missing (s), (l), (g), (aq) symbols; describing the limewater test without the CaCO₃ formula; assuming every metal oxide is basic; confusing soap (a sodium salt of a fatty acid, which forms scum in hard water) with detergent.