SACE Chemistry Practice Questions

1. (1 mark) CO₂(aq)+H₂O(l)⇌H₂CO₃(aq) links dissolved carbon dioxide to carbonic acid. 2. (1 mark) H₂CO₃⇌H⁺+HCO₃⁻ provides increased hydrogen ions through the connected equilibria, lowering pH. 3. (1 mark) H⁺+CO₃²⁻⇌HCO₃⁻ reduces carbonate availability as acidity rises. 4. (1 mark) CaCO₃(s)⇌Ca²⁺+CO₃²⁻ can shift toward dissolution when carbonate is consumed. 5. (1 mark) The shell may therefore lose calcium carbonate; this is an equilibrium-based tendency whose extent depends on the full solution conditions, not an assertion that every shell instantly dissolves. Further worked reasoning: The three equilibria form one causal chain rather than three unrelated facts. Added CO2 increases dissolved CO2; hydration and acid dissociation then increase H3O+. When H3O+ reacts with CO3^2- to form HCO3-, the carbonate concentration is lowered. In the CaCO3 dissolution expression, Ksp = [Ca^2+][CO3^2-] for the idealised solid equilibrium. Removing carbonate makes the ionic product smaller, so more solid can dissolve until the equilibrium condition is approached again. A single combined acid-dissolution equation, CaCO3(s) + H+(aq) -> Ca^2+(aq) + HCO3-(aq), displays the same net consequence while the separate equations show why it occurs. The phrase "closed sample" matters. Carbon cannot escape to the atmosphere during the stated observation, although it can move among CO2(aq), H2CO3, HCO3- and CO3^2-. Charge and atom checks support the equations: the net dissolution equation has one calcium, one carbon, three oxygen and one hydrogen on each side, and total charge +1 on both sides. The shell is not a catalyst; its CaCO3 is a reactant in the dissolution process. A strong response also distinguishes direction from size. Lower pH supports a shift toward protonated carbonate species and a greater tendency for CaCO3 dissolution, but no equilibrium constants, starting concentrations or shell area are supplied. Therefore a numerical mass loss cannot be calculated. The five marks are earned by the connected chemical reasoning and equations, not by claiming that the whole shell necessarily disappears. A diagram can connect added CO2, increased H3O+, decreased CO3^2- and shell dissolution, provided the reversible equilibria and conditional conclusion remain explicit.