State an equation that shows why rain water is naturally acidic.

Outline the process responsible for the production of each acid and state an equation to show its formation.

Acid rain has caused damage to limestone buildings and marble statues. State an equation to represent the reaction of acid rain with limestone or marble.

\({\text{C}}{{\text{O}}_2} + {{\text{H}}_{\text{2}}}{\text{O}} \rightleftharpoons {{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}/{\text{C}}{{\text{O}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}} \rightleftharpoons {{\text{H}}^ + } + {\text{HCO}}_3^ - \);

Do not penalize absence of reversible sign.

Do not accept CO₂ + H₂O \( \to \) 2H⁺ + CO₃^2–.

Acid 1:

\({\text{(HN}}{{\text{O}}_{\text{2}}}{\text{/HN}}{{\text{O}}_{\text{3}}}{\text{)}}\) high temperature in internal combustion/jet engine;

reaction between \({{\text{N}}_{\text{2}}}\) and \({{\text{O}}_{\text{2}}}\) at high temperature/lightning;

Accept either of the above for first mark.

\({\text{2N}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}} \to {\text{HN}}{{\text{O}}_3} + {\text{HN}}{{\text{O}}_2}/{\text{4N}}{{\text{O}}_2} + {{\text{O}}_2} + {\text{2}}{{\text{H}}_2}{\text{O}} \to {\text{4HN}}{{\text{O}}_3}\);

Acid 2:

\({\text{(}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{3}}}{\text{/}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{)}}\) from burning of coal / smelting plants / sulfuric acid plants / volcanic activity;

Do not accept combustion of fossil fuels.

\({\text{S}}{{\text{O}}_2} + {{\text{H}}_2}{\text{O}} \to {{\text{H}}_2}{\text{S}}{{\text{O}}_3}/{\text{S}}{{\text{O}}_3} + {{\text{H}}_2}{\text{O}} \to {{\text{H}}_2}{\text{S}}{{\text{O}}_4}\);

Allow H₂SO₃/H₂SO₄ to be Acid 1 and HNO₂/HNO₃ to be Acid 2.

\({\text{CaC}}{{\text{O}}_{\text{3}}} + {\text{2HN}}{{\text{O}}_{\text{3}}} \to {\text{Ca(N}}{{\text{O}}_{\text{3}}}{{\text{)}}_{\text{2}}} + {\text{C}}{{\text{O}}_{\text{2}}} + {{\text{H}}_{\text{2}}}{\text{O}}\);

Accept equation with H₂SO₃ or H₂SO₄ or ionic equations.

Do not accept equations with H₂CO₃.

Most candidates correctly identified carbon dioxide as the source of natural water acidity and wrote an acceptable equation. Many also identified sources of nitric and sulphuric acid, though these equations often proved trickier, with many candidates writing equations for the formation of the oxide from which the acid is derived. Balanced equations for the reaction with limestone also proved to be a challenge, with carbonic acid often appearing as a product.

Most candidates correctly identified carbon dioxide as the source of natural water acidity and wrote an acceptable equation. Many also identified sources of nitric and sulphuric acid, though these equations often proved trickier, with many candidates writing equations for the formation of the oxide from which the acid is derived. Balanced equations for the reaction with limestone also proved to be a challenge, with carbonic acid often appearing as a product.

Most candidates correctly identified carbon dioxide as the source of natural water acidity and wrote an acceptable equation. Many also identified sources of nitric and sulphuric acid, though these equations often proved trickier, with many candidates writing equations for the formation of the oxide from which the acid is derived. Balanced equations for the reaction with limestone also proved to be a challenge, with carbonic acid often appearing as a product.