State what is meant by the term acid deposition.

Describe, using equations, the mechanism of acid deposition caused by the oxides of nitrogen, including their initial formation.

All shellfish have a calcium carbonate shell. Discuss, including a balanced equation, the long-term effect of acid deposition on these organisms.

Balanced equation:

process by which acidic (substances) leave atmosphere/return to Earth / OWTTE;

Do not allow acid rain.

Formation:

\({{\text{N}}_2}{\text{(g)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2NO(g)}}/{\text{2NO(g)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2N}}{{\text{O}}_2}{\text{(g)}}\);

Acid deposition: Award [2 max] for any two of the following.

\({{\text{H}}_2}{\text{O(g)}} + {{\text{O}}_3}{\text{(g)}} \to {\text{2HO}} \bullet {\text{(g)}} + {{\text{O}}_2}{\text{(aq)}}/{{\text{H}}_2}{\text{O(g)}} + {\text{O}} \bullet {\text{(g)}} \to {\text{2HO}} \bullet {\text{(g)}}\);

\({\text{HO}} \bullet {\text{(g)}} + {\text{N}}{{\text{O}}_2}{\text{(g)}} \to {\text{HN}}{{\text{O}}_3}{\text{(aq)}}\);

\({\text{HO}} \bullet {\text{(g)}} + {\text{NO(g)}} \to {\text{HN}}{{\text{O}}_2}{\text{(aq)}}\);

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

Allow radicals represented without dot (if consistent throughout), but penalize inconsistency once only in 17 (b)(ii) and 18 (b).

Ignore state symbols.

Award [2 max] if no free radical used in describing mechanism.

shells become thinner as some of the calcium carbonate shell reacts / OWTTE;

Accept “dissolving of marine carbonate shells”.

\({\text{CaC}}{{\text{O}}_3}{\text{(s)}} + {\text{2HN}}{{\text{O}}_3}{\text{(aq)}} \to {\text{Ca (N}}{{\text{O}}_3}{{\text{)}}_2}{\text{(aq)}} + {{\text{H}}_2}{\text{O(l)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}}/\)

\({\text{CO}}_3^{2 - }{\text{(s)}} + {\text{2}}{{\text{H}}^ + }{\text{(aq)}} \to {\text{C}}{{\text{O}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{O(l)}}/\)

\({\text{CaC}}{{\text{O}}_3}{\text{(s)}} + {\text{2}}{{\text{H}}^ + }{\text{(aq)}} \to {\text{C}}{{\text{a}}^{2 + }}{\text{(aq)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{O(l)}}/\)

\({\text{CaC}}{{\text{O}}_3}{\text{(s)}} + {{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{(aq)}} \to {\text{CaS}}{{\text{O}}_4}{\text{(aq)}} + {\text{C}}{{\text{O}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{O(l)}}\);

Ignore state symbols.

Allow equations with H₂SO₃ and HNO₂.

Do not accept H₂CO₃ instead of H₂O and CO₂.

Candidates confused acid rain with acid deposition in Q18 and the mechanism of acid deposition was patchy. The effect on the carbonate shell of shellfish was understood and most candidates illustrated the reaction with sulfuric acid.

Candidates confused acid rain with acid deposition in Q18 and the mechanism of acid deposition was patchy. The effect on the carbonate shell of shellfish was understood and most candidates illustrated the reaction with sulfuric acid.

Candidates confused acid rain with acid deposition in Q18 and the mechanism of acid deposition was patchy. The effect on the carbonate shell of shellfish was understood and most candidates illustrated the reaction with sulfuric acid.