State what is meant by the term acid deposition.

Industrial processes, such as the burning of coal, generate non-metallic oxides of carbon and nitrogen into the atmosphere. State balanced equations for the reactions by which these oxides are produced and then removed from the atmosphere.

Oxide of carbon:

Produced:

Removed:

Oxide of nitrogen:

Produced:

Removed:

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.

Oxide of carbon:

Produced: \({\text{C(s)}} + {{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{C}}{{\text{O}}_{\text{2}}}{\text{(g)}}\);

Accept a correctly balanced equation for the combustion of a hydrocarbon fuel.

Removed: \({\text{C}}{{\text{O}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{O(l)}} \to {{\text{H}}_2}{\text{C}}{{\text{O}}_3}{\text{(aq)}}/{\text{6C}}{{\text{O}}_2}{\text{(g)}} + {\text{6}}{{\text{H}}_2}{\text{O(l)}} \to {{\text{C}}_6}{{\text{H}}_{12}}{{\text{O}}_6}{\text{(aq)}} + {\text{6}}{{\text{O}}_2}{\text{(g)}}\);

OR

Produced: \({\text{2C(s)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{2CO(g)}}\);

Removed: \({\text{2CO(g)}} + {\text{2NO(g)}} \to {{\text{N}}_{\text{2}}}{\text{(g)}} + {\text{2C}}{{\text{O}}_2}{\text{(g)}}/{\text{2CO(g)}} + {{\text{O}}_2} \to {\text{2C}}{{\text{O}}_2}{\text{(g)}}\);

Oxide of nitrogen:

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

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

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

OR

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

Removed: \({\text{2}}{{\text{H}}_2}{\text{O(l)}} + {\text{4NO(g)}} + {{\text{O}}_2}{\text{(g)}} \to {\text{4HN}}{{\text{O}}_2}{\text{(aq)}}/{\text{2NO(g)}} + {{\text{O}}_2} \to {\text{2N}}{{\text{O}}_2}{\text{(g)}}\) /

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

Ignore state symbols.

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.

Many correct answers. Some candidates oversimplified the term as acid rain.

While this question received many correct answers it is worrying that a number of candidates used incorrect formulae for common substances (such as N instead of \({{\text{N}}_{\text{2}}}\)) and a few did not balance their equations.

Most candidates realized that the acid would react with the carbonate rendering the shell weaker and therefore obtained at least one mark. The second mark was scored by a smaller number of candidates. Some candidates lost the second mark by using an acid not found in acid deposition or using the wrong formula for the salt product.