Explain, writing an appropriate equation, why, even in an unpolluted environment, rainwater is still slightly acidic.

(i)     Outline the major source of this gas, including an equation.

(ii)     Describe, including an equation, a chemical method used to control the emission of this pollutant.

(iii)     Identify a compound, to which nitrogen monoxide is eventually converted, that is responsible for acidity in lakes and rivers.

dissolved carbon dioxide / \({\text{C}}{{\text{O}}_2}{\text{(g)}} + {{\text{H}}_2}{\text{O(1)}} \rightleftharpoons {{\text{H}}_2}{\text{C}}{{\text{O}}_3}{\text{(aq)}}\);

\({{\text{H}}_2}{\text{C}}{{\text{O}}_3}{\text{(aq)}} \rightleftharpoons {{\text{H}}^ + }{\text{(aq)}} + {\text{HCO}}_3^ - {\text{(aq)}}/{\text{C}}{{\text{O}}_2}{\text{(aq)}} + {{\text{H}}_2}{\text{O(1)}} \rightleftharpoons {{\text{H}}^ + }{\text{(aq)}} + {\text{HCO}}_3^ - {\text{(aq)}}\);

(i)     internal combustion engine / high temperature combustion;

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

(ii)     catalytic converters / exhaust recirculation;

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

(iii)     nitric acid/\({\text{HN}}{{\text{O}}_{\text{3}}}\) / nitrous acid/\({\text{HN}}{{\text{O}}_{\text{2}}}\);

Many candidates identified that dissolved carbon dioxide causes the rain water to be acidic but did not show with the help of equation partial dissociation of carbonic acid.

The source of nitrogen monoxide and the method used to control its emission appeared to be well known and many could include appropriate chemical equations. Very few candidates could identify that nitric or nitrous acid is responsible for acidity in lakes and rivers. Some candidates formed \({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\), \({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{3}}}\), \({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}\) and \({{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}\) from nitrogen monoxide.