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Date May 2012 Marks available 5 Reference code 12M.3.sl.TZ1.E2
Level SL Paper 3 Time zone TZ1
Command term Describe, Identify, and Outline Question number E2 Adapted from N/A

Question

One major environmental problem that affects many countries is acid rain.

Nitrogen monoxide pollution is a major contributor of acid rain.

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

[2]
a.

(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.

[5]
b.

Markscheme

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

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

a.

(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/\({\text{HN}}{{\text{O}}_{\text{2}}}\);

b.

Examiners report

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.

a.

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.

b.

Syllabus sections

Core » Topic 8: Acids and bases » 8.5 Acid deposition
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