Question
Outline how one calcium compound in the lime cycle can reduce a problem caused by acid deposition. [1]
Answer/Explanation
Ans
«add» Ca(OH)2/CaCO3/CaO AND to «acidic» water/river/lake/soil
OR «use» Ca(OH)2/CaCO3/CaO in scrubbers «to prevent release of acidic pollution»
Accept any correct name for any of the calcium compounds listed.
Question
Some of the most important processes in chemistry involve acid-base reactions.
Describe the acid-base character of the oxides of each of the period 3 elements, Na to Cl.
State one example of an acidic gas, produced by an industrial process or the internal combustion engine, which can cause large-scale pollution to lakes and forests.
Suggest one method, other than measuring pH, which could be used to distinguish between solutions of a strong acid and a weak acid of the same molar concentration. State the expected results.
Answer/Explanation
Markscheme
Na, Mg: basic;
Al: amphoteric;
Do not accept amphiprotic.
Si to Cl: acidic;
Award [1] for stating oxides become more basic towards left/Na and more acidic towards right/Cl.
Do not penalize incorrect formulas of oxides.
\({\text{N}}{{\text{O}}_{\text{2}}}\)/nitrogen dioxide / \({{\text{N}}_{\text{2}}}{{\text{O}}_{\text{4}}}\)/dinitrogen tetroxide / \({\text{S}}{{\text{O}}_{\text{2}}}\)/sulfur dioxide / \({\text{S}}{{\text{O}}_{\text{3}}}\)/sulfur trioxide;
Do not accept NO/NOx/CO2/CO.
measure electrical conductivity;
strong acids are good conductors/better conductors than weak acids / weak acids are poor conductors;
OR
react with magnesium or a named active metal/metal carbonate/hydrogen carbonate/bicarbonate;
Do not accept Na/K
strong acids react faster/more gas bubbles (per unit time)/more heat produced / weak acids react slower/less gas bubbles (per unit time)/less heat produced;
Do not accept answers based on titration curves as they are based on pH.
Accept Neutralization: weak acid would produce less energy/less temperature increase compared to a strong acid.
Examiners report
The majority of candidates gave the correct answers to (a), but a few were confused about the acid-base character of the oxides of aluminium and silicon.
Part (b) proved to be a difficult question. Not many candidates gave the name or formula of an acidic gas produced by an industrial process. Some wrong answers were: CO, SO, \({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\), CFCs, Methane, \({\text{N}}{{\text{H}}_{\text{3}}}\).
There were a few good answers to (c); measuring the conductivity or the reaction with magnesium or calcium carbonate was a possible method for distinguishing between a strong and a weak acid of the same concentration.