| Date | May 2011 | Marks available | 1 | Reference code | 11M.2.sl.TZ2.2 |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Deduce | Question number | 2 | Adapted from | N/A |
Question
The element antimony, Sb, is usually found in nature as its sulfide ore, stibnite, \({\text{S}}{{\text{b}}_{\text{2}}}{{\text{S}}_{\text{3}}}\). This ore was used two thousand years ago by ancient Egyptian women as a cosmetic to darken their eyes and eyelashes.
One method of extracting antimony from its sulfide ore is to roast the stibnite in air. This forms antimony oxide and sulfur dioxide. The antimony oxide is then reduced by carbon to form the free element.
Deduce the oxidation number of antimony in stibnite.
Deduce one other common oxidation number exhibited by antimony in some of its compounds.
Deduce the chemical equations for these two reactions.
Markscheme
+3;
Do not accept 3, 3+ or the use of Roman numerals.
+5 / –3;
Penalize incorrect format only if not penalized in (a)(i).
\({\text{S}}{{\text{b}}_2}{{\text{S}}_3} + {\text{4}}\frac{1}{2}{{\text{O}}_2} \to {\text{S}}{{\text{b}}_2}{{\text{O}}_3} + {\text{3S}}{{\text{O}}_2}/{\text{2S}}{{\text{b}}_2}{{\text{S}}_3} + {\text{9}}{{\text{O}}_2} \to {\text{2S}}{{\text{b}}_2}{{\text{O}}_3} + {\text{6S}}{{\text{O}}_2}\);
\({\text{2S}}{{\text{b}}_2}{{\text{O}}_3} + {\text{3C}} \to {\text{4Sb}} + {\text{3C}}{{\text{O}}_2}/{\text{S}}{{\text{b}}_2}{{\text{O}}_3} + {\text{3C}} \to {\text{2Sb}} + {\text{3CO}}\);
Ignore state symbols.
Examiners report
This question proved difficult to candidates as the antimony was unfamiliar to them. However they were expected to just apply what they already knew about other members of the group such as nitrogen and phosphorous. Those that could calculate the oxidation state of antinomy in stibnite often forgot to add the + charge.
Writing the chemical equations proved difficult for candidates but again many picked up 1 out of 2 marks as ecf was applied.
